How big a home battery do you need for an outage?

What a home battery has to do

During an outage a battery has two jobs. It must supply enough energy (kilowatt-hours) to cover everything you want running for as long as you want, and it must supply enough power (kilowatts) to run those things at once and to start motors. People often buy on energy alone and then find that the inverter trips on the well pump. This calculator keeps both in view. It adds up a list of critical loads into kilowatt-hours a day, multiplies by the days of backup, corrects for efficiency and for the part of the rated capacity you can really use, and counts how many battery units of your chosen model that takes. It also sums the running watts and the biggest start-up surge for the inverter, and estimates the solar panels needed to refill the battery each day.

The method

Daily energy (Wh) = Σ watts × hours a day × number of devices. Drawn from the battery = daily kWh × days ÷ efficiency. Rated (nameplate) capacity = drawn ÷ usable share. Units = rated capacity ÷ the kWh of one unit, rounded up, and also at least running kW ÷ one unit's kW. Peak watts = Σ running watts + the largest single (starting − running) watts, assuming all loads are on together and only one motor starts at a time. Solar kW = daily kWh ÷ efficiency ÷ (sun hours × (1 − losses)). All of this is arithmetic we derived; none of it is a code requirement. Compare with the battery runtime calculator, which answers "how long does this battery run this one load", and the generator size calculator, which adds up watts but not energy.

Worked example

A refrigerator of 150 W running 8 hours a day (starting at 600 W), eight 10 W LED lights for 6 hours, a 20 W router around the clock and 60 W of phone and laptop charging for 4 hours. The daily energy is 2,400 Wh = 2.4 kWh. One day of backup at 90 percent efficiency means 2.67 kWh drawn, and with 100 percent usable that needs 2.67 kWh of rated capacity: one 13.5 kWh unit covers it many times over, and could carry the same list for about 5.1 days. All the loads together are 310 W, and the refrigerator's start adds 450 W for a moment, a peak of 760 W. To refill the day's 2.4 kWh with 4 sun hours and 14 percent losses takes about 0.8 kW of panels. With a lead-acid battery limited to 50 percent depth of discharge, the same single day needs 5.3 kWh of rated capacity. Add an electric well pump and an hour of heating and the numbers climb: that is why the load list is the part to get right.

Limits of this method

• Hours and watts are yours. Refrigerators, freezers and furnace fans cycle on and off; the hours per day are the compressor run time, not 24. Measure with a plug-in watt meter, or use a clamp or a smart-panel reading. We give no typical wattages, because they depend on the model.
• Rated capacity is not a promise. The Tesla datasheet's 13.5 kWh is quoted at 25 °C at the beginning of life; datasheet values are given for 25 °C and a new battery, and real capacity changes with age and temperature. Lead-acid batteries deliver less when drained fast (Trojan rates capacity at the 20-hour rate). Leave a margin.
• Depth of discharge varies by product. We found no government table of DoD by chemistry; blogs disagree. Use the datasheet and warranty of the actual battery.
• Solar in an outage. A grid-tied solar system normally shuts down in an outage unless it is part of a backup-capable system. Cloud and winter cut the sun hours, so the recharge figure is a best case.
• Installation and safety. Home batteries and backup panels connect to your service and must be installed to the National Electrical Code and local rules by a licensed electrician; never backfeed a house with a generator through an ordinary outlet.

Planning tool only: not a substitute for a licensed electrician, the battery maker's sizing tool or the installer's load calculation. Tell them what must keep running; check the inverter's surge rating against your largest motor.

How do I size a home battery for a power outage?

List what must keep running, with its watts and the hours it runs per day, and add up the watt-hours. Divide by the efficiency of the battery and inverter, and by the share of the battery's rated capacity you can actually use. Multiply by the days you want to ride out. The example list in the form (refrigerator, eight LED lights, router, charging) totals 2.4 kWh a day; at 90 percent efficiency that is 2.67 kWh drawn from the battery for one day and 8 kWh for three days.

What is depth of discharge and what should I enter?

Depth of discharge (DoD) is the share of a battery's rated energy you can use without shortening its life or tripping its limits. Enter the number from the datasheet or warranty of the battery you are looking at, and read how the maker defines its capacity: some, like the Tesla Powerwall 3 datasheet, quote the battery energy (13.5 kWh) as the usable figure, so you enter 100 percent. Other batteries quote nameplate capacity and a DoD limit: Trojan Battery recommends discharging its flooded lead-acid batteries only 20 to 50 percent for the longest life (up to 80 percent is possible), while Battle Born rates its LiFePO4 battery for 100 percent. Blog tables of typical DoD for lithium chemistries disagree, so we do not preset them.

What is the difference between kW and kWh?

kWh is energy: how long you can keep things running. kW is power: how many things you can run at once. A battery with a 13.5 kWh capacity and an 11.5 kW inverter (the figures of one current product's datasheet) can run 11.5 kW of loads at once, but only for about an hour at that power; with the example list drawing 0.31 kW when everything runs together, the same battery lasts for many hours. You need both numbers to be enough: capacity for the hours and inverter power for the peak.

What about motor start-up surge?

Motors in refrigerators, well pumps and air conditioners draw several times their running watts for a moment when they start. The inverter must deliver that surge. The calculator adds the largest single start-up (starting watts minus running watts) on top of the total running watts: 310 W running plus 450 W gives a peak of 760 W in the example. A datasheet states the surge limit in some form (the Powerwall 3 datasheet lists a load start capability of 185 amps LRA, locked-rotor amps); check it against the nameplate of your largest motor. Starting watts are on the appliance label or in its manual.

How many solar panels do I need to recharge the battery?

To refill what you use in a day from the sun you need about daily kWh ÷ round-trip efficiency ÷ (peak sun hours × (1 − system losses)) kilowatts of panels. With 2.4 kWh a day, 90% efficiency, 4 peak sun hours and the PVWatts default loss of 14%, that is 0.8 kW. This is our own derivation. Peak sun hours depend on your location and season, and in an outage in cloudy winter weather they can be much lower; use the solar panel calculator on this site and NREL PVWatts for a real location. A battery with solar needs a hybrid inverter or a system designed for islanding during an outage.

Can a battery run my whole house, or just essentials?

For whole-house backup, the loads of electric heat, air conditioning, dryers, ovens and EV charging dominate the list. Add the big ones and the answer jumps: adding a 1,500 W load for 2 hours and an 800 W pump with a 1,600 W start brings the two-day requirement of the example to 22.7 kWh and 2 unit(s) of 13.5 kWh. A common approach is to back up essentials (refrigerator, lights, internet, a few outlets, sometimes a well pump or furnace fan) and leave the heavy loads off. A generator can add days: see the generator size calculator.

Sources: Tesla, Powerwall 3 Datasheet (2025; 13.5 kWh nominal battery energy, 11.5 kW continuous AC in the US, 185 A LRA, 89% solar-to-battery-to-home efficiency); Trojan Battery FAQ and Battle Born Batteries manual, as used on the battery runtime page; NREL PVWatts (system losses 14%). Related tools: battery runtime calculator, generator size calculator, solar panel calculator, electricity cost calculator. How we check our calculators.