Running vs. surge watts: sizing your inverter
Running (continuous) watts are what a device draws while it's operating normally. Surge (starting) watts are the brief spike some devices draw as they start, especially anything with a motor or compressor. Your inverter has to handle both, or it will shut off.
The classic story: someone buys a power station, plugs in a fridge or a power tool, hears a beep, and everything goes dark. The power station had plenty of capacity. What it couldn't handle was the startup surge.
Why some devices surge
An electric motor at a standstill has very little resistance to current, so for a fraction of a second it draws far more than it will once it's spinning. Compressors in fridges, freezers and air conditioners have it worse, because they start against pressure. Devices without motors, such as LED lights, laptops, phone chargers and heating elements, have little or no meaningful surge.
How much surge to plan for
Exact figures vary widely between models, and manufacturers don't always publish them. For planning, these multipliers are commonly used. Treat them as rough guides and check the manual where you can.
| Device type | Typical startup multiplier |
|---|---|
| Lights, electronics, chargers, heaters | ~1× (no real surge) |
| Fans, small motors | ~1–2× |
| Power tools (saws, drills) | ~2× |
| Refrigerators, freezers, sump pumps | ~3× or more |
| Air conditioners, well pumps | often 3× or more; check specs |
"Inverter" fridges and air conditioners (which use variable-speed compressors) usually start much more gently than older fixed-speed models.
How to size the inverter
- Continuous rating: add up the running watts of everything that might run at the same time. Add about 25% headroom, since running an inverter right at its limit makes it hot and less efficient.
- Surge rating: take that total running load and add the extra startup draw of the single largest motor. Usually only one motor starts at any given moment.
Example: a fridge (150 W) and a sump pump (800 W), plus 100 W of lights and electronics. Total running: 1,050 W, so look for at least about 1,300 W continuous. Surge: the pump is the largest motor, so 1,050 + 800 × 2 = 2,650 W. Our power station size calculator does this automatically when you pick devices from the list.
Why surge ratings on spec sheets vary
Manufacturers measure surge differently. Some quote a peak that lasts milliseconds, others a level the inverter can hold for a second or more. Two power stations both claiming "4,000 W surge" may not behave the same with a stubborn compressor. If you plan to start a large motor, it's worth looking for owner reports of that specific combination, or giving yourself a generous margin.
Pure sine wave vs. modified sine wave
Most reputable portable power stations use pure sine wave inverters, which produce power similar to a wall outlet. Some cheap standalone inverters use modified sine wave output. That can make motors run hotter and noisier, and some electronics and chargers may not work properly. For fridges, pumps, CPAP machines and anything sensitive, choose pure sine wave.
Tips if your inverter keeps tripping
- Start the motor load first, before turning on other devices, so the surge has the most headroom.
- Avoid starting two motor loads at once.
- Avoid long, thin extension cords. They add voltage drop and can make starting harder.
- Check whether the power station has a "power lifting" or similar mode. Some units use it to run resistive loads like kettles at reduced power. It doesn't help motor surges.