Before choosing a backup battery, answer one question: what do you want to keep running, and for how long?
Watts vs. watt-hours
- Watts (W) = how much power a device uses at a moment. This decides the output power your battery needs.
- Watt-hours (Wh) = power × time. This decides the capacity your battery needs.
Typical consumption
| Device | Power | Energy per day |
|---|---|---|
| Wi-Fi router | 10–20 W | 0.25–0.5 kWh |
| LED light bulb | 5–10 W | 0.05 kWh (5 h) |
| Phone charge | — | 10–20 Wh |
| Laptop | 30–65 W | 0.2–0.4 kWh (6 h) |
| Fridge (modern) | 60–150 W running | 0.5–1.5 kWh |
| TV | 50–100 W | 0.2–0.4 kWh (4 h) |
| CPAP machine | 30–60 W | 0.3–0.5 kWh (8 h) |
| Electric heater | 1,000–2,000 W | Not practical on battery |
Check the label on your own devices, as figures vary.
The calculation
- List the devices you need.
- Multiply each device's watts by the hours you'll use it.
- Add everything up.
- Divide by 0.85 to account for inverter losses.
Example (12-hour outage): router (15 W × 12 h = 180 Wh) + fridge (≈500 Wh) + 3 lights (8 W × 3 × 5 h = 120 Wh) + phones and laptop (≈200 Wh) = 1,000 Wh ÷ 0.85 ≈ 1,200 Wh.
Quick sizing guide
- 300–700 Wh: phones, router, lights, laptop. Good for short outages and camping.
- 1,000–2,000 Wh: adds a fridge and essentials for most of a day.
- 3,000–5,000 Wh+: whole-day backup for several appliances or longer outages. Expandable systems are ideal here.
Don't forget output power
Your battery's inverter must handle the combined watts of everything running at the same time, plus the start-up surge of devices like fridges.
FAQ
Can I recharge during an outage?
Yes, with solar panels, if your system supports solar input.
How long does a charged battery stay ready?
Most modern batteries lose little charge in storage, but top up every few months. Not sure which type to choose? Read Portable power station vs. home backup battery.