How many hours will a fridge run from a battery? And a boiler, a router and a TV together? The answer takes a minute once you know two numbers: how much energy the battery delivers and how much your appliances draw on average. Below: a consumption table for typical appliances, ready calculations for batteries from 1 to 10 kWh, and ways to stretch the charge.
The formula
Runtime = usable capacity × inverter efficiency ÷ average power
- Usable capacity is the part the battery delivers without harm: for LiFePO4, about 90 % of nominal.
- Inverter efficiency is 0.85–0.92.
- Average power is not the maximum but the hourly average: a fridge, for example, runs in cycles.
Example: a 12.8 V 100 Ah LiFePO4 battery holds 1.28 kWh. About 1.15 kWh is usable, and about 1 kWh remains after the inverter. A fridge averages 60 W, so it will run for about 17 hours.
How much appliances draw
| Appliance | Running power | Average draw | Note |
|---|---|---|---|
| Wi-Fi router | 6–12 W | the same | runs around the clock |
| Fibre terminal | 4–8 W | the same | |
| LED bulb | 7–12 W | depends on hours | |
| Laptop | 30–65 W | up to 90 W while charging | |
| Phone charging | 5–20 W | ||
| LED TV | 40–120 W | depends on screen size | |
| Fridge | 80–150 W | 40–60 W (0.9–1.5 kWh a day) | start-up 3–5 times higher |
| Chest freezer | 70–120 W | 30–50 W | |
| Gas boiler | 60–150 W | electronics, pump, fan | |
| Circulation pump | 25–100 W | ||
| Well pump | 0.5–1.5 kW | depends on water use | start-up 3–7 times higher |
| Microwave | 0.8–1.2 kW | minutes a day | |
| Electric kettle | 1.8–2.2 kW | 3–5 min per boil | |
| Induction hob zone | 1.5–3.5 kW | ||
| Washing machine | 0.3–2 kW | up to 1 kWh per wash | mostly water heating |
| Air conditioner | 0.7–2.5 kW | ||
| Desktop PC | 150–400 W | ||
| CCTV camera | 3–10 W |
How long the battery will last
Runtime for typical sets of appliances by usable battery capacity (including 0.9 inverter efficiency).
| Appliances | Average power | 1 kWh | 2.5 kWh | 5 kWh | 10 kWh |
|---|---|---|---|---|---|
| Router and terminal | 15 W | 60 h | 150 h | over a week | over 2 weeks |
| Router, lights, laptop | 110 W | 8 h | 20 h | 41 h | 82 h |
| Plus fridge and TV | 250 W | 3.5 h | 9 h | 18 h | 36 h |
| House: boiler, fridge, lights, router, TV | 375 W | 2.4 h | 6 h | 12 h | 24 h |
| Plus well pump and microwave | 700 W | 1.3 h | 3.2 h | 6.4 h | 13 h |
The first row is telling: a big inverter is inefficient for a single router — see below. For connectivity, a separate 12 V mini UPS is better.
Don't forget the inverter's own consumption
An inverter draws power even with no load — typically tens of watts for 5–12 kW models. At 50 W, a 10-hour outage costs 0.5 kWh — half of a 1 kWh battery. For small loads this cuts runtime sharply, so:
- use a separate DC UPS for the router and cameras;
- in an apartment, size the inverter to the real load rather than "with plenty of headroom";
- if the inverter has a power-saving mode, turn it on.
How to stretch the charge
- Don't run heavy appliances at the same time. A kettle, a microwave and a washing machine together can overload the inverter and drain the battery in an hour.
- Cook on gas if you can — the kettle and electric stove are the biggest household consumers.
- Keep the fridge closed: shut, it holds the cold for hours.
- Charge laptops and power banks while the grid is up.
- Switch to LED bulbs — about eight times less than incandescent.
- Don't heat water electrically from the battery: 2 kW for an hour is the entire capacity of a small battery.
Summary
Work out the average power of what really has to run and divide the usable capacity by it — that's your hours. An apartment with a fridge needs 1.5–2.5 kWh for 6 hours; a house with a gas boiler needs about 5 kWh for 8 hours. A detailed calculation for your own appliances is in how to calculate battery capacity.