Home battery size calculator

How to size a home battery

A battery has two limits, just like a generator. Its energy, in kWh, decides how long it lasts. Its power, in kW, decides how much can run at the same time. You size for both, then allow for the energy the battery keeps in reserve and loses along the way. Here is the math for the default outage list.

1. 1

   Add up the energy per day

   Watts × hours ÷ 1,000 for each item: refrigerator 1.2 kWh, furnace fan 5.25, sump pump 1.05, microwave 0.25, lights 0.6, TV 0.32, internet 0.72, charging 0.23. Total: 9.6 kWh a day.

2. 2

   Scale to the outage

   A 24-hour outage needs the full 9.6 kWh at your outlets. A 12-hour outage would need half.

3. 3

   Allow for depth of discharge and losses

   The battery only drains to 90% of its capacity, and about 5% is lost on the way out (the square root of 90% round-trip efficiency). 9.6 ÷ (0.90 × 0.95) = 11.3 kWh of battery.

4. 4

   Check the power

   If every item ran at once, they would draw 3,360 W, or 3.4 kW. A battery rated 5 kW continuous covers that.

5. 5

   Round up to whole batteries

   One 13.5 kWh battery clears both limits. It delivers about 11.5 kWh, enough for about 29 hours of this list. At $1,150 per kWh it costs about $15,525 installed.

kWh vs kW: energy and power

Think of kWh as the size of the tank and kW as the width of the hose. A battery with lots of kWh but a small inverter can run a fridge for days, but it trips when the well pump and the microwave start together. Motors also pull two to three times their running power for a second when they start. Battery inverters handle a short surge above their continuous rating, but how much varies by model. Well pumps and central air are the usual trouble spots; a soft-start kit on the AC often solves it.

What common loads use in a day

These are the calculator's defaults. The best numbers come from your own appliances' labels or a plug-in power meter.

The pattern is clear: lights, internet and the fridge are cheap to back up. Anything that heats or cools, such as central air, electric heat, a water heater or an electric range, is what turns a one-battery job into a three-battery job. Many installers set up a backed-up "essentials" subpanel so those big loads stay off during an outage.

Depth of discharge and round-trip efficiency

Depth of discharge is how much of the battery's capacity you actually use. Lithium iron phosphate (LFP) home batteries are commonly rated for 90% to 100%. Some spec sheets already list "usable" kWh; if yours does, set depth of discharge to 100%. Many owners also keep a backup reserve, say 20%, during normal daily cycling so there is always something in the tank when the grid fails.

Round-trip efficiency is the share of the energy you put in that comes back out as AC power, typically around 90% for current lithium systems. The calculator charges half of the loss on the way in and half on the way out. For time-of-use, that means each kWh you shift costs 1 ÷ 0.9 = 1.11 kWh of charging.

Solar and time-of-use savings

A battery earns money when the power it stores is worth less than the power it replaces. That happens under time-of-use rates with a big evening peak, and under net billing, where solar exports earn far less than retail (California's net billing tariff is the best-known example). In the default example, shifting 10 kWh a day from a $0.10 charging value to a $0.45 peak saves 10 × (0.45 − 0.10 ÷ 0.9) = $3.39 a day, or about $1,237 a year. On a $15,525 battery that is a payback of about 12.6 years.

Under full net metering with flat rates, a battery saves almost nothing, because the grid already acts as free storage. Run our solar payback calculator first, then use this one to see if storage adds enough. The prices in this mode are examples; take yours from your utility's tariff sheet.

What batteries cost, and the tax credit

EnergySage's marketplace put a typical 13.5 kWh system at $15,647 installed before incentives in the first half of 2026, about $1,160 per kWh. State averages ranged from $563 per kWh in Alabama to $1,586 in Louisiana. Extra batteries on the same install usually cost less per kWh than the first, because the gateway, permits and labor are shared.

There is no federal tax credit for an owned battery installed in 2026. Battery storage of 3 kWh or more qualified for the 30% Residential Clean Energy Credit (section 25D) starting in 2023. The 2025 budget law ended that credit early, and the IRS now says it "is not available for any property placed in service after December 31, 2025." There is no separate exception for batteries. Leased or third-party-owned systems fall under different business credit rules. Some states and utilities pay battery incentives, often in exchange for letting the utility draw on the battery during grid emergencies.

Battery vs generator

A portable generator with an interlock costs about $1,500 and runs as long as you feed it fuel. A 22 kW standby generator runs $10,000 to $15,000 installed and can carry central air. A battery costs more for less runtime, but it is silent, switches over in a split second, makes no carbon monoxide and can work every day to cut your bill. For outages of a day or two, especially with solar to recharge, a battery works well. For week-long outages or big electric loads, a generator, or a battery plus generator, is the safer plan.

Tips before you buy

• Measure, don't guess. A plug-in meter on the fridge for a day, and your utility's hourly usage download, beat any table.
• Winter outages cost more. A gas furnace still needs its blower, and an all-electric home with resistance heat may need far more storage than any battery can hold.
• Ask about solar recharge. With panels, a battery can refill on sunny days and stretch an outage for several days. Without them, it is a one-shot backup.
• Hire a licensed installer. Batteries connect to your main panel and must meet the National Electrical Code and the fire code rules for energy storage (NFPA 855), with a permit and inspection.
• Read the warranty. A common pattern is 10 to 15 years with a cycle or energy-throughput limit. Daily time-of-use cycling uses it up faster than backup only.

Sources and assumptions

• Federal credit: IRS, Residential Clean Energy Credit (page updated July 4, 2026): battery storage qualified from 2023; "not available for any property placed in service after December 31, 2025."
• Installed cost: EnergySage, solar battery cost (updated July 13, 2026; quotes from the first half of 2026). The $1,150 per kWh default rounds the typical 13.5 kWh price.
• Example battery specs: a 5 kWh modular battery listed at 3.84 kW continuous, 7.68 kW for 3 seconds, 15-year warranty up to 6,000 cycles. Cited as a reference, not a recommendation.
• Appliance energy method and refrigerator duty cycle: DOE Energy Saver, Estimating Appliance and Home Electronic Energy Use. Motor running watts match our generator calculator (Generac and Champion wattage charts). CPAP: 56 W, the typical power draw in ResMed's AirSense 11 specifications as listed by CPAP.com (peak 73 W); without the heated humidifier it uses less. Internet: 30 W rounds up the 9 to 29 W ready-state draw in Xfinity, internet equipment energy usage. Both read November 1, 2026. The charger value is a HomeOutlay estimate.
• Generator prices and fuel: estimates from our generator size calculator (5,000 W portable about $700, interlock about $800, 22 kW standby $10,000 to $15,000 installed, about 10 gallons of gasoline a day at half load), checked against HomeGuide, transfer switch installation cost (interlock $400 to $850 installed) and HomeGuide, whole house generator cost (portables $500 to $2,500); gasoline $4.27 from the EIA, week of October 26, 2026.
• Net billing example: California Public Utilities Commission net billing tariff. The $0.45 peak and $0.10 charging values are placeholders, not a specific tariff.
• HomeOutlay assumptions: 90% depth of discharge and 90% round-trip efficiency by default; losses split evenly between charging and discharging; every ticked load counted as running at the same time for the power check; no solar recharge during an outage; prices as of October 2026.

Common questions