Solar Battery Sizing Calculator

How many kWh of battery do you actually need for backup? Pick what you want to power and for how long — we'll size it and recommend specific batteries.

1. What do you want to back up?

How many hours of backup? Most US outages are under 4 hours. Hurricane-prone areas plan for 24-72h.

Battery depth-of-discharge Modern home batteries use 100% usable LFP chemistry.

Climate Hot or very cold climates reduce effective battery capacity.

Have solar to recharge? (kW) If you have solar, daytime production extends backup runtime significantly.

Pick at least one load above to size your battery.

Selected load—

Energy needed (load × hours)—

Climate / DoD adjustment—

Recommended battery size—

Estimated installed cost—

2. Battery options that fit your needs

How to size a battery — what to actually think about

Battery sizing comes down to three numbers: how much power you draw (kW load), how long you want to run on battery (hours), and how much capacity you actually get from the battery's nameplate rating (depth of discharge × climate derating).

The formula

Total load (kW) = sum of appliances you'll be running

Energy needed (kWh) = load (kW) × backup hours × duty cycle (fridges cycle on/off)

Battery size needed (kWh) = energy needed ÷ (depth of discharge × climate derating)

With solar recharge: you can effectively double or triple runtime in long outages if days are sunny

Three common sizing tiers

Essentials-only (5-10 kWh): Refrigerator, lights, internet, phone chargers, garage door. About one Powerwall or one Enphase IQ Battery.

Whole home, short outage (15-25 kWh): Above + HVAC for sleeping hours + most outlets. One Powerwall 3 + one Powerwall 3 expansion, or three Enphase IQ Batteries.

Off-grid or hurricane-zone (40-60 kWh): Multi-day autonomy with HVAC, water pumping, well, and EV charging. Typically 4-6 Powerwalls or equivalent.

FAQ

How many kWh of battery storage do I need?

For essential loads only (refrigerator, lights, internet, a few outlets), 5-10 kWh covers 8-12 hours of backup. For full-home backup including HVAC, you typically need 20-30 kWh. For 3 days of off-grid autonomy, plan on 40-60 kWh. Most US homeowners install 10-20 kWh.

Is a Tesla Powerwall worth it?

A Tesla Powerwall 3 (13.5 kWh, $9,300-$13,000 installed) is worth it if you have frequent outages, time-of-use rate arbitrage opportunities, or live in a state with NEM 3.0 (California) where exporting solar to the grid pays poorly. For just emergency backup without arbitrage, a smaller battery (5-10 kWh) is usually a better value.

How long will a battery power my house?

It depends on usage. A typical US home draws 1-2 kW continuously. A 10 kWh battery powers an average home for 5-10 hours; a 20 kWh battery for 10-20 hours. With essentials-only loads (fridge + lights), the same battery lasts 2-3× longer.

What's the difference between Powerwall, Enphase, and Franklin?

Tesla Powerwall 3 (13.5 kWh, AC + DC coupled, integrated solar inverter) — most popular, lowest cost per kWh. Enphase IQ Battery 5P (5 kWh modular units, microinverter-compatible) — flexible sizing, best for Enphase solar systems. Franklin Whole Home (15 kWh) — premium build, longer warranty, costs more.