Learn · Battery Sizing
"Whole-home backup" is a marketing line, not a sizing strategy. Here's how to think about your real needs — outage hours, essential loads, solar pairing — and end up with a battery sized for your house, not the brochure.
Two numbers, not one
Every battery is described by two specs and they answer two different questions:
kWh · "how much"
How much total energy the battery holds. Like the size of your gas tank. A 13.5 kWh battery (one common size) holds 13.5 units of electricity.
kW · "how fast"
How much energy the battery can push out at one time. Like horsepower. A 5 kW battery can run 5 kW of load simultaneously. Try to draw more and it'll trip.
A battery rated 13.5 kWh / 5 kW kicks out 5 kW of power for ~2.7 hours before it's empty. Need more hours? Bigger kWh. Need to run more loads at once? Bigger kW (or more units in parallel).
Sizing strategy
Cover the things you'd be miserable without: fridge, well pump, internet, a few lights, the heat pump or boiler controls. Skip the dryer, oven, EV charger. Most New England houses can run this stack on ~5 kWh per day.
Typical size: 10–15 kWh / 5 kW — one battery. Outage coverage: 2–3 days for the loads above.
Cover everything except your biggest single load (often the central AC, the electric range, or the EV charger). This is the practical "whole home" — the lights, the receptacles, the fridge, the well pump, the heat pump on a low setting. Just not everything at once.
Typical size: 20–40 kWh / 10 kW — two or three units in parallel. Outage coverage: ~24 hours of normal-ish living.
Run the house exactly the way you would on grid power. Every load, every appliance. This is what "whole-home backup" actually means — and it's a lot more battery than most people expect.
Typical size: 40–80 kWh / 15+ kW. Outage coverage: 12–24 hours. Cost: often $30k–$60k installed (before any rebates).
When solar changes the math
If you pair a battery with solar, the battery doesn't have to last the whole outage on its own. As long as the sun comes up, the panels can recharge the battery during the day and you can run essentials at night.
In a multi-day outage, this is a huge difference. A solar+battery house can keep going indefinitely on essentials. A battery-only house empties out and that's it.
If you're installing solar and a battery together, you can often size the battery smaller than you'd think — because the solar covers daily refill. The battery just has to hold enough for the longest stretch without sun. A 13.5 kWh battery on a 10 kW solar house can carry most New England outages.
Reality check
Worth knowing what you're sizing for. The average residential outage in MA, NH, VT, and ME is under 4 hours. The 90th percentile is around 24 hours. The bad multi-day events (ice storms, hurricanes) are rare but real.
Median residential outage
90th percentile outage
Worst-case storm event (1–2x/decade)
For most homeowners, sizing for the 90th percentile (a one-day outage) is the right balance of cost and coverage. Sizing for the rare multi-day storm is what solar+battery does well — and what battery-alone really can't.
Common mistakes
The brochure says "whole-home backup." Then the panel inventory shows 200 amps of installed loads and the battery is 5 kW. Math doesn't work. Always check the battery's kW rating against what's actually plugged in.
Startup current matters. A well pump or AC compressor pulls 3–6x its running power for a half second. If the battery's kW rating doesn't cover that surge, it'll trip every time the pump starts.
Most installers will size by gut. Ask for a load study — a written sheet of which circuits are backed up, how many kW they peak at, and which loads are excluded. If the installer can't produce one, you're buying a guess.
Get the Right Size
Try our Battery Backup Calculator to ballpark your loads in 5 minutes, or book a remote consult to get more in depth information on your specific home