A refrigerator hums, a furnace fan kicks on, the router blinks, and the clocks reset. What size battery backup do I need for a typical house? The honest answer depends far less on square footage than on which loads you refuse to live without.
The math takes about ten minutes: add up the watts of everything you want running, multiply by the hours you need, and leave room for the fact that batteries don’t hand back everything you put in.
The Load Math That Decides Your Battery Size
Two homes on the same street can land three times apart, because the deciding factor is whether the air conditioner, electric range, or electric water heater stays in the backup plan.
- Refrigerator: 150–400 running watts, roughly 1–2 kWh per day.
- Gas furnace fan: 300–800 watts while running.
- Well pump: 1,000–2,000 watts, with a much higher startup surge.
- Sump pump: 800–1,200 watts, intermittent and weather-dependent.
- Router plus modem: under 30 watts, continuous.
- LED lighting, 20 bulbs: roughly 150 watts total.
- Space heater or portable AC: 1,500 watts and up, continuous.
Two numbers matter beyond the watts. Surge — the brief spike when a motor starts — can run two to three times the running draw, and undersized inverters trip on it.
How Many kWh Does A Typical House Actually Need?
Common sizing tiers land between 5 kWh and 20 kWh, set by which appliances stay on. These are working ranges, not one manufacturer’s spec sheet.
| Backup Tier | Capacity | What It Runs |
|---|---|---|
| Essentials only | 5 kWh | Fridge, router, lights, phones, a few small devices for 12+ hours |
| Essentials plus comfort | 10 kWh | Adds furnace fan, sump pump, TV, and microwave |
| Whole-home, no heavy loads | 13–20 kWh | Everything except AC, electric range, and electric water heater |
| Whole-home with AC | 20–30 kWh | Adds central air and higher overnight runtime |
| Multiple outage days | 30 kWh plus | Adds a solar recharge loop or generator assist |
| Well pump included | Sized separately | Inrush current often demands a larger inverter, not more battery |
| Sump pump in wet season | Add 2–4 kWh | Duty cycle climbs sharply during storms |
To skip the spreadsheet and buy a matched system, our tested roundup of the best battery backup for power outage walks through how these tiers perform in real outages. For the underlying engineering, Tesla’s own Powerwall 3 specifications and independent testing notes are worth reading before you commit to a capacity number, since continuous output limits often constrain a system before its stored kWh do.
Breaker Panel Limits And Peak Load
Capacity is only half the answer — your breaker panel decides the ceiling on what the battery can deliver at one moment. A 200-amp service can carry a full home, but a battery system’s continuous output typically caps far below that, so loads get managed rather than run simultaneously.
Whole-home systems lean on load management: they sense when backup starts and drop the electric range, water heater, or EV charger before running the rest. If your plan is “everything, exactly as it was,” price in that load-management hardware and the extra capacity, because a battery that stores 20 kWh and outputs only 10 kW cannot run your house the way the grid does.
Verify peak output in writing. The number that matters is continuous kilowatts, not the surge rating quoted in headlines, and the two are frequently confused in marketing copy.
Where To Start Sizing Tomorrow
Grab your last twelve months of electric bills and find the highest single-month kilowatt-hour figure — that month sets your upper bound. Divide it by 30, then by 24, and you have an average draw.
Subtract the loads you would live without, add 20% headroom for aging and cold weather, and round up to the next available capacity. That final figure, not your square footage, is the size that will carry you through an outage.
FAQs
Does house square footage determine battery size?
Only loosely. Two 2,000-square-foot homes can need 5 kWh or 25 kWh depending on whether they heat with gas or electricity, run central air, or have a well pump. Size from your loads and billing history, not the floor plan.
Can I add capacity later instead of sizing big now?
Usually yes, and it is often smarter. Many home battery systems are modular, so starting with essentials and adding units later spreads the cost. Check that the inverter and wiring can accept additional capacity before buying the small system.
How long will a 10 kWh battery run my house?
Runtime scales directly with how many watts you leave on, so cutting loads extends it more than buying capacity does.
References & Sources
- Clean Energy Reviews. “Tesla Powerwall 3 Review.” Source for home battery capacity ranges, continuous output limits, and load management behavior.
