RV inverter size calculator

How to use the calculator

1. Tick what runs at the same time Size depends on the 120 V things you run together, not everything you own. The tool starts with a coffee maker, a laptop and a TV ticked.
2. Type in your own watts Each box holds a figure from a maker spec or the US Department of Energy (DOE) list. Your appliance’s label wins. Use “Add your own” for anything else.
3. Enter your battery bank Type the total amp hours (Ah) of your house batteries. The default is 200 Ah, such as two 100 Ah batteries wired in parallel.
4. Pick the battery type Choose lithium (LiFePO4) or flooded/AGM lead-acid. This sets how much of the bank the tool lets you use.
5. Read the result You get the inverter size, the load, the battery current and the runtime.

How the calculator works

The tool adds the watts of every ticked item, multiplies by 1.25 and rounds up to the next common size: 300, 600, 1,000, 1,500, 2,000, 2,500, 3,000, 4,000 or 5,000 W. Battery current is the load divided by 12 V x 0.90, since we assume the inverter is 90% efficient. Runtime is the usable part of your bank divided by that current: 80% of a lithium bank, 50% of a lead-acid bank.

With the defaults: 1,200 W + 50 W + 120 W = 1,370 W. Then 1,370 W x 1.25 = about 1,713 W, so the tool picks a 2,000 W inverter. On the battery, 1,370 W / (12 V x 0.90) = 1,370 / 10.8 = about 127 A. A 200 Ah lithium bank gives 200 x 0.80 = 160 Ah, and 160 Ah / 127 A = about 1.3 hours. On 200 Ah of lead-acid you get 100 Ah, about 0.8 hours.

Tick the 1,500 W microwave as well and the load becomes 2,870 W. With the margin that is about 3,588 W, so the answer jumps to 4,000 W, pulling about 266 A.

Why we add 25% on top of your load

The 1.25 factor is our own safety margin, not a maker rule. We base it on three points in Victron’s inverter sizing guide:

• Size for the largest combined load. The inverter must cover the highest continuous demand of everything running at once.
• Full rating only up to 25 °C (77 °F). Output drops as the air around the inverter gets hotter. Victron’s Phoenix 12/1200 datasheet shows 1,000 W continuous at 25 °C but 850 W at 40 °C (104 °F).
• Surge is short. Victron says its inverter/chargers can deliver about double their continuous rating for short periods, but Victron still says to plan for start-up loads.

The name on the box can mislead too. Victron’s Phoenix 12/3000 (230 V version) is rated 3,000 VA, but its continuous output at 25 °C is 2,400 W, which is 80% of the number in its name.

The 90% efficiency sits at Renogy’s stated floor for its 2000W inverter (over 90%) and below Victron’s 91% to 93% maximum. Renogy advises no more than 80% depth of discharge on its lithium batteries; Trojan recommends 50% or less on flooded lead-acid.

Typical watts of things people run on an inverter

The DOE figures are nameplate ranges; the tool uses the top of each.

Can you run an RV air conditioner on an inverter?

An inverter can carry the running load, but we can’t promise it will start the compressor. Dometic lists the Brisk II 13,500 BTU at 12.4 A for the compressor plus 2.5 A for the fan: 14.9 A x 115 V = about 1,714 W running. With our margin that is about 2,143 W, so the tool says 2,500 W.

The start is the problem. Dometic lists the compressor’s locked rotor current at 68 A, the current a stalled motor pulls, and a compressor can pull close to it for a moment as it starts. That is 68 x 115 = about 7,820 W. With surge at about double the rating (Renogy’s 2000W lists a 4,000 W peak), a 3,000 W inverter has roughly 6,000 W to give. That falls short, which is why people add a soft starter. Micro-Air says its EasyStart Breeze cuts starting current by up to 75%, and Coleman-Mach claims up to 75% for its Soft Start, depending on the AC model. Those are maker claims, not test results, so a soft starter may help but is no guarantee.

On the battery, 1,714 / 10.8 = about 159 A, so a 200 Ah lithium bank runs it about 1 hour. For all-afternoon cooling, use the generator size calculator. If the AC trips breakers on shore power too, read RV AC keeps tripping the breaker.

What size inverter do I need to run a 30 amp RV?

We don’t think most owners need one that covers the full service. Southwire’s 30 A surge protector lists 3,600 W maximum (30 A x 120 V). Covering all of it with our margin means 3,600 x 1.25 = 4,500 W, so a 5,000 W inverter pulling 3,600 / 10.8 = about 333 A at full load. Size for what you run together on battery instead. To check plugged-in loads, try the 30 amp load calculator. For a 50 amp coach, Victron says to use two 120 V units for North American 50 A split-phase power.

Common questions