AC Load
Other Simultaneous Loads
Inverter Sizing
Every electric motor draws a large inrush current when it first starts. For an AC compressor, this surge can last two to five seconds and reach two to five times the normal running current. If the power inverter cannot supply this surge, it trips on overload protection and the AC never starts. Sizing an inverter purely to the AC's rated running wattage will almost always fail on startup.
Inverter AC vs non-inverter AC startup surge: A variable-speed inverter AC ramps the compressor gradually, producing a soft surge of roughly 1.5 to 2 times the running load. A fixed-speed non-inverter compressor fires at full torque immediately, producing a hard surge of 3 to 5 times the running load. For the same 1.5 ton tonnage, the non-inverter AC may need a power inverter twice the size of the one that comfortably runs the inverter AC.
| Load | Surge factor (times running watts) | Duration |
|---|---|---|
| Inverter AC compressor | 1.5 to 2.0 | 1 to 3 seconds |
| Non-inverter AC compressor | 3 to 5 | 2 to 5 seconds |
| Refrigerator / freezer | 2 to 3 | 1 to 2 seconds |
| Ceiling fan | 1.2 to 1.5 | Under 1 second |
| LED lights, electronics | 1.0 to 1.1 | Negligible |
An inverter AC running at 1,000 W, a refrigerator at 150 W, fans and lights at 150 W. Surge factor 1.8 for the inverter AC, 2.5 for the fridge. 20% safety margin.
What size inverter for a 1.5 ton AC?
For an inverter-type 1.5 ton AC at 1,000 W with typical household loads and a 20% margin, a 3,000 to 3,500 VA inverter. For a non-inverter 1.5 ton AC, 5,000 VA or larger due to the much harder startup surge.
Why does AC need a bigger inverter than its wattage?
Compressors surge at startup to 2 to 5 times the running load. An inverter sized only for running watts trips on every AC startup. The inverter must handle the peak surge plus all other running loads.
What is the difference between VA and watts for inverters?
VA is apparent power including reactive current. Watts is real power consumed. Motor loads have a power factor below 1, so VA exceeds watts. A 1,000 W motor load needs about 1,100 to 1,250 VA from the inverter.
Do inverter ACs need smaller inverters?
Yes, significantly. An inverter AC surges at 1.5 to 2 times running load; a non-inverter AC surges at 3 to 5 times. For the same tonnage, an inverter AC may need a power inverter half the size of one needed for a non-inverter model.
Should I add other loads when sizing?
Yes. Add all appliances running simultaneously. The inverter must handle all running loads plus the AC startup surge at the same time. A refrigerator can also surge on startup, so include it separately.
Surge factors used are typical values. Actual startup surge varies by AC model, compressor design and installation conditions. Allow extra margin for older or larger compressors.