What Can You Actually Run on a Home Battery During an Outage? Backup Load Calculation

The short answer: a home battery keeps on whatever fits inside two independent limits — its energy capacity in kWh (how long appliances run) and its inverter output in kW plus surge rating (how many run at once). Exceed the first and runtime shrinks; exceed the second and the system trips or shuts down immediately, regardless of charge. This spoke of our Home Battery Buying Guide gives you the exact calculation, a fill-in appliance table, realistic wattage numbers, worked examples for 5/10/20 kWh systems, and the motor-surge rules that catch most first-time buyers. Spend fifteen minutes here with your electricity bill and a notepad, and you will never need to trust a salesperson's "it powers a whole house" claim again.

Step 1 — Understand the Two Numbers (kWh and kW)

The answer first: size kWh from what you run and for how long; size kW from what runs simultaneously. Energy is watts multiplied by hours: a 100 W fan running eight hours consumes 0.8 kWh. Power is the instantaneous draw: the same fan plus a 200 W fridge plus a 1,000 W kettle needs 1,300 W (1.3 kW) of continuous inverter capacity at that moment. Buyers routinely purchase enough energy but too little power, then discover the inverter trips under perfectly normal evening loads. The distinction is the foundation of the whole kWh sizing calculator, and it determines whether you need a bigger inverter before a bigger battery.

Step 2 — Fill in the Appliance Table

Walk through your home room by room and list everything you expect to use in an outage. Use the table below as a reference; real values are printed on the appliance label or in the manual, and nameplate watts for motors exceed typical running watts. Multiply running watts by the hours the appliance is actually on — fridges, freezers and air conditioners cycle, so an 8-hour evening means maybe 3 hours of compressor time.

Note the last two rows: resistive heating appliances are energy monsters. A single 10-minute kettle boil costs about 0.3 kWh — tolerable occasionally — but an electric boiler reheating a tank can devour an entire 5 kWh battery in one cycle. In markets with frequent outages, households routinely switch cooking and water heating to gas specifically so the battery budget survives for refrigeration and communications.

Step 3 — Choose Your Backup Tier

The answer first: almost nobody needs "whole house." The professional approach is to divide loads into three tiers and engineer for the tier you can actually afford.

Most families in outage-prone markets land on Tier 2 with a 10 kWh battery and a 3-5 kW hybrid inverter — it preserves food, sleep, connectivity and basic hygiene at a price that pays back against generator fuel; the full size comparison is in the 5kWh vs 10kWh vs 20kWh sizing guide.

Step 4 — A Worked Example

Take a family in a region with 4-8 hour evening outages. Tier-2 list: ten LED lamps at 8 W for five hours (0.4 kWh); router 24 h (0.4 kWh); fridge cycling, 1.4 kWh/day; two fans six hours (0.7 kWh); TV four hours (0.5 kWh); phones (0.2 kWh); pressure pump running intermittently (0.6 kWh). Daily critical energy totals about 4.2 kWh; adding inverter losses (~8%) and 25% headroom for a second day or a heatwave pushes the requirement to ~5.8 kWh — a 10kWh wall battery comfortably covers one full day plus a margin, while a 5kWh unit covers a shorter evening outage. Simultaneous running watts: lights 80 + router 15 + fridge 180 + fans 140 + TV 120 + pump 600 (intermittent) peaks around 1,135 W, so a 2 kW inverter suffices for Tier 2 — but verify surge: when the fridge and pump compressors start together, a 5 kW surge rating keeps the breaker from tripping.

Step 5 — Respect the Surge Rules

The answer first: motors draw 3-6 times their running watts for 1-3 seconds at startup, and your inverter's surge (peak) rating — not its continuous rating — must absorb that spike. Inductive loads — fridge compressors, air conditioners, well pumps, washing machines — are the risk. Practical rules: stagger motor starts (do not let the fridge, AC and pump all cycle together at dusk); put the well pump or AC on a soft-starter if the inverter is marginal; and if you must choose, spend on inverter headroom before overspending on kWh you cannot push through it. Off-grid system calculators apply the same logic — see the step-by-step off-grid sizing calculator.

Runtime Reality for 5, 10 and 20 kWh

Figures assume 90-95% round-trip efficiency, LiFePO4 usable DoD of 90%+ and normal ambient temperature; derate 15-25% in extreme cold or heat as detailed in the buying guide climate section. If outages in your area last days rather than hours, also read the guide to batteries for frequent power outages — recharge strategy (solar, grid windows or generator) becomes as important as capacity. For apartments, camping and device-level backup, a 2kWh portable station covers Tier 1 instantly with no installation; the trade-offs are compared in portable station vs home battery.

The Four Load-Audit Mistakes to Avoid

1. Using nameplate watts as running watts. A fridge label says 250 W but the compressor cycles; measure or assume 30-40% duty cycle — otherwise you over-size the battery and overspend.
2. Ignoring simultaneous loads. Daily kWh can look fine while evening peak watts exceed the inverter. Calculate the worst-case moment, not just the day.
3. Forgetting surge. Two motors starting together is the classic inverter shutdown; verify peak rating, not only continuous.
4. Skipping losses and headroom. Deduct 5-10% for the inverter, derate for temperature, and keep 20-30% reserve for clouds, aging cells and guests.

Once your load table is done, the next thing to verify is whether a candidate battery's datasheet actually supports your numbers — usable capacity, continuous and surge power, and DoD all matter, which is exactly what the companion article home battery specs explained: reading a LiFePO4 datasheet covers line by line.

Frequently Asked Questions

Can a 10kWh home battery run a whole house?

A 10kWh LiFePO4 battery can run an average family home's essential circuits — fridge, lights, fans, router, TV, device charging and an occasional small pump — for roughly 12-24 hours, but not the whole house simultaneously: a single split air conditioner draws 1-2 kW running and 3-6 kW at startup, and an electric kettle or oven adds 2-3 kW more. Whole-house backup including air conditioning and electric cooking normally needs 20kWh of storage plus an inverter rated for 5-8 kW continuous output.

How long will a fridge run on a home battery?

A modern 300-500 litre fridge consumes only 1-2 kWh per day because its compressor cycles on for roughly 20-40% of the time even in warm weather, despite a 100-250 W nameplate. A 5kWh wall battery therefore keeps a fridge running for well over a full day alongside LED lighting, a router and phone charging, and a 10kWh unit covers the fridge alone for 4-6 days. Inverter losses of 5-10% and cold/hot ambient temperature slightly reduce the real figure.

Can a home battery run an air conditioner?

Yes if the inverter's surge rating covers the compressor startup spike, typically 3-6 times running current for 1-3 seconds. A 9000 BTU split unit draws about 700-1000 W running, a 12000 BTU unit 1000-1500 W, and an 18000 BTU unit 1800-2500 W. Plan for an inverter with at least 5 kW continuous / 8-10 kW surge, and remember energy: cooling a room for 8 hours consumes 4-12 kWh, so air conditioning practically demands a 10-20kWh battery.

What will a 5kWh battery power during a blackout?

A 5kWh battery covers Tier-1 essentials for a typical evening or overnight outage: LED lighting (50-150 W total), a fridge cycling intermittently (about 1-2 kWh/day), wifi router and modem (10-20 W), TV or laptop (50-150 W), phone charging and a few fans. It cannot comfortably carry simultaneous cooking, air conditioning or electric water heating. The 5kWh size suits apartments, short 2-6 hour outages and single-room backup; for recurring long outages choose 10kWh or more.

Does the inverter limit what I can run even if the battery has enough kWh?

Absolutely, and this is the most common sizing mistake. kWh is energy capacity (how long); the inverter's kW continuous and surge ratings decide how many appliances run at once. A 10kWh battery behind a 2 kW inverter shuts down the moment the fridge compressor and a kettle start together no matter how much charge remains. Add the running watts of every appliance used simultaneously, allow 20-25% margin, and separately verify the surge rating against motor startup spikes.