Inverter Size Calculator

Last updated: September 3, 2026

Quick answer Add up running watts, make sure the biggest start surge fits under the inverter's peak rating, and add headroom. At 12 V every 100 W of AC costs ≈9.5 battery amps — a 1,500 W load pulls ≈140 A. That number kills more inverter plans than the wattage does.

Your 120V loads (toggle & edit)

Inverter targets
Select your loads

How this calculator works

Continuous rating: total running watts × 1.2 headroom (cheap protection against naming a unit that runs pinned at 100%). Surge rating: the single biggest start surge must fit under the inverter's peak (most makers allow 2× for a few seconds).

Battery amps: DC amps = AC watts ÷ system volts ÷ efficiency. That 88% default reflects typical small pure-sine inverters (85–92%); your unit's manual lists its own figure. Wiring matters too — big inverters need short, thick battery cables.

Worked example

Microwave (1,000 W run) + coffee maker (900 W): 1,900 W running → 2,280 W with headroom → a 2,400–3,000 W inverter. Running the microwave alone pulls 1,000 ÷ 12 ÷ 0.88 ≈ 95 A from the batteries — a 100 Ah lithium handles that continuously, a 100 Ah lead-acid does not happily. Voltage drop and BMS limits, not inverter wattage, are usually the real ceiling.

Assumptions & sources

  • Preset wattages: typical nameplate values for consumer appliances; verify against your devices' labels.
  • Microwave cooking-vs-input gap: cooking wattage (quoted in ads) is lower than electrical input; labels state the real draw.
  • Inverter efficiency 85–92% by class: manufacturer spec sheets.
  • Surge allowance 2× for seconds: standard inverter specification practice.

Frequently asked questions

What size inverter do I need for an RV microwave?
A "1,000 W" microwave draws roughly 1,300–1,500 W from the wall (cooking wattage ≠ input wattage), so plan on a 2,000 W inverter to run it with anything else on. Check the microwave's label for its input rating — it is higher than the cooking power everyone quotes.
Why does my inverter pull so many battery amps?
Power must balance: watts out ≈ watts in, minus inverter losses. At 12 V, every 100 W of AC load costs about 9.5 A from the battery (100 ÷ 12 ÷ 0.88). A 1,500 W hair dryer pulls ≈140 A — more than many battery banks' continuous rating, and why inverters and shore power live in different worlds.
What is surge rating?
Motors and compressors draw several times their running power for a fraction of a second at start. Inverters publish a surge (peak) rating they can sustain for a few seconds. Your appliances' biggest surge — usually a fridge or AC compressor — must fit under it.

Related tools & guides

tool

Battery runtime calculator

reference

RV appliance wattage chart

guide

Lead-acid vs lithium

reference

30A vs 50A watts