What size inverter do I need?
An inverter turns the battery’s direct current into alternating current for the house. Choosing one by adding up the watts of every appliance almost always gives one that is too big; choosing it by average use gives one that trips the first time the fridge starts.
1. Continuous power
Add up only what can run at the same time. If the iron and the microwave are never used together, count only the larger of the two.
| Appliance | Power |
|---|---|
| Fridge | 150 W |
| Television | 80 W |
| LED lighting | 60 W |
| Laptop | 65 W |
| Microwave | 1100 W |
| Total at once | 1455 W |
2. Start-up surge
Motors and compressors (fridges, water pumps, non-inverter air conditioners) draw several times their rated power for a fraction of a second when they start. The inverter has to handle that peak; the datasheet calls it peak or surge power, usually twice the continuous rating.
Example. The 150 W fridge may draw 600 to 900 W at start-up. With everything else on: 1455 − 150 + 900 = 2205 W peak.
3. Headroom
An inverter that always runs near its limit gets hot and wears out sooner. Add 25 %: 1455 × 1.25 ≈ 1820 W. A model rated 2000 W continuous and 4000 W peak covers the example comfortably.
4. Pure sine wave
Modified-sine inverters are cheaper, but they make motors hum, heat up transformers, and some chargers fail on them. For a home, pure sine wave is the right choice.
5. Battery voltage
At a lower voltage the same power needs more current: I = P ÷ V. For 2000 W:
| Battery | Approximate current |
|---|---|
| 12 V | 167 A (about 185 A with losses) |
| 24 V | 83 A |
| 48 V | 42 A |
At 170–185 A you need very thick cables, large fuses and flawless connections. As a rule of thumb: up to about 1500 W, 12 V; up to about 3000 W, 24 V; above that, 48 V.
Summary
- Add up what runs at the same time, not everything in the house.
- Check the start-up surge of motors.
- Leave 25 % headroom.
- Choose pure sine wave, and a battery voltage that fits the power.