Inverter Sizing Calculator Formula
Understand the math behind the inverter sizing calculator. Each variable explained with a worked example.
Formulas Used
Minimum Inverter Rating
min_inverter_w = continuous_load_w * surge_multiplierDC Input Current (continuous)
dc_current_a = continuous_load_w / (dc_voltage * eff_fraction)DC Power Draw
dc_power_w = continuous_load_w / eff_fractionVariables
| Variable | Description | Default |
|---|---|---|
continuous_load_w | Continuous Load(W) | 500 |
surge_multiplier | Surge Multiplier(x) | 2 |
inverter_efficiency_pct | Inverter Efficiency(%) | 90 |
dc_voltage | DC Input Voltage(V) | 12 |
eff_fraction | Derived value= inverter_efficiency_pct / 100 | calculated |
How It Works
Inverter Sizing
Formulas
Inverter Rating = Continuous Load x Surge Multiplier
DC Current = AC Load / (DC Voltage x Efficiency)
Motors and compressors draw 2-3x surge at startup. Size the inverter for the surge, not just continuous load.
Worked Example
500 W continuous load, 2x surge, 90% efficient inverter on 12 V DC.
- 01Minimum inverter = 500 x 2 = 1000 W
- 02DC current = 500 / (12 x 0.9) = 46.3 A
- 03DC power = 500 / 0.9 = 556 W
Frequently Asked Questions
Pure sine vs modified sine?
Pure sine wave is needed for sensitive electronics, motors, and medical equipment. Modified sine is cheaper but can cause noise and overheating.
Why account for surge?
Motors, compressors, and power tools draw 2-3x rated current at startup. The inverter must handle this briefly.
What wire gauge for DC input?
At 500 W on 12 V the DC current is ~46 A. Use at minimum 4 AWG copper wire for short runs.
Ready to run the numbers?
Open Inverter Sizing Calculator