An inverter that handles a load perfectly well all day long can trip the very moment a fridge compressor kicks in. That is not a fault. It is the difference between two ratings that most people only discover after buying the wrong size.
The calculator below works out both figures from your load and tells you which one is actually the constraint. Our roundup of power inverters covers the units themselves.
The Two Ratings
| Rating | What it means |
|---|---|
| Continuous | What it supplies indefinitely |
| Surge or peak | A brief burst, usually seconds |
| Running watts | What an appliance draws normally |
| Starting watts | What a motor demands at startup |
| Headroom | Margin above your actual load |
| Duty cycle | How long surge can be sustained |
Why Motors Behave Differently
An electric motor sitting at rest has no rotational momentum at all, and getting it turning in the first place demands far more current than simply keeping it turning does.
The figure is commonly cited as three times running watts for typical household motors, and considerably higher for compressors and air conditioners.
That burst lasts a fraction of a second to a couple of seconds, which is why inverters advertise a surge rating separately rather than folding it into one number.
Resistive loads have no equivalent. A kettle, a toaster, or an incandescent bulb draws essentially the same current from the instant it switches on.
The practical consequence is that a system running lights and laptops sizes very differently from one running a fridge. Our note on what appliances you can run off grid covers the load list.
Only the Largest Motor Matters
Motors very rarely start together
Size to the biggest one, not the sum.
Everything else just counts as running
Their normal draw is what stacks alongside the surge.
Simultaneous starts are the exception
A well pump and a fridge together is worth allowing for.
Sequencing avoids the problem entirely
Staggering high-draw appliances keeps peaks apart.
Surge Ratings Are Not All Equal
Two inverters advertising the same surge figure can behave very differently, because the rating means little without a duration attached.
Some manufacturers state surge capacity for several seconds, others for a fraction of a second, and the second kind will still trip on a compressor.
Low frequency inverters using a transformer generally sustain surge longer than high frequency designs, which is much of why they cost more and weigh considerably more.
For a system with a well pump or air conditioning, that difference matters more than the headline surge number.
Checking the specification sheet for surge duration rather than surge watts alone is the step most buyers skip. Our roundup of RV inverter chargers covers units built for motor loads. Protection sizing follows from the inverter figure, and fuse and breaker sizing takes it from there.
Soft Start Devices Change the Math
Where the surge figure is the binding constraint, reducing the surge is frequently cheaper than upsizing the inverter.
Soft start devices ramp a motor up gradually rather than applying full voltage instantly, which substantially reduces the startup spike.
They are most common on air conditioners, where the difference can be large enough to bring a unit within reach of a much smaller inverter.
The tradeoff is another component to install and another point of failure, and they do not eliminate the surge entirely.
For a single problem appliance in an otherwise modest system, the arithmetic usually favors the soft start. Our note on power stations for outages covers the hardest case.
Battery Current Is the Other Constraint
An inverter sized correctly on the AC side can still fail because of what it demands from the battery.
Power drawn on the AC side has to come through the DC side, and at low system voltages that means very high current.
A 2,000W load at 12V pulls roughly 170 amps from the battery before accounting for inverter losses, which is more than many battery management systems will allow.
Cable between battery and inverter has to carry that current over a short run, which is why those cables are dramatically thicker than anything else in a system.
Higher system voltage is the clean answer. The same load at 48V pulls about a quarter of the current, which is much of why larger systems move away from 12V. Our note on what size wire you need covers the cable side.
Headroom Is Not Wasted Capacity
Running an inverter continuously near its rated output generates heat, and heat is what shortens component life.
Efficiency also falls at the extremes, so an inverter working flat out delivers a smaller share of what it draws from the battery.
Idle consumption pulls the other way. A large inverter powering almost nothing still draws standby current, and overnight that adds up against the bank.
Somewhere between roughly a quarter and three quarters of rated output is the comfortable band on most units.
Sizing with 25 percent headroom above your realistic peak is the common recommendation, and it also leaves room for one more appliance later. Our roundup of inverter cooling fans covers managing the heat.
Pure Sine Versus Modified Sine
Waveform is a separate decision from sizing altogether, and it interacts with motor loads in ways worth knowing about.
Modified sine wave inverters approximate AC with a stepped waveform, which is cheaper and adequate for simple resistive loads.
Motors run hotter and less efficiently on modified sine, and some refuse to start at all. Electronics with sensitive power supplies can behave unpredictably.
Medical equipment including CPAP machines generally requires pure sine, and manufacturers frequently state this explicitly.
For anything beyond lights and heating elements, pure sine is the safer specification regardless of what the sizing calculation returns. Our roundup of power stations for CPAP covers that requirement.
Before You Buy
Add up running watts for everything at once
Nameplate figures, not estimates.
Identify the single largest motor separately
That is the one that sets the surge.
Check surge duration, not just surge watts
A fractional-second rating will not start a compressor.
Leave some headroom above the peak
Around 25 percent is the usual figure.
Common Mistakes to Avoid
Sizing to the running watts alone
Motors demand several times that at startup, and the surge is what trips the unit.
Adding every motor’s surge together
They rarely start simultaneously. Size to the largest one plus everything else running.
Comparing surge ratings without duration
The same number sustained for two seconds and for a fraction of one are different products.
Choosing a modified sine unit for motor loads
Motors run hot on it, some will not start, and sensitive electronics can misbehave.
Sources
US Department of Energy, on inverters and off-grid system components. National Electrical Code Article 690, on inverter installation requirements. Underwriters Laboratories 1741, on inverter safety standards. National Renewable Energy Laboratory, on inverter efficiency and performance.
Recommended Reading
See our note on how many watt hours you need, our roundup of energy efficient appliances, our note on power station versus generator, and a roundup of inverter remote displays.
Inverter Sizing FAQ
What size inverter do I need?
Enough continuous rating for everything running at once with headroom, and enough surge rating for your largest motor at startup. The calculator returns both.
Why does my inverter trip when the fridge starts?
Motors draw several times their running watts for a moment at startup. The inverter is handling the running load fine and failing on the surge.
Do I add up every motor’s starting watts?
No. Motors rarely start at the same instant. Size to the largest one starting while everything else runs normally.
How much headroom should I leave?
Around 25 percent above your realistic peak. Running near the rated output continuously generates heat and shortens the unit’s life.
Are all surge ratings comparable?
No, and this is the detail buyers miss. A surge rating means little without a duration. Check the specification sheet rather than the headline figure.
What is a soft start device?
It ramps a motor up gradually rather than applying full voltage instantly, reducing the startup spike. Frequently cheaper than upsizing the inverter.
Pure sine or modified sine?
Pure sine for anything beyond simple resistive loads. Motors run hot on modified sine, some will not start, and medical equipment generally requires pure sine.
Is a bigger inverter always safer?
Not entirely. Larger units draw more standby current, which matters overnight when almost nothing is running.
