Home Battery Sizing Guide
Sizing a home battery is two questions: how much energy (kWh) you need to ride through an outage, and how much power (kW) your circuits draw at once. Most California households land in one of two camps: 5-10 kWh for critical loads, or 20-30+ kWh for whole-home backup. This guide walks the kWh math, the critical-loads method, and how solar and PSPS patterns change the answer. Cost guide only -- your installer sizes the final system.
Orange County, CA · Updated 2026 · Costs shown as ranges — your quote will vary by contractor and scope
Energy vs. power: the two numbers that matter
A battery has two ratings and shoppers mix them up. Energy (kWh) is the size of the tank -- how long things run. Power (kW) is the width of the pipe -- how much can run at once. A 13 kWh battery with 5 kW of continuous output runs a 1 kW load for about 11 usable hours (after efficiency losses), -- the pipe is too narrow for a 7 kW AC regardless of the tank.
Usable capacity is smaller than the nameplate: most systems reserve 5-10% at the bottom to protect the cells, and round-trip efficiency is about 85-90%. A 13.5 kWh nameplate battery delivers roughly 11-12 kWh you can actually use. Ask installers for usable kWh and continuous kW -- those numbers, not the brand name, determine what your house can do.
The critical-loads method: 5-10 kWh
Start with what must stay on: refrigerator (1-2 kWh/day), Wi-Fi and phones (under 0.5 kWh), some lights, medical devices. A typical critical-loads list draws 3-6 kWh per day -- a 10 kWh battery covers roughly a day and a half, or a full day with cloudy-day margin.
Well pumps, sump pumps, and electric water heaters are the budget-killers: a well pump can draw 2 kW while running and needs the battery's surge capacity to start. With a pump or medical equipment on the list, size power (kW) first and energy (kWh) second -- an undersized inverter that cannot start your pump is a paperweight in an outage.
Whole-home backup: 20-30+ kWh and the honest math
Whole-home backup means the battery carries everything, including the 3-5 kW draw of central air conditioning. A home using 25-30 kWh a day needs 27-33 kWh of usable storage for one full day -- two to three 13 kWh units, $30,000-$45,000 installed.
Most households do not need it. The cost jump from one battery to three buys a lot of hotel nights, and NEM 3.0 self-consumption -- the daily bill-saving use -- rarely justifies more than one or two units. Partial-home backup with a critical-loads subpanel is the value play for roughly 8 in 10 buyers; whole-home is for medical needs, home businesses that cannot go dark, or buyers who simply want it and can afford it.
Sizing for solar + PSPS outages
Solar changes the math: a 6-8 kW array generates 24-32 kWh on a sunny day, refilling a 13 kWh battery daily while the house runs. That turns one battery into indefinite critical-loads coverage during multi-day PSPS outages -- sun permitting. Size for the overnight gap, not the whole outage.
In fire zones with 2-3 day PSPS events, the winning combo is one 13-14 kWh battery plus enough solar to recharge it daily, with a small portable generator as the cloudy-day backstop. Skip the third battery and put the $12,000 toward the generator and a transfer switch -- it covers the edge cases the batteries cannot, for a fraction of the price.
Home battery sizing cheat sheet
| Backup goal | Typical size |
|---|---|
| Phones + Wi-Fi + lights (evening) | 5 kWh |
| Critical loads, 1 day | 8 – 10 kWh |
| Critical loads, 2 days | 13 – 16 kWh |
| Critical loads + well pump | 13 – 16 kWh (check kW!) |
| Whole-home, 1 day (no AC) | 20 – 25 kWh |
| Whole-home with AC, 1 day | 27 – 35 kWh |
| Daily NEM 3.0 self-consumption | 10 – 14 kWh |
| PSPS + solar recharge combo | 13 – 14 kWh + generator |
DIY vs. hiring a pro
Do the sizing homework before the sales call. Pull 12 months of hourly usage from your utility portal and find your evening peak draw (kW) and your overnight consumption (kWh, roughly 4 PM to 8 AM). List every circuit you want backed up with its running watts -- the fridge label, the pump plate, the medical device specs. Real numbers get you sized once and sized right; 'about average' gets you sold the bigger unit.
What drives the price up or down
- Backup scope -- critical loads vs. whole home
- Peak power draw -- the kW your loads need at once
- Outage length -- PSPS history in your zone
- Solar pairing -- daily recharge shrinks the battery
- Well pumps/medical -- size kW first
- Efficiency losses -- usable kWh is less than nameplate
How to keep the cost down
- Size for overnight, not the whole outage, when you have solar
- Critical-loads subpanel beats whole-home on value 8 times in 10
- Ask for usable kWh and continuous kW, not brand names
- One battery + small generator beats three batteries for PSPS
- Check surge capacity before backing up pumps or AC
- Revisit size after an efficiency pass -- LEDs and insulation shrink loads
Frequently asked questions
How many kWh battery do I need for my house?
For critical loads (fridge, Wi-Fi, lights, phones): 8-10 kWh covers about a day. For whole-home backup including AC: 27-35 kWh for one day. For daily NEM 3.0 bill savings: 10-14 kWh. Start from your actual overnight kWh, not rules of thumb.
What is the difference between kW and kWh in a battery?
kWh is energy -- how long things run. kW is power -- how much can run at once. A battery can have plenty of kWh but too little kW to start a well pump or central AC. Size kW for your biggest simultaneous loads, kWh for how long they run.
Can one battery run a whole house?
Not typically -- one 13.5 kWh unit with 5 kW continuous output covers critical loads comfortably but cannot carry a whole home with central AC. Whole-home backup usually needs two to three units plus the electrical work to support them.
How long will a 10 kWh battery last in an outage?
About a day of critical loads (fridge, lights, Wi-Fi, phones at 3-6 kWh/day), or roughly 8-10 hours if you add heavier loads. With solar recharging it daily, one battery can cover critical loads indefinitely through multi-day outages.
Should I get a battery without solar?
You can, but the economics are weaker: without solar the battery charges from the grid, so savings come only from time-of-use shifting (charging cheap overnight, discharging at peak). Batteries paired with solar under NEM 3.0 pay back meaningfully faster.