Home Battery Backup Calculator: what size battery do you need?

Capacity and output are two different numbers

Capacity is how much energy a battery holds, measured in kilowatt-hours (kWh). Think of a fuel tank. Output is how much power the battery can deliver at one moment, measured in kilowatts (kW) or watts (W). Think of the width of the fuel line.

A battery can pass one test and fail the other. A 10 kWh battery with a 2 kW inverter can run a refrigerator for days, but it cannot start a well pump that needs 3 kW for a moment. A 6 kW inverter on a 2 kWh battery can run a heavy load, but only for a short time.

This calculator reports three numbers:

• Usable capacity needed (kWh). Daily energy times outage days, divided by 0.90 depth of discharge and 0.90 inverter efficiency. A battery is not drained to zero, and the inverter wastes some energy turning battery power into household power. Both losses are already included, so this is the battery size to look for.
• Continuous output (kW). The running watts of every checked load that runs at the same time.
• Surge (kW). The worst single start-up: one load’s surge plus the running watts of everything else that can be on at that moment.

Motors cause most surge problems. A compressor or pump can pull several times its running watts for a fraction of a second as it starts. If the inverter cannot supply that, it shuts off, even with a full battery. That is why surge gets its own number.

Typical appliance watts

These are the starting figures the calculator fills in. Labels on your own equipment are better. Central air conditioners are filled in at 1,200 running watts per ton of cooling, and surge at three times running watts.

Refrigerators and freezers cycle on and off. A fridge that draws 150 W while running may only run a third of the day, so enter about 8 hours rather than 24.

Portable power station or whole-home battery

A portable power station is a battery and inverter in one box. You plug appliances into its outlets, or connect it to your house through an inlet and a transfer switch. You can buy one and use it the same day. Larger models can be expanded with extra batteries and can output 240 V, but you still decide, outage by outage, which things are plugged in.

A whole-home battery is installed by a licensed installer. It is wired into your electrical panel, usually with a gateway that disconnects your house from the grid and switches over on its own. It can cover the circuits you choose, or the whole house, and it often works with rooftop solar. It also needs a permit, and in many areas an approval from your utility.

Neither is better in every case. Portable systems suit people who want a lower starting cost, a short list of essential loads, or the option to take the battery with them. Installed systems suit people who want automatic switchover, who have larger loads such as central air conditioning, or who are adding solar. Questions worth asking either way:

• Which circuits or appliances will it run, and which will it not?
• Does it switch over automatically, or do you have to do something?
• Does it make noise, and where can it be placed safely?
• What does the warranty cover, and for how long?
• Who installs it, and who services it?

The shortlist above includes one installer-only reference system so you can compare an installed battery with the portable options on the same three numbers.

What 240 V loads mean for backup

Most North American homes have split-phase service: two 120 V lines that together give 240 V. Everyday outlets use one line at 120 V. Big appliances such as a well pump, central air conditioner, electric water heater, electric range and EV charger use both lines at 240 V.

A battery that only has 120 V outlets cannot run those loads. You need a battery system with a true 240 V output, such as a 30 amp 120/240 V outlet on a large power station, a paired-unit kit, or an installed whole-home system. Even then the output has a ceiling. A 30 amp, 240 V outlet tops out around 7.2 kW.

The calculator flags every 240 V load you check, and marks any product without a 240 V output as not meeting your needs. Some products that reach 240 V only do so when two units are joined, so read the maker’s page carefully.

Connecting a battery to your house wiring is electrical work. A licensed electrician should confirm any panel, transfer switch or 240 V work. Never feed power into a house through a dryer outlet or a homemade cable. It can energize lines outside your home and injure utility workers.

If your large loads exceed what you can afford to supply, you have options. Use the stagger setting so only the largest runs at once. Move a load to fuel, such as a gas water heater or a camp stove. Or leave it off the backup list and plan around it.

How solar changes the math for multi-day outages

Without solar, the battery has to hold every day of the outage. Three days at 10 kWh a day means 30 kWh of energy, before losses.

With solar, panels refill the battery on sunny days, so after day one the battery only has to cover the shortfall. The calculator estimates daily recharge as solar kW times peak sun hours times 0.80. The 0.80 allows for real-world losses such as heat, wiring and dirt. A 5 kW system with 5.5 peak sun hours recharges about 22 kWh a day.

The requirement then becomes day-one energy plus the daily shortfall on each remaining day, and never less than one day of energy. If solar covers the full daily use, you only need to size for one day. If your use is 30 kWh and solar gives 22, the shortfall is 8 kWh on each remaining day.

Peak sun hours measure how much sunlight an area gets, expressed as hours of full-strength sun. The calculator starts at 4.5, or 5.5 for ZIP codes in California, Arizona, Nevada, New Mexico and Texas. Treat those as starting points, and edit the value if you know your local figure.

Several things can undo the benefit:

• Many grid-tied solar systems shut down during an outage for safety. Solar only helps if the system is set up to keep running when the grid is down.
• Clouds, snow on panels and winter days cut output, sometimes sharply.
• A battery can only accept so many watts of charge. Check the maker’s solar input limit.
• Loads that run in the evening still come from the battery, so charge level at sunset matters.

For a long outage, solar is best treated as extending your battery, not replacing it.

How to size for a medical device

This is general information, not medical advice. If a device protects someone’s health, talk to the equipment supplier or the person’s clinician about backup power.

Start with the label. Find the watts or amps and voltage printed on the device or its power adapter, and enter those watts. A CPAP machine often draws well under 100 W, though a heated humidifier or hose adds to that. Oxygen concentrators commonly draw 300 W or more. Because the prefilled figure in the calculator is only a CPAP-class example, always replace it with your own.

Then think about the hours. A CPAP runs overnight, while an oxygen concentrator may run around the clock. Enter realistic hours, then add a margin beyond what the calculator gives you. Extra margin is safer than a battery that runs out at night.

A few practical steps:

• Check the device maker’s guidance on power sources. Some devices need a pure sine wave supply.
• Keep the battery charged and test it before you need it.
• Keep medical loads on their own outlet or circuit so they are not switched off with the rest.
• Ask your utility whether it keeps a list of customers who rely on powered medical equipment.
• Have a plan for what to do if the power stays off longer than your battery lasts.