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Five example setups, calculated
With 125% headroom, a surge rating of twice the continuous rating, and one motor starting at a time. All ticked appliances run together. The last column is the battery current at the inverter’s full rated load on 12 V.
| Setup | Running | Biggest surge | Inverter size | Battery current at 12 V |
|---|---|---|---|---|
| Laptop, lights and phone chargers | 100 W | 100 W | 150 W | 15 A |
| Weekend camper: fridge, lights, laptop, TV | 230 W | 530 W | 300 W | 29 A |
| Workshop: drill and angle grinder | 1,500 W | 3,300 W | 2,000 W | 196 A |
| Cabin: fridge, well pump, TV, lights | 480 W | 1,080 W | 600 W | 59 A |
| Camper kitchen: microwave only | 1,200 W | 1,200 W | 1,500 W | 147 A |
- The workshop needs a 2,000 W inverter and about 196 A at 12 V. That is a job for 24 V or 48 V.
- The microwave alone has a modest running load, but it needs an inverter about as big as the cabin’s whole fridge, pump and TV setup.
How the inverter size is calculated
- Running load (W) = watts × quantity for every ticked appliance.
- Surge moment (W) = running load + the extra start-up power of the biggest motor load. The extra is watts × (surge − 1). If several motors may start together, all the extras are added.
- Continuous size needed = the larger of running load × headroom, and surge moment ÷ the inverter’s surge ratio.
- Inverter size = the next standard size up: 150, 300, 500, 600, 800, 1,000, 1,200, 1,500, 2,000, 2,500, 3,000, 4,000 or 5,000 W.
- Battery current (A) = inverter watts ÷ (0.85 × battery voltage). The 0.85 covers conversion loss.
It does not model idle draw (many inverters use 10–30 W just by being on), very long surges, or the specific surge rating of a particular inverter. Check both ratings on the product you buy.
Using it anywhere in the world
- Match the inverter’s output voltage and frequency to your appliances. Outlet voltage is about 100–127 V in North America and parts of Japan, and 220–240 V in most other countries, at 50 or 60 Hz depending on the country.
- Watts are the same whichever voltage you use, so the calculator works worldwide. Heating appliances are rated higher on 230 V circuits, so check the label on your own kettle or heater.
- Numbers follow your own region’s format.
Common sizing mistakes
- Sizing for the running load only. A fridge compressor or pump can need several times its running power for a second.
- Adding up everything you own. Untick the appliances you will not run together.
- Buying far too big. An oversized inverter wastes more power at idle and costs more, and it forces thicker cables.
- Forgetting the battery side. A 2,000 W inverter at 12 V pulls about 196 A, so the cable, fuse and battery must handle it.
- Mixing up continuous and peak ratings. Listings often headline the peak figure.
Common questions
What size inverter do I need to run a fridge?
A typical 100 W compressor fridge can need about 400 W for a moment when its compressor starts. With a surge rating of twice the continuous rating, the calculator suggests a 300 W inverter. Many people choose one size up for margin, because a fridge’s real start-up power varies with its age and model.
What is surge or peak power?
Motors and compressors need extra power for a fraction of a second when they start. Inverters list a continuous rating for steady loads and a higher surge rating for these moments. Both numbers must be high enough for your appliances.
Do I need a pure sine wave inverter?
For laptops, TVs, fridges, motors and medical devices, yes, it is the safer choice. Modified sine wave inverters are cheaper and run simple heaters and lights, but some electronics hum, overheat or stop working on them.
Can I run a microwave, kettle or hair dryer on an inverter?
Yes, if the inverter’s continuous rating is high enough, but they use a lot of power. A 1,200 W microwave needs about a 1,500 W inverter and about 147 A from a 12 V battery, so you need thick cable, a strong battery and short use. They run for minutes, not hours.
How big a battery and cable does an inverter need?
The battery current is inverter watts ÷ (0.85 × battery voltage). A 1,000 W load pulls about 98 A at 12 V and about 49 A at 24 V. Use that current in the wire gauge and fuse calculator for the cable and fuse, and the battery runtime calculator to see how long the battery will last.
Should I buy a 120 V or a 230 V inverter?
Choose the output voltage and frequency your appliances are made for, which is on their labels. North America mainly uses 120 V at 60 Hz, and most of the rest of the world uses 220–240 V at 50 Hz, with exceptions. Some inverters can be switched between standards, so check the product before you buy.
Related calculators
Once you have the size, use the wire gauge and fuse calculator for the battery cable, and the battery runtime calculator for how long it will run. To plan the whole system, see the solar and battery calculator.
Planning for a blackout? The power outage calculator sizes a battery or generator for your essentials.
Camping with shore power or a generator? See the RV amp load calculator and the generator runtime calculator.
Need typical watts for your appliances? See the appliance wattage chart.