Inverter size calculator
Size a power inverter from your appliance list: continuous watts, motor start-up surge, the next standard size and DC current from a 12, 24 or 48 V battery.
Results
| Recommended continuous rating (+25%) | 2,138 W |
|---|---|
| Next standard inverter size | 3,000 W |
| Continuous load | 1,710 W |
| Start-up surge needed | 2,760 W |
| DC current at continuous load | 158.3 A |
- Over 150 A from a 12 V battery needs very heavy cable (2/0-4/0 AWG for short runs) and high-current fusing. A 24 V or 48 V system is usually cheaper and safer at this power level.
- Choose an inverter whose surge rating (often 2× continuous for a few seconds) covers the start-up peak. Low-frequency transformer inverters handle motor starts better than high-frequency ones.
About this calculator
An inverter has to handle two numbers: the continuous power of everything running at once, and the brief surge when a motor or compressor starts. A fridge that runs at 150 W can pull over 1,000 W for a fraction of a second at start-up. If the inverter can't supply it, the fridge never starts.
This calculator adds up your loads, applies a 25% continuous margin so the inverter isn't running flat-out and overheating, works out the start-up peak and shows the DC current the inverter pulls from your battery. That current decides your cable and fuse sizes.
How to use it
- Add every appliance that might be on at the same moment. Leave out the ones you'll never run together.
- Use running watts for each appliance, plus starting watts for anything with a motor or compressor.
- Select your battery voltage.
- Pick an inverter at or above the standard size whose surge rating covers the start-up peak, then size cable from the DC current.
The formula
Continuous load = Σ running W × quantity.
Recommended rating = continuous × 1.25.
Surge = continuous + the largest (starting − running) difference.
DC current = continuous W ÷ (battery V × efficiency).
Worked example (default values)
Inputs
| Appliances that may run at the same time | Microwave (1000 W cooking): 1500 W; Refrigerator (running while compressor on): 150 W; Laptop: 60 W |
|---|---|
| Battery voltage | 12 V |
| Inverter efficiency at load | 90 % |
Results
| Recommended continuous rating (+25%) | 2,138 W |
|---|---|
| Next standard inverter size | 3,000 W |
| Continuous load | 1,710 W |
| Start-up surge needed | 2,760 W |
| DC current at continuous load | 158.3 A |
Tips and common mistakes
Microwaves draw more than their label. A "1000 W" microwave is rated by its cooking power. It typically pulls about 1,400-1,600 W from the outlet. Check the rear label for the input watts.
Idle draw: big inverters can use 20-50 W just switched on. That's over 1 kWh per day. If you'll run small loads most of the time, pick a model with a low-power search mode, or run small loads from DC.
Frequently asked questions
What size inverter do I need to run a microwave?
For a typical 1,000 W (cooking) microwave drawing about 1,500 W, use a 2,000 W pure sine wave inverter. On 12 V that pulls about 140 A, so short, heavy cables are essential.
Can I run a refrigerator on a 1000W inverter?
Usually yes, if the inverter's surge rating covers the compressor's start-up (often 800-1,500 W for a few hundred milliseconds) and the fridge is the main load.
Pure sine wave or modified sine?
Pure sine wave for anything with motors, compressors, chargers, audio gear or medical devices. Modified sine wave is only suitable for simple resistive loads.
Related calculators
- DC wire size calculator (12 V, 24 V, 48 V)
Find the right copper wire gauge (AWG) for a 12 V, 24 V or 48 V DC circuit from current, cable length and allowed voltage drop, checked against ampacity.
- DC fuse & breaker size calculator
Pick a DC fuse or breaker size: at least 125% of continuous current, no larger than what your wire can safely carry. Covers 12 V, 24 V and 48 V circuits.
- Battery bank runtime calculator
How long will a 100Ah (or any) battery run your load? Accounts for chemistry, usable depth of discharge, inverter losses and the Peukert effect.
- Watts ↔ amps calculator
Convert watts to amps or amps to watts for DC, single-phase AC and three-phase AC circuits at any voltage and power factor.
More wiring & dc electrical calculators
Off-grid solar sizing, step by step
- Rough out the whole system · Off-grid solar system calculator
- Size the inverter from your loads (you are here)
- Plan the battery bank · Battery bank series & parallel calculator
- Check battery runtime · Battery bank runtime calculator
- Size the solar array · Solar panel size calculator
- Check panel string voltage · Solar string voltage calculator (series & parallel)
- Choose the charge controller · Solar charge controller size calculator
- Size the DC cables · DC wire size calculator (12 V, 24 V, 48 V)
- Pick the fuses · DC fuse & breaker size calculator
Estimates for planning only, based on the assumptions shown. Real-world performance varies with equipment, temperature and installation. Updated October 19, 2026. See our methodology.