Surge and Continuous Ratings on an Inverter

An inverter has two limits, and they answer two different questions. One is how much it can supply all day. The other is how much it can supply for a moment.

An inverter in one paragraph

A battery stores direct current. Most household appliances want alternating current. An inverter does the switch from one to the other. The reference page on power inverters describes the family of devices that do this. The size of the inverter, not only the battery, decides how many watts reach your appliance.

The two ratings

Continuous rating. This is the power the unit can supply for as long as it runs. It matches the idea of running watts, which one maker's guide describes as the power a device draws continuously¹. It's the figure to compare with the total of everything you leave switched on.

Surge rating. Also called peak or starting power. Starting watts, by the same guide, are a short burst of two or three seconds at start-up¹. On the supply side, the surge rating is how much the unit is built to give during such a burst.

So the surge rating is higher than the continuous one. It exists to cover the short spike that a motor produces.

How long does the surge last

That depends on the unit, and I can't give you a figure that holds for every product. The spec sheet or manual for your own unit is the place to read it. Look for words like "surge duration", "peak for" or a number of seconds beside the surge figure. If the sheet doesn't say, ask the maker's support team before you rely on it.

What is clear from the sources is how short the need is. Appliances need extra power for two or three seconds when starting¹. A surge rating with no stated time is a loose promise. Treat it with care.

What happens at the limit

Makers say a unit may protect itself. One support page says a startup surge may exceed the limit and trigger protection. The result can be an error or shutdown, even when the rated power is within range². That's the symptom behind the page on when an inverter overloads.

The same page names air conditioners, pressure washers and compressors as drawing a very high surge at startup². It adds that with several devices, the total load and combined startup surges shouldn't exceed the rated output².

Protection of this kind is a normal idea in electrics. Overcurrent is a larger than intended current, and fuses, circuit breakers and current limiters are commonly used protection against it³. In an inverter, that protection is built in, and it's what stops the output when a burst is too big.

A note on waveform

Not all inverters produce the same shape of output. The reference page on inverters says most AC motors will run on modified-sine-wave inverters with an efficiency reduction of about 20% owing to the harmonic content⁴. So the type of output can matter for motors. Your appliance manual should say what it expects.

Reading a spec sheet

Take your unit's sheet and look for four things:

• the continuous output, in watts;
• the surge or peak output, in watts, and any time given with it;
• whether the figures are for a single outlet or the whole unit;
• the output type.

Then check the appliance side. The label page shows where to find its numbers, and the first page in this set covers how to add them.

Who does the connecting

A plug-in portable unit with its own outlets is something you use as its manual describes. A battery system wired into your home, through the switchboard or a transfer switch, is different. That's licensed electrician work, and the rules for it vary by country and state. Check with your local electrical regulator or your installer. Never feed power back into the house through an ordinary outlet.