Inverter size calculator

Enter the watts of everything you run at once, and the running and startup watts of your biggest motor load. You get the continuous and surge ratings to shop for, the battery current it draws, and a check of the inverter you have in mind.

Inputs

Add up the running (not startup) watts of every AC device that can be on together. The nameplate or the manual gives it.

The fridge, pump, compressor, power tool or air conditioner with the largest startup kick. Part of the total above.

Often 2–7 times the running watts for a moment. Check the nameplate or manual; an air conditioner with a soft-start has almost none.

Battle Born suggests adding 20% so the inverter is not at its limit.

Decides the current the battery and cables must carry.

The continuous (not peak) rating of the inverter you are considering. Victron’s 12/1200 is 1,000 W continuous at 25 °C.

Its peak or surge rating, which it can hold for seconds. Victron’s 12/1200 peaks at 2,200 W.

Inverter size (continuous)

1,450W

1,200 W running plus 20% headroom. Look for a surge rating of at least 2,400 W.

The inverter you are checking covers both: 2,000 W continuous against 1,440 W, and 4,000 W surge against 2,400 W.

Surge needed
2,400 W
Surge as a multiple
1.7×
Battery current at 12 V
133 A
Battery to support it
150 Ah or more
Surge and continuous, needed and offered

Running 1,200 W of loads with 20% headroom calls for an inverter rated at least 1,450 W continuous. When the biggest motor starts (1,800 W) on top of the rest, the inverter must surge to 2,400 W. At 12 V that draws about 133 A from the battery. The bars compare these with the inverter you are checking: The inverter you are checking covers both: 2,000 W continuous against 1,440 W, and 4,000 W surge against 2,400 W.

Safety first

An inverter draws a very large current from the battery, often more than 100 A on a 12 V system, and it makes mains voltage that can shock or start a fire.

  • Fuse every circuit, especially the cable between the battery and the inverter. Use the fuse and cable size the inverter maker specifies, as close to the battery as you can.
  • Follow the manufacturer’s instructions for the inverter and the battery, including the cable size, grounding and ventilation.
  • Follow your local electrical code and the standard for your vehicle or boat, including any rule on AC wiring and ground-fault protection.
  • Have a qualified person check the installation before you use it, particularly if the inverter is wired into the RV or house wiring.

How it works

continuous needed = running watts × (1 + headroom)
surge needed      = running watts − biggest motor running + biggest motor startup
battery current   = continuous needed ÷ 0.90 ÷ battery voltage
  • Running watts are the loads that can be on together. Their sum is the continuous power the inverter must give.
  • Surge assumes the biggest motor starts while everything else is already running, the worst case.
  • Headroom is 20% by default (Battle Born). It is applied to the continuous rating. Battle Born’s RV guide says to add it to the total “including the surges”; the calculator applies it to the continuous rating alone and shows the raw surge so you can decide.
  • Efficiency is 90% here; Victron’s Phoenix datasheet gives 89–92% at best, and less at light loads.

Worked example

You run 1,200 W of loads, of which a fridge compressor is 600 W running and 1,800 W starting.

  • Continuous: 1,200 × 1.2 = 1,440 W, so look for an inverter rated 1,450 W or more (a 1,500 W model).
  • Surge: the other 600 W plus the compressor’s 1,800 W = 2,400 W, the same as Battle Born’s example.
  • A Victron 12/1200 (1,000 W continuous, 2,200 W peak) is too small on both counts; a 2,000 W inverter with a 4,000 W surge is comfortable.
  • At 12 V, 1,440 W draws 1,440 ÷ 0.9 ÷ 12 = 133 A from the battery, which needs a bank of at least 150 Ah.

Tips from the road

  • Be honest about what runs together. The microwave and the air conditioner rarely need to be on at once, and the inverter can be sized for one of them.
  • Look for a soft start. Battle Born notes that newer RV air conditioners with soft-start or inverter technology have little to no startup surge, which can shrink the inverter you need.
  • Turn it off when you are not using it. An inverter uses power just being on, several watts for the small Victron models, which adds up over a day.

FAQ

What size inverter do I need?

Add up the running watts of everything that can be on at the same time, then add 20% headroom. Battle Born's example of 1,500 W becomes 1,800 W. Then check that the inverter's surge rating covers the biggest motor starting on top of the rest.

What is the difference between continuous and surge watts?

Continuous is what the inverter can deliver all day. Surge (or peak) is what it can deliver for a few seconds, which is what a compressor, pump or air conditioner needs to start turning. Battle Born notes that microwaves, air conditioners and some power tools need extra power for a few seconds too.

How do I find a device's startup watts?

Look on the nameplate or in the manual for locked-rotor or starting amps, and multiply by the voltage. Motors often need several times their running watts for a moment. An air conditioner with a soft-start has almost none. If the manual is silent, use a clamp meter at the moment it starts.

How much current does an inverter draw from the battery?

Watts ÷ efficiency ÷ battery voltage. At 12 V a 2,000 W inverter at full load and 90% efficiency draws 185 A, which needs thick cable and a large fuse. At 48 V it is 46 A. That is why bigger inverters use higher battery voltages.

How big a battery does my inverter need?

Battle Born says at least 100 Ah for every 1,000 W of inverter. A smaller battery will sag under load and may shut down on its own protection before the inverter reaches its limit.

Sources

  1. How to Size an Inverter (And When Surge Limiting Is Needed), Battle Born Batteries. Continuous rating covers the total running watts; surge rating covers the highest startup load; 1,200 W running and 2,400 W of startup; at least 100 Ah of battery for every 1,000 W of inverter.
  2. What Size Inverter Do I Need for My RV?, Battle Born Batteries. Add up the watts you need at once, including surges, and add 20%; 1,500 + 300 = 1,800 W.
  3. Phoenix Inverter VE.Direct 250 VA to 1600 VA data sheet, Victron Energy. Continuous power at 25 °C and peak power, for example 1,000 W and 2,200 W for the 12/1200; maximum efficiency 89–92%.

Formula and sources last checked September 30, 2026. How we test formulas.

  • Off-grid solar system builderList your loads and size the whole off-grid system at once, with battery amp-hours, solar watts, inverter size, main cable gauge and a 24-hour charge chart.
  • Amp-hour and battery runtime calculatorHow long a battery runs a load. Enter amp-hours, voltage, battery type and watts. Covers LiFePO4 and AGM, inverter losses and lead-acid discharge-rate loss.
  • DC wire size and voltage drop calculatorFind the AWG wire size for a 12, 24 or 48 V DC circuit from current and distance. Checks voltage drop and ampacity with NEC and Coast Guard tables.
  • Battery bank size calculatorHow many amp-hours of battery you need for an RV, van, boat or cabin, from your daily watt-hours and days of autonomy, with a 24-hour state-of-charge chart.
  • Power station runtime calculatorHow long a portable power station runs your fridge and other appliances, and whether it can start them. Runtime bars for each appliance and for all together.
  • Solar charge controller size calculatorWhat size solar charge controller you need, MPPT or PWM: its amp rating, the voltage it must take on a cold morning and the PV fuse, from your panels.

Safety note: Estimates only. Equipment ratings vary by manufacturer, so always follow the manufacturer’s instructions and ratings, and ask a qualified professional when safety is at stake.