A Watt-Hour Budget for Your Pet's Equipment: The Math, and the Four Things the Division Leaves Out

To estimate how long a power station will run your pet’s equipment, divide the station’s published watt-hours by the equipment’s rated watts. The result is a ceiling, not a promise: conversion loss, unusable battery capacity and cold all shorten the real run, while a thermostat can stretch it. Read the wattage off your own equipment, plan under the number you calculate, and never treat it as an assurance for anything an animal or a person depends on.

Read this first.

• Generators never run inside. The US Consumer Product Safety Commission (CPSC) says: “NEVER use inside a home or garage, EVEN IF doors and windows are open.” A battery power station is a different product with different rules (see below).
• A person’s medical device is not part of this calculator. If someone in your household depends on powered equipment, that question belongs to the device’s labeling, the equipment supplier and the prescribing clinician.
• If an animal is in distress, call your veterinarian or an emergency clinic. At that point the power budget has stopped being the problem you are solving.

This page is the arithmetic underneath our portable power station comparison, which is where the which-unit question lives. You start from the equipment already in your house, read its numbers off its own data plates, and end with a watt-hour budget that belongs to your household.

A power station’s headline number is watt-hours, which is stored energy. Your equipment’s headline number is watts, which is how fast it spends that energy. Divide the first by the second and hours fall out. That is the whole calculation, and it is not the answer, because both inputs are softer than they look.

What this page will not do

• It will not tell you a battery is enough. Enough is a guarantee about an outage of unknown length, in a room of unknown temperature, with equipment this page has not seen.
• It will not supply your equipment’s wattage. A number borrowed from a similar product is wrong in a way that compounds across every hour of the calculation.
• It will not rank a human medical device against an animal’s equipment. That is a clinical decision. Human life outranks animal welfare wherever the two compete for the same battery.
• It will not give the hour at which you move the animal instead of powering it. No published source gives one (see the end of this page).

Step one: read the number off your equipment

Every honest power budget starts at a data plate: the small printed panel, sticker or molded label carrying the model number, the voltage, and either the wattage or the current in amps. On a refrigerator it is usually inside the cabinet or on a side or rear panel. On an aquarium or terrarium accessory it is often on the plug-in transformer rather than the device in the tank. On a fan it is typically on the motor housing or the base.

Danby’s product page for its 1.6 cubic foot compact refrigerator has a customer FAQ that asks what wattage and amps the unit uses. Its answer: “Information pertaining to watts and amps can be found on the rating plate located on the side of the unit.”

Some manufacturers do publish the figure. Exo Terra’s Heat Mat page says the line is “Available in 4, 8, 16 and 25 Watts.” Zoo Med’s instruction sheet for its ReptiCare Ceramic Heat Emitter begins “Use appropriate wattage Heat Emitter for your terrarium.” It recommends 60 watt for a 10-20 gallon terrarium, 100 watt for 30-40 gallon terrariums, and 150 watt for 50-100 gallon terrariums or large wire Iguanariums. It then adds: “Because of variables like ambient room temperature and ventilation, more or less heat may be required.” The manufacturer is giving you a rated wattage and telling you the wattage that suits your room may not be the one on the shelf.

What to write down, per device

One line per powered item, filled in before a season rather than during an outage:

• Model number, exactly as printed. Sizes within one product line differ in wattage: Exo Terra’s heat mats run from 4 to 25 watts.
• Volts and either watts or amps. Volts times amps gives volt-amps, not watts. The two match only for a purely resistive load such as a heat mat or heating element: a plate reading 120 V and 0.5 A is a 60 W device by that multiplication. For anything with a motor or compressor (an air pump, a filtration pump, a box fan, a refrigerator), the volt-amp product is an upper bound on real watts unless the plate or manual separately publishes a watt rating or power factor. Using volt-amps in place of an unpublished watt figure makes your runtime conservative rather than optimistic.
• Where the figure came from: the plate, the manual, or the manufacturer’s page. A year from now you will not remember.
• Whether the device is thermostat-controlled or runs continuously. This is the biggest fork in the next section.
• Whether the manufacturer publishes a starting or surge figure. The Westinghouse generator manual quoted below says manufacturers rarely publish it.

There is no federal energy label on pet equipment

For a household appliance you may be used to the yellow EnergyGuide label. The Federal Trade Commission’s Energy Labeling Rule, 16 CFR Part 305, sets label content product type by product type. As of the eCFR text on September 29, 2026, its label-content sections cover refrigerators, refrigerator-freezers and freezers, clothes washers, dishwashers, water heaters, room and portable air conditioners, pool heaters, central air conditioners, heat pumps and furnaces, ceiling fans, lighting products, plumbing products and televisions. Aquarium heaters, air pumps, substrate heaters, ceramic heat emitters and box fans are not among them.

So for the pet half of your budget there is no standardized energy figure to look up. There is a nameplate and there is the manufacturer.

Step two: nameplate watts is a ceiling, not a consumption rate

A device’s rated wattage describes how much it draws while it is drawing. It does not describe how much energy it uses over an hour, because many loads in a pet household are switched off for part of every hour by a thermostat or compressor control.

Zoo Med’s sheet describes the thermostat it recommends this way: “This thermostat will control all your terrarium heating devices and shut them off if your terrarium gets too warm.” A 100 W emitter held off for half the hour has used 50 watt-hours in that hour, not 100. Exo Terra says its mats can be used with an Exo Terra Thermostat and/or with additional heat sources for higher-temperature species.

Refrigeration behaves the same way. Danby’s FAQ answers “My compressor comes on often. Is this normal?” with “This is normal.” The compressor cycles, so its running watts are the wrong input for an energy budget and the right input for a capacity check.

The one category with a test-derived energy figure

Under the Energy Labeling Rule, section 305.14 requires labels for refrigerators, refrigerator-freezers and freezers to carry the model’s estimated annual energy consumption. Section 305.2 defines that as the energy likely to be consumed annually in representative use, as determined by tests prescribed under the Act.

Here it is worked on one named model. The EnergyGuide label Danby publishes for its DCR016A3WDB compact refrigerator (label revision code 202402) states an Estimated Yearly Electricity Use of 207 kWh. In the units a power budget uses:

• 207 kWh is 207,000 watt-hours per year.
• Divided by 365 days, that is about 567 watt-hours per day.
• Divided by 24 hours, that is about 24 watts averaged across the whole day, cycling included.

The gap between that average and any running-watts figure is the duty cycle, made visible.

The caveats are on the label and in the rule. The label says: “Compare ONLY to other labels with yellow numbers.” The rule’s definition rests on representative use under a prescribed test, and an outage is not that test. Our reasoning, not the label’s: in an outage the room is likely warmer, the door gets opened, and a warm cabinet works harder. Treat the label figure as the best-case daily energy and plan above it. It is also a figure for that model, and yours is a different appliance until you have read its own label.

If you want your own number

The only way to know what a device draws in your room is to measure it with a plug-in energy meter that reports watt-hours accumulated over time. Run it for a full day on the equipment you plan to back up, in the season you expect an outage, and write down the watt-hours. That beats every published figure on this page.

Step three: the division, and the four things it ignores

With a load in watts and a pack in watt-hours, capacity divided by load equals hours. Jackery’s knowledge article on watt-hours presents its version as “Working Time = Capacity Wh × 0.85 / Operating wattage of the device”. That page does not say what the 0.85 accounts for, and it does not present the result as a promise. Its disclaimer says appliance runtimes are for reference only and that “Actual runtime may vary under different conditions.” Here are the conditions.

1. Conversion loss between the cells and the outlet

Household equipment runs on alternating current and the cells store direct current. The inverter in between converts one to the other, and some energy is lost in the conversion.

No specification table cited here publishes an inverter efficiency for a given unit. EcoFlow’s RIVER 2 Pro product page and manual do not, and Jackery’s 0.85 is the only conversion factor among the sources cited here, offered as a rule of thumb in a consumer article rather than as a specification for a named model. Treat it as a manufacturer’s planning convention, not a measured property of your unit.

2. The capacity on the box is not the capacity you can withdraw

A pack’s published watt-hours describe the cells. What you can pull out is decided by the battery management system, which stops the discharge before the cells are empty to protect them. A footnote to EcoFlow’s RIVER 2 Pro specification table lists its protections as over-voltage, overloading, over-temperature, short circuit, low-temperature, low voltage and overcurrent. Low voltage is the cutoff that ends your run.

The units cited here publish nominal capacity and rated output, not usable capacity or depth of discharge. So the withdrawable figure is lower than the nameplate by an amount the manufacturer has not disclosed. Build the plan so the gap does not matter, rather than applying an invented derate.

Do not try to bypass, disable or work around a low-voltage cutoff, a battery management limit or an over-temperature protection to squeeze out the last of a pack.

3. Temperature is published, and it is not one number

This factor is most likely to bite a pet owner, because backup power tends to live in a garage, utility room or shed. EcoFlow publishes four temperature rows for the RIVER 2 Pro:

Published row	Figure
Discharge Temperature	14°F to 113°F
Charge Temperature	32°F to 113°F
Optimal Operating Temperature	68°F to 86°F
Storage Temperature	14°F to 113°F (68°F to 86°F is best)

From 14°F up to 32°F, that unit is inside its published discharge window and outside its charging window. A pack in a freezing garage may run your equipment and then refuse to take a charge, which is the wrong failure to discover on the second night of an outage. EcoFlow’s troubleshooting table lists a “Battery charging low temperature” indicator and says normal operation resumes automatically once the battery temperature is restored to above 3°C.

The optimal range of 68°F to 86°F means that outside that band the unit works, but not at its best. EcoFlow does not publish how much capacity you lose at 20°F, and neither do we. The manual says to strictly follow the operating temperature it specifies, and states the risk at both ends: “If the temperature is too high, it may result in a fire or explosion”, and if it is too low, “the product performance may be severely reduced, or the product may cease to work.” These figures are for one model. Yours publishes its own, and those govern.

4. Duty cycle, running the other way

A thermostat-controlled load uses less than its nameplate suggests, so a nameplate-based division understates your hours. That does not cancel the first three factors. It means your real figure is bracketed rather than known, which is why an outage plan is built on margin rather than precision.

A worked budget, with every assumption stated

The point of the table is that every cell has a named provenance and a named limitation, which is the format your own worksheet should copy. These are figures published by three manufacturers for named products. They are not stand-ins for your equipment.

Item	Published figure	What kind of figure it is	Where it came from
Exo Terra Heat Mat line	4, 8, 16 and 25 watts	Rated wattage: a ceiling while energized; these mats can be run under a thermostat	Exo Terra Heat Mat 4W product page
Zoo Med ReptiCare Ceramic Heat Emitter	60, 100 and 150 watt sizes, recommended by terrarium size	Rated wattage, with the maker’s qualifier that room temperature and ventilation change what is required	Zoo Med instruction sheet CE-60/100/150/250
Danby DCR016A3WDB compact refrigerator	Estimated Yearly Electricity Use 207 kWh	Test-derived annual energy under a prescribed procedure, cycling included; not a running-watts figure	Danby EnergyGuide label, revision 202402
EcoFlow RIVER 2 Pro	Capacity 768Wh; AC output pure sine wave, 800W total (surge 1600W)	Nominal cell capacity and rated output; no usable-capacity or inverter-efficiency figure is published	EcoFlow RIVER 2 Pro product page

Assumptions. Capacity is the manufacturer’s nominal watt-hour figure, because no usable figure is published. Conversion loss is represented by Jackery’s 0.85 planning factor, used because it is the only manufacturer-published factor cited here, not because it is verified for any unit. Ambient temperature is assumed inside the optimal band, which a garage in winter is not. Duty cycling is assumed away for the heater: it draws full rated wattage every minute, which is deliberately pessimistic for a thermostat-controlled load. Any of these can be wrong in your house.

One 25 W substrate heater, treated as continuous, on a 768 Wh nominal pack. The raw quotient is 768 divided by 25, which is 30.7 hours. With the 0.85 factor, 768 times 0.85 divided by 25 is 26.1 hours. That is a ceiling under these assumptions. It is not a floor or a guarantee, and it says nothing about whether 26 hours of substrate heat is the right plan for your animal, which is a veterinary question.

The same pack against a refrigerator described by its EnergyGuide figure. Using the Danby label’s 207 kWh a year, daily energy is about 567 watt-hours. With the 0.85 factor, the pack holds about 653 watt-hours of planning energy, a ratio of about 1.15 to one day of that model’s test-procedure use with nothing else drawing. At the label’s own average, that works out to about 27.6 hours. The label is a standardized annual average. Our reasoning: an outage is likely warmer and the door gets opened, so real daily energy is probably higher by an amount neither the label nor this page can quantify, and that ceiling gets shorter. If your plan turns on where inside that day the pack runs out, measure rather than calculate.

Loads add before they divide, and mixing two kinds of number has to be named. A 25 W mat at full rated wattage alongside the refrigerator’s 24 W EnergyGuide average is 49 W, and 768 times 0.85 divided by 49 is 13.3 hours. That is not a ceiling on the same basis as the 26.1-hour figure. The mat’s 25 W assumes full draw every minute. The refrigerator’s 24 W has the compressor’s duty cycle built in and is a test average that an outage will likely exceed. One half of the 49 W is a pessimistic ceiling and the other an optimistic floor, added as if they were the same kind of number. Treat 13.3 hours as bracketed, and measure the refrigerator’s own draw with a plug-in meter if the difference matters. A third load shortens the runtime in proportion.

Surge is a separate question

Watt-hours decide how long. Watts decide whether it starts at all. Anything with a motor or compressor demands more current for the first moment than once running, and if that exceeds what the inverter can deliver, the load never starts, however full the pack is.

The Westinghouse WGen5500 generator manual gives a rule-of-thumb starting-power multiplier for motor-driven devices: 1.2 for small hand-held or portable devices and 3.5 for larger stationary devices. It also says manufacturers rarely publish starting power demand, so it is often necessary to estimate it. That is one generator maker’s estimate for sizing a generator, and our generator sizing for pet loads page uses it there. It is not a specification for your device or for a power station, so do not carry it onto this calculation.

Instead, read what is published. Power stations publish a rated output and a separate peak output: EcoFlow lists the RIVER 2 Pro at 800W total (surge 1600W). On the appliance side, look for a published starting wattage, locked-rotor current or inrush figure in the manual or on the plate. If the manufacturer does not publish one, ask the manufacturer. If the pack will not start the appliance, that is information about a pairing, found by testing before an outage rather than during one.

Some inverters produce a pure sine wave and some a modified sine wave. Whether a particular pump, controller or medical device tolerates the latter is a statement only that device’s manufacturer can make.

The refrigerator warning in the manual

If you keep refrigerated pet medication, this failure mode is not prevented by correct watt-hour arithmetic. EcoFlow’s RIVER 2 Pro manual carries a numbered safety instruction: when the power supply is connected in the normal mode to a refrigerator, “the power supply may be automatically shut down due to the power fluctuation property of the refrigerator.” If the refrigerator stores medicines, vaccines or other high-value items, the manual highly recommends setting the AC output to “Never Turn Off” through the app, and says users should pay attention to the power supply’s consumption.

The remedy is a setting, named in the manual, that has to be changed in advance, and the manual puts the monitoring burden on the owner. Your own unit’s manual may describe the same behavior under a different setting name, or not at all. Plug the appliance into the pack on a quiet afternoon and watch it for an hour or two rather than assuming a full battery equals a running fridge. Whether medication stays usable after a temperature excursion is a separate question for your veterinarian and the medication’s storage instructions. Our pet medication and refrigeration in a power outage page covers it.

Human medical devices come first, and are not in this calculator

If someone in your household uses a powered medical device, three named parties settle what it needs, whether it can run on battery and what happens if it stops: the device’s own labeling, the durable medical equipment supplier and the prescribing clinician.

EcoFlow’s RIVER 2 Pro manual says the product “is not recommended for powering medical emergency equipment related to personal safety”, including medical-grade (hospital-version) CPAP ventilators and artificial lungs (ECMO). It adds that the unit can normally work with a home ventilator (home-version CPAP) and does not have to be continuously monitored by professionals, then hands the decision back: “Please follow your doctor’s instructions and consult with the manufacturer for restrictions on the use of the equipment.” For general medical equipment, it says to monitor the power status so the power does not run out.

FDA’s guidance page on medical devices during natural disasters (current as of April 16, 2024) tells readers to notify their electric company and fire department that they have a medical device that needs power, and to read the user instructions or call the distributor or manufacturer to find out whether the device can be used with batteries or a generator. Its General Safety section adds: “If you depend on your device to keep you alive, seek emergency services immediately.” It continues: “If possible, notify your local Public Health Authority to request evacuation prior to adverse weather events.”

FDA also publishes a fill-in booklet whose Section 2 is a list of questions for the device manufacturer or medical provider. They include whether the device can operate on another power source, whether you could be harmed if it stops for a short period and for how long, and how long it will last on battery power. Those questions go to a person who knows the device.

For a pet household the consequence is plain. If a person in your home depends on powered equipment, the backup power is spoken for. The animals’ plan is built from whatever is left, and if not enough is left, the answer is not to shave the human margin. Solve the animal’s problem a different way: relocate the animal, arrange boarding, insulate an enclosure instead of heating it, or leave earlier. Our board or evacuate together decision and the aquarium and reptile outage guide cover routes that use no watt-hours.

The animal side is a veterinary question

Which of your animal’s equipment counts as life support depends on the species, the individual animal, its age and its condition, and your veterinarian is the person qualified to say. A filtration pump that is a convenience on one system is the oxygenation path on another. A heat source that is optional in July is not in January.

Instead of a shed order, fill in this worksheet once, in writing, with your vet’s input on the first question, and keep it with your outage plan:

1. What changes for this animal if this device is off for one hour, and who told me so? An answer with no source is a guess.
2. Is that change reversible when power returns? A tank that cools and rewarms is a different problem from one whose biological filtration has died.
3. Is there a non-powered substitute? Insulation, a pre-chilled or pre-warmed mass, a smaller enclosure, a warmer or cooler room, a manual water change. Every load you can answer this way stops competing for the battery.
4. What is this device’s measured or published draw, and where did the figure come from?
5. At what point does this animal need to be somewhere else rather than on backup power? See the last section: this is a question for your vet, and there is no published number.

Any concern about your animal’s health warrants a call to your veterinarian. If the animal is in distress, call an emergency clinic instead of continuing with the battery plan.

Where the battery stops and the generator starts

If the load is bigger than any battery in the house, you have reached a different product with different, non-negotiable rules.

A generator never runs inside. The CPSC safety alert on generators and carbon monoxide (publication CPSC468 0322) says: “NEVER use inside a home or garage, EVEN IF doors and windows are open.” It also says: “Only use OUTSIDE and far away from windows, doors, and vents.” Its recommended practice: “DO operate portable generators outside only, at least 20 feet away from the house, and direct the generator’s exhaust away from the home and other buildings.” CPSC also says not to operate a portable generator inside a home, garage, basement, crawlspace or shed, or on a porch, never to leave a car running in an attached garage, and not to use a generator near doors, windows or vents.

No enclosure, canopy, tent, cracked door, large room, short run or fan makes indoor operation acceptable. CPSC recommends asking retailers for a generator that shuts off automatically when high carbon monoxide concentrations are present, and the never-indoors instruction sits on the same page unchanged. A shutoff feature is an added protection, not a permission. Our generators, carbon monoxide and pets page covers this in full, and quiet generator versus battery power station covers the choice between the two.

Carbon monoxide alarms. CPSC’s alert says: “DO install battery-operated CO alarms or CO alarms with battery backup outside sleeping areas, and on each floor of your home. Test alarms monthly.” CPSC also says never to ignore a sounding CO alarm: get outside immediately, then call 911. Where an alarm goes is set by its manufacturer’s instructions and the code your jurisdiction has adopted. NFPA publishes a 2025 edition of NFPA 72, the National Fire Alarm and Signaling Code. Follow the instructions in the box, and ask your local fire or building department which edition it enforces.

Do not connect a generator to house wiring yourself. The Portable Generator Manufacturers’ Association’s release of September 12, 2025 says a transfer switch, or an interlock device installed by a qualified electrician, are the ways to connect a portable generator to a home’s panel: “These are the only safe ways to use a portable generator to power an entire residence.” Plugging a generator into a wall outlet creates an electrocution risk for utility workers and neighbors on the same transformer. It also bypasses some of the built-in household circuit protection and can cause short circuits, overloads, fires and damage to appliances and wiring. A licensed electrician and your local authority own the rest of that subject.

A battery pack is a different product. ANSI/CAN/UL 2743, the Standard for Portable Power Packs (Third Edition, September 24, 2025), classifies packs as suitable for outdoor use, temporary outdoor use, or indoor use only. Outdoor packs have no restrictions. Temporary outdoor packs are for limited wet conditions and are always stored indoors. Indoor-only packs are stored and used indoors and never outdoors. The standard adds: “A power pack with a booster function is not considered indoor use only under any conditions.” Which class your pack carries is settled by its own documentation. Nothing a generator’s placement rule says gives a battery pack permission, and nothing a pack’s indoor listing says gives a generator permission. EcoFlow’s manual tells owners to “Store the product in a dry and ventilated place” and to “Keep this product out of reach of children and pets.”

Site the pack away from the tank. An indoor listing answers which room the pack may sit in, not where in that room. A tank, sump or reservoir is standing water with a splash risk, a siphon risk and a hose or pump line that can fail. Neither UL 2743 nor EcoFlow’s manual names a tank, so this is our reasoning, not a published protocol: read the dry-place instruction as a siting rule for that spot, and put the pack above and away from the tank, any sump, any drip loop and any hose run, on a surface a leak or siphon cannot reach. Keep the cords out of reach of animals too.

UL 2743 also says it does not cover power banks (covered by UL 2056) or packs permanently connected to premises wiring or able to supply it (covered by UL 9540). A product that connects to your house wiring is outside the portable-power-pack category, and the electrician conversation above applies.

The hour you move the animal: nobody publishes one

No source cited on this page gives a threshold, in hours, at which an owner stops powering an animal’s enclosure and relocates the animal. The federal and standards material here does not address relocation timing, so this page publishes no number. Instead, ask your veterinarian before the season, about your animal and species: what are the signs that this animal needs to be somewhere else rather than on backup power, and where is that somewhere else? Write the answer next to the worksheet. Our aquarium and reptile power outage guide covers relocation as a decision, including what moving an animal costs it, and sheltering in place with pets through power outages sets the wider picture.

Two related timing questions live elsewhere. How long an automatic feeder lasts in a power outage works from named products’ published backup figures. Whether refrigerated medication is still usable after an excursion is a cold-chain question, covered in pet medication and refrigeration in a power outage.

Do this on a quiet afternoon

Nothing above works if it is attempted for the first time in the dark. The exercise takes an evening and lasts for years.

• Walk the house with a notebook and read every data plate: model number, volts, and watts or amps, for every powered item any animal depends on. Include the plug-in transformers, where tank and terrarium figures often sit.
• Mark each item as thermostat-controlled or continuous. That column changes the arithmetic more than any other.
• For any covered appliance, find its EnergyGuide label or the manufacturer’s estimated yearly electricity use, and convert it to watt-hours per day.
• Look up your power station’s published capacity, rated output, surge output and temperature rows. Write down where the pack is stored and what temperature that place reaches in your worst season.
• Read your pack’s manual for its statements on operating location, medical equipment and refrigerators. Change any setting the manual names before you need it, not during.
• Run the arithmetic for your real combination of loads, write down the assumptions beside the answer, and plan under the answer rather than on it.
• Test the pairing. Plug the actual equipment into the actual pack for an hour or two and watch what happens, especially anything with a compressor.
• If the pack sits near a tank, sump or reservoir, site it above and away from any splash, siphon or hose-failure path, on a surface a leak cannot reach, and route its cords where neither water nor an animal can reach them.
• Put the human medical question to the prescribing clinician and the equipment supplier, using FDA’s list of questions, and settle it independently of the animals’ plan.
• Ask your veterinarian which of your animal’s equipment is time-critical, and what the signs are that the animal needs to be moved rather than powered.
• Confirm you have carbon monoxide alarms per CPSC’s instruction if any combustion appliance or generator could ever enter the picture, and test them monthly.

Where to go next

This page is the arithmetic spoke of our sheltering in place through power outages hub.

• Choosing hardware: portable power stations compared for pet loads is the buying-guide answer with manufacturer specifications, and Jackery versus EcoFlow for pets narrows it to two brands. For a tracker or other low-draw device, see solar charger versus power bank.
• Fuel instead of cells: quiet generator versus battery power station compares the categories, and generators, carbon monoxide and pets is the safety ground to read first.
• The specific loads: aquariums and reptiles in a power outage, pet medication and refrigeration in a power outage, insulin cooler cases for a diabetic pet and automatic feeders and fountains without power.
• Heat and cold without the grid: dog cooling gear during a power outage and battery-powered fans for a pet carrier.

Then read three data plates this week and write the numbers down. A household that knows its own watts before an outage is doing arithmetic on real inputs, which is the only kind that holds up in a dark house.