Solar Inverter Sizing Calculator
Size a solar inverter from your continuous load and surge demand, and check the DC to AC ratio against your array.
Results
Continuous rating versus surge
An inverter carries two numbers. The continuous rating is what it can hold indefinitely; the surge rating is what it delivers for a few seconds while a motor starts. A fridge compressor or a well pump can draw three to five times its running wattage at the moment it kicks in.
Sizing on running watts alone is the classic mistake: the system works until the pump starts on a dull afternoon and the inverter shuts down on overload.
The DC to AC ratio
Arrays are routinely oversized against the inverter, typically between 1.1 and 1.3 to one. The array only reaches nameplate output for a short part of the year, so a slightly undersized inverter clips a handful of peak hours and spends far more time working in its efficient band.
Common mistakes
Sizing on the sum of every nameplate in the house produces an inverter twice as large as it needs to be. Loads do not all run at once. What matters is the realistic simultaneous peak, plus the surge of the largest motor.
Questions people ask
Does an oversized inverter waste power? Yes, a little. Inverters draw standby power whether or not anything is running, and a large unit idling on a small load sits well below its efficiency peak.
Pure sine or modified sine? Pure sine, unless the entire load list is resistive. Motors, compressors, and most modern electronics run hot or misbehave on a modified waveform.