Inverter Size Calculator

What size inverter do I need for my RV?

The size that runs what you switch on at the same time, with room to spare, rather than the sum of every plug aboard. Add the running watts of the loads that overlap, multiply by 1.2 because inverters derate warm and a rating is a ceiling, and check that the biggest motor's start surge fits under the inverter's surge rating (usually twice continuous or more, for a few seconds). Then buy the smallest standard size that clears both. The combinations people ask about, computed:

Running watts from the load list above; start = running + the single biggest surge; class = smallest of 300 / 600 / 1,000 / 1,500 / 2,000 / 2,500 / 3,000 W that clears running × 1.2 and the start at a 2× surge rating. A residential fridge counts in whichever of its two states is worse: the compressor starting, or the 435 W defrost heater running.

That is why "2,000 W" is the answer for most rigs and "3,000 W" for the ones that make coffee while the microwave runs: microwave + coffee maker + tv is 2,210 W running, 2,652 W with the margin, and a 2,000 W inverter is 210 W short before the margin. Stagger them and the microwave alone is 1,260 W, which a 2,000 W inverter carries comfortably. The tool shows both readings: what you own against what's ticked, and what to buy if it doesn't clear.

A residential fridge is the load people undersize. Whirlpool's tech sheet for a 26 cu ft model lists a 101 W compressor with a 9.3 A locked-rotor current, 1,116 W at 120 V for the first few cycles of every start, and a 435 W defrost heater that runs with the compressor off but alongside the TV and the laptop. Sized for the worse of the two, the fridge with a TV and a laptop is 545 W running and 1,226 W to start: a 1,000 W class. The load list has both states; tick the one you're checking.

Can I run a microwave on an inverter?

The number on a microwave's door is cooking power; the label on the back is what it draws, about 1.4× more, because a magnetron and its transformer are not efficient. Any pure-sine inverter sized to that label figure runs it. A 900 W RV microwave is 1,260 W at the plug; a 1,200 W countertop unit is 1,680 W. So the RV microwave wants a 2,000 W inverter and the countertop one a 2,500 W. A modified-sine inverter runs a microwave at reduced power with a louder transformer, and a microwave at 1,260 W pulls 109 A from a 12 V lithium bank, which is the check in the next section.

How many amps does an inverter draw from the battery?

Watts ÷ (battery voltage × efficiency). The 120 V side hides it, but every watt at the outlet is ten times the amps on the 12 V side plus the inverter's losses: 2,000 W through a 90% inverter on a 12.8 V lithium bank is 174 A; 3,000 W through an 85% unit on a 12 V lead-acid bank is 294 A. That current sizes the cable, the fuse and the battery rating:

Full-load draw at 12.8 V (lithium) or 12 V (lead) nominal; fuse ≈ 125% of full-load draw rounded to a standard ANL / Class T size, and the inverter manual's figure beats this. The wire-size calculator sizes the cable for these amps over your run.

The 24 V column is the quiet argument for 24 V on anything over 3,000 W: half the amps, a quarter of the cable losses, and battery discharge ratings that stop being the limit. Below 2,000 W, 12 V is fine and everything in the rig already speaks it.

Can my batteries feed this inverter?

Every lithium battery has a maximum continuous discharge current set by its BMS. Battle Born's BB10012 data sheet rates its 100 Ah battery at 100 A continuous, and batteries in parallel add their ratings. A 2,000 W inverter at full tilt pulls 174 A; two 100 Ah batteries supply 200 A, so it clears. A 3,000 W inverter pulls 260 A from the same pair, and the BMS opens the moment the microwave and the kettle overlap. "The 3,000 W inverter ran my microwave for years" and "the BMS tripped" can both be true: the two owners had different bank sizes.

Inverter full-load draw at 12.8 V and 90% vs the bank's summed BMS rating. "Over" means the BMS can trip at full load; the fix is another battery, not a bigger inverter. Two 6 V batteries in series count as one string.

Lead-acid has no BMS to trip, so it fails quietly instead: Peukert. A 1,000 W load on a 200 Ah flooded bank pulls 93 A, and at that rate the "200 Ah" label behaves like about 103 Ah. Half the bank is gone before the 50% floor is even counted. The result prints the effective capacity at your draw; the runtime page turns it into minutes.

No-load power at 12 V from each maker's data sheet; Victron's continuous ratings are at 25 °C. Renogy lists "< 2.5 A", here × 12 V. Victron's search and ECO modes put the inverter to sleep and pulse to look for a load, at the cost of a wake-up delay.

Inverter or inverter/charger?

An inverter/charger replaces the converter too: one box that charges from shore or the generator, passes shore power through when it's there, and inverts when it isn't, with a transfer switch built in. It costs more and its charger is usually larger than a stock converter (a 12/3000 charges at 120 A), which matters if the charge-time page says your bank could take more than your converter gives. A plain inverter wired to one or two outlets is the right answer for a microwave and a coffee maker; the whole-rig inverter/charger is the right answer for a residential fridge, because that fridge is on 24 hours a day and the transfer switch is what keeps it running when the pedestal drops.