Home backup battery calculator
List the essentials you want to keep running — fridge, lights, internet — or start from your bill’s daily kWh, and say how long the outage might last. The calculator gives the usable kWh and how many batteries make it up.
Enter your measurements
More options: Inverter eff.
- Battery units
- 1 unit
- 12 V 100 Ah LiFePO4 batteries (at 80% usable)
- 8 batteries
- 48 V 100 Ah LiFePO4 modules (at 80% usable)
- 2 modules
- Average load during the outage
- 282 W
- Running watts if everything is on at once
- 990 W
Fridge 200 W × 8 h + Chest freezer 150 W × 8 h + LED lights (6 bulbs) 60 W × 5 h + Router and modem 20 W × 24 h + Phones and laptop 60 W × 3 h + Furnace fan (winter) 500 W × 6 h = 6.76 kWh a day × 24 h ÷ 24 = 6.8 kWh of use
÷ 90% inverter = 7.5 kWh usable battery → 1 unit of 10 kWh
or 8 batteries (1.024 kWh usable each) · 2 modules (4.096 kWh usable each)
- Battery size covers how long; the inverter’s continuous and surge rating covers what can run at once — a well pump or central air needs a much bigger inverter than lights and a fridge.
Quick answers for common sizes
| Example | Answer |
|---|---|
| How many kWh to power a house for a day | 28.4 kWh a day — about 31.6 kWh of battery |
| How many 100Ah batteries to power a house | 31 × 12 V 100Ah LiFePO4 (8 for essentials) |
Battery needed for longer outages
| Outage to cover | Usable battery needed | Battery units | 48 V 100 Ah LiFePO4 modules (at 80% usable) |
|---|---|---|---|
| 4 | 1.3 kWh | 1 unit | 1 module |
| 8 | 2.5 kWh | 1 unit | 1 module |
| 12 | 3.8 kWh | 1 unit | 1 module |
| 24 | 7.5 kWh | 1 unit | 2 modules |
| 48 | 15 kWh | 2 units | 4 modules |
| 72 | 22.5 kWh | 3 units | 6 modules |
The calculator starts with an essentials list — fridge, freezer, lights, internet, phone charging and a furnace fan — which comes to about 7.5 kWh of battery for a 24-hour outage, a fraction of a whole house. Switch Back up to your bill for the whole-house figure. Worked examples: how many kWh to power a house for a day and how many 100Ah batteries to power a house.
How it is worked out
kWh per day = Σ watts × hours (essentials), or bill kWh per day × share kWh of use = kWh per day × hours of outage ÷ 24 usable battery = kWh of use ÷ inverter efficiency units = usable battery ÷ usable kWh per unit, rounded up
The EIA puts the average US residential customer at 863 kWh a month in 2024 — about 10,359 kWh a year, or 28.4 kWh a day — but it varies a lot: Louisiana averaged 1,202 kWh a month, Hawaii 495. Your own bill is the right starting point.
Whole house vs essentials
| Plan | Share of a 28.4 kWh day | Battery for 24 h (90% inverter) |
|---|---|---|
| Whole house | 100% | 31.6 kWh |
| Most things, no heating or cooling | 50% | 15.8 kWh |
| Essentials | 25% | 7.9 kWh |
| Bare minimum | 15% | 4.7 kWh |
Battery building blocks
- Home battery units list a usable kWh figure and a continuous power rating on their spec sheets. Use the usable figure here.
- 48 V rack modules at 51.2 V and 100 Ah hold 5.12 kWh, about 4.1 kWh usable at 80%.
- 12 V 100 Ah LiFePO4 batteries hold 1.28 kWh, about 1.02 kWh usable — handy for small backup systems but a lot of batteries for a whole house.
- Lead-acid gives about half its rating for long life (Trojan recommends 20–50% discharge), so it takes roughly twice the rated capacity.
Batteries, generators and solar together
A battery alone covers a known number of hours. Add solar and the battery refills during the day; add a generator and you can recharge through a long, cloudy outage. The generator size calculator and solar system size calculator cover those parts.
CalculatorBattery Bank Size Calculator →Building your own bank from 12, 24 or 48 V batteries?What you’ll need
- Plug-in electricity usage monitorBudget price
Plugs in between the wall and a 120 V appliance and shows real watts and kWh over a day — the honest way to fill in a load list, especially for fridges that cycle on and off.
See options on Amazon (opens in a new tab) - 12 V 100 Ah LiFePO4 batteryPremium price
About 1.28 kWh rated in a box roughly the size of a group 24–31 lead-acid battery at around half the weight. Check the BMS discharge limit and whether it can charge below freezing.
See options on Amazon (opens in a new tab) - Battery monitor with shuntMid-range price
Counts amp-hours in and out, so you see true state of charge instead of guessing from voltage — which barely moves on a LiFePO4 battery until it is nearly empty.
See options on Amazon (opens in a new tab)
As an Amazon Associate we earn from qualifying purchases. Links to products are affiliate links — they cost you nothing extra and help keep these calculators free.
Frequently asked questions
How many kWh does it take to power a house for a day?
The 2024 US average was about 28.4 kWh a day (EIA), so roughly 31.6 kWh of usable battery after inverter losses. The essentials list on this calculator needs about 7.5 kWh. See how many kWh to power a house for a day.
How many batteries does it take to power a house?
For an average house for a day, about 31.6 kWh usable: four 10 kWh units, eight 48 V 100 Ah modules or about 31 12 V 100 Ah LiFePO4 batteries. See how many 100Ah batteries to power a house.
How long will a home battery last in an outage?
Divide its usable kWh by your average load in kW. A 10 kWh battery running essentials at 300 W average lasts about 30 hours after a 90% inverter; running a whole average house (about 1,183 W average) about 7.6 hours.
Can a home battery run central air or a well pump?
Only if its inverter’s continuous and surge ratings cover them. Battery kWh decides how long; the inverter decides what can start and run.
Related guides
Sources
- U.S. Energy Information Administration — Electric Sales, Revenue, and Average Price, Table 5A (2024 data, released October 2025) (opens in a new tab) — 2024 average residential customer: 863 kWh a month (about 10,359 kWh a year, 28.4 kWh a day); Louisiana 1,202 kWh a month, Hawaii 495.
- Champion Power Equipment — Generator Wattage Chart (opens in a new tab) — Typical running and starting watts for household, RV and job-site appliances (a manufacturer’s planning chart — your nameplate wins).
- Victron Energy — Lithium Battery Smart manual, technical data (opens in a new tab) — LiFePO4 12.8 V: cycle life 2,500 at 80% DoD, 3,000 at 70%, 5,000 at 50% (to 80% of capacity); 92% round-trip efficiency; 12.8 V 100 Ah: recommended continuous discharge ≤100 A (1C), maximum 200 A; capacity about 20% lower at 0 °C; charge only between +5 °C and +50 °C.
- Victron Energy — Energy Unlimited (book) (opens in a new tab) — Lead-acid charge efficiency 80–95% (85% suggested as a starting value); battery energy efficiency about 75% for full cycles, about 89% in partial state of charge; inverter efficiencies above 90%.
- OSHA Fact Sheet — Using Portable Generators Safely (opens in a new tab) — Never attach a generator to a building’s wiring unless a qualified electrician has installed a transfer switch; 3–4 ft of clear space around the unit.
- U.S. Consumer Product Safety Commission — Portable Generator Hazards (Safety Alert 5123, 2021) (opens in a new tab) — Never run a generator indoors or in a garage, even with doors open; place it outside far from doors, windows and vents; CO alarms; never “backfeed” by plugging into a wall outlet.