Key takeaways
- An inverter has two ratings: continuous (running) and surge (peak).
- Motors draw 2–3× their running watts for a moment at startup.
- Size continuous to running load × 1.25; size surge to your largest motor.
- Pick a pure sine wave unit — and remember it pulls all that power from the battery.
Continuous watts vs surge watts
Every inverter ships with two power ratings, and they answer two different questions. Continuous watts — sometimes called running or rated watts — is the power the inverter can supply steadily, hour after hour, without overheating. This is the number that has to cover everything you have switched on at the same time: lights, laptop, fridge, water pump, and so on.
Surge watts — also called starting or peak watts — is a much larger figure the inverter can deliver for only a second or two. It exists because of motors. The compressor in a fridge, the motor on a well pump, and the compressor in an air conditioner all pull a huge inrush of current the instant they kick on, typically 2 to 3 times their running watts, before settling back down. Resistive loads like a microwave, a TV, or an LED light draw the same at startup as they do running, so they have no real surge.
If you only size for continuous watts, the inverter runs your gear fine until the well pump tries to start — then the surge demand exceeds what it can deliver, and it shuts down on overload. That is why both numbers matter.
How to size your inverter
The 1.25 multiplier on continuous watts gives you headroom so the inverter isn't pinned at 100% — that keeps it cooler and leaves room for the load you forgot. Surge is sized to the single largest motor, not the sum of all of them, because they rarely start at the same instant. Run your own list through the solar inverter sizing calculator, and use the off-grid load calculator to total the running and peak watts in the first place.
Common appliance running vs starting watts
| Appliance | Running watts | Starting (surge) watts |
|---|---|---|
| Refrigerator | 150 | 1,200 |
| Well pump (1 HP) | 1,000 | 3,000 |
| Microwave | 1,000 | 1,000 |
| RV air conditioner (15k BTU) | 1,500 | 4,500 |
| TV | 80 | 80 |
Notice the pattern: the microwave and TV surge equals their running watts, while the fridge, pump, and air conditioner spike far higher. Your inverter's surge rating has to clear the tallest of those spikes.
Worked example
Say you run a fridge, some lights, a laptop, and a water pump that can total about 1,500 W of running load at once. Continuous needed = 1,500 W × 1.25 = 1,875 W. Your largest motor is the pump, with a starting surge of 2,200 W. So you want an inverter rated at roughly 2,000 W continuous (which clears 1,875 W) with a surge rating of at least 2,200 W. A common 2,000 W pure sine unit that surges to 4,000 W satisfies both conditions with room to spare.
Pure sine vs modified sine, and the battery link
Inverters come in two waveform types. Pure sine wave output matches grid power and runs everything cleanly, including motors, electronics, and medical gear. Modified sine wave is cheaper but can cause buzzing, motor heat, and problems with sensitive electronics — fine for simple resistive loads, risky for anything with a motor or microprocessor. For an off-grid home system, pick pure sine.
Finally, remember the inverter draws everything it outputs from your battery bank. A 2,000 W load at 12 V pulls nearly 170 A from the battery, so a bigger inverter ties directly into a bigger bank and heavier cabling. Size the two together using the battery bank sizing calculator so the battery can actually feed the inverter at full draw.
Frequently asked questions
What size inverter do I need?
Add up the running watts of everything that runs at the same time, multiply by 1.25 for margin, and pick an inverter whose continuous rating meets or beats that number. Then confirm its surge rating is at least the starting watts of your largest motor.
What is the difference between continuous and surge watts?
Continuous (running) watts is the power an inverter supplies indefinitely. Surge (peak) watts is the brief burst it delivers for a second or two to start motors, which draw 2–3× their running watts at startup. Both ratings matter — check each.
Will a 2,000 W inverter run a fridge or RV air conditioner?
A 2,000 W continuous inverter with a ≥2,200 W surge easily starts and runs a fridge (≈150 W running, 1,200 W surge). A 15k BTU RV air conditioner needs ≈1,500 W running and 4,500 W surge, so it requires an inverter that surges to at least 4,500 W.