Inverter Size Calculator
Add your AC appliances - get continuous wattage, surge peak, DC current, and pure-sine recommendation for 12V, 24V, or 48V off-grid systems.
Inverter Size Calculator
65W × 1 = 65W · surge ≈ 81W
80W × 1 = 80W · surge ≈ 400W
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Result
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Surge Factor by Appliance Type
Startup surge is what kills undersized inverters. Pick the largest appliance, multiply by its surge factor, and make sure your inverter peak rating covers it on top of other steady loads.
| Category | Surge × continuous | Pure-sine? | Examples |
|---|---|---|---|
| Resistive | 1.0x | Optional | Heater, kettle, incandescent bulb |
| Electronics | 1.25x | Required | Laptop, TV, modem, audio |
| Compressor fridge | 5.0x | Required | Camper fridge, freezer |
| Motor (light) | 3.0x | Required | Drill, mixer, blender |
| Pump / AC compressor | 4.0x | Required | Water pump, AC, heat pump |
Frequently Asked Questions
What size inverter do I need?
Sum continuous AC wattage, multiply by 1.25, then pick the smallest standard size. Add appliance-specific surge factors for the worst case startup. The calculator does both above.
Do I need a pure sine wave inverter?
For modern electronics, motors, and compressor fridges - yes. Modified-sine is only OK for purely resistive loads (kettle, incandescent).
How much surge do I need?
Resistive 1x, electronics 1.25x, motor tools 3x, compressor fridges ~5x, pumps 4x. Worst surge stacked on steady loads sets your inverter peak rating.
How do I calculate DC current draw?
AC watts / (efficiency × DC volts). 90% efficient 2000W on 12V = 185A continuous. Same on 48V = 46A. Higher DC voltage makes big inverters cheaper and safer.
Can I oversize the inverter?
25-50% headroom is normal. Massively oversizing wastes idle current and burns budget on bigger cables, fuses, and batteries.
What is no-load idle draw?
Inverter pulls 0.5-2A on 12V even with no load, just to stay alert. Search mode drops idle to under 0.1A.
Should I run my fridge on the inverter?
A DC compressor fridge is more efficient than AC through an inverter. AC fridges need ~5x surge headroom at startup.
What size inverter for a 200Ah battery?
200Ah at 12V = 2400 Wh stored. 1500-2000W inverter is a typical match. Bigger inverters are pointless if the bank can not sustain them.
How to Size a Pure-Sine Inverter
The right inverter handles your loads at steady state and absorbs every startup surge without tripping. Get this wrong and your fridge cycle starts the inverter shutting down. Get it right and the system just works.
Continuous load is the floor
Sum every AC watt that runs simultaneously. Add a 25% safety factor on top to keep the inverter from running flat-out indefinitely.
Surge sets the ceiling
Compressor fridges can pull 5x rated watts for a few hundred milliseconds at startup. Most inverters advertise a peak rating of about 2x their continuous size. If your worst surge plus other steady loads exceeds that 2x peak, you need a bigger inverter.
DC current pulls the cables
At 12V, 2000W = 185A. That is heavy AWG 2/0 cable territory. The same 2000W at 48V is 46A - much friendlier. Higher DC voltage almost always pays for itself once you cross the 2 kW mark.
Pure sine is the default today
Pure sine costs only slightly more than modified sine and works with everything. Outside resistive heating, there is rarely a reason to specify modified sine in a new build.
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