How to size an inverter
Worked example
A microwave (1,000 W input), a laptop (60 W) and a fridge (140 W running, 600 W starting) could all run together: 1,200 W. With 25% headroom you need at least 1,500 W continuous. The surge needed is 1,200 − 140 + 600 = 1,660 W, which almost any 1,500 W pure sine inverter handles (they typically surge to 2× for a second or so).
Remember that microwaves draw roughly 1.5–2 times their cooking power from the socket. Use the input watts on the label.
Pure sine or modified sine?
Choose pure sine wave. Modified sine inverters are cheaper but can make motors run hot, upset chargers and cause buzzing in audio equipment.
Don't forget the DC side
A 2,000 W inverter can pull about 190 A from a 12 V battery. It needs very thick, short cables and a properly rated fuse; see the inverter current calculator and cable size calculator.
Frequently asked questions
What size inverter do I need for a microwave?
Check the microwave's input watts (often 1,200–1,500 W for an 800 W oven) and add 25%: a 2,000 W pure sine inverter is the usual choice.
What size inverter to run a fridge?
A domestic fridge runs at 100–200 W but can surge to 600–1,000 W when the compressor starts. A 600–1,000 W pure sine inverter with good surge capacity usually works.
Can an inverter be too big?
Only in cost and idle consumption. Larger inverters use more power when switched on with no load, so don't oversize massively.