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Inverter Size Calculator

Calculate recommended continuous and peak surge inverter wattage for home backup, RV, and off-grid solar setups.

Continuous Running Load (Watts)

Peak Starting Surge (Watts)

Motors/compressors draw 2x to 4x running watts at startup

Continuous Safety Margin

Recommended Inverter Sizing

Minimum Continuous Rating

960 Watts

Includes 20% safety headroom buffer

Minimum Peak Surge Rating

1,800 Watts

Required for motor and compressor inrush

Nearest Standard Inverter Size

1,000 VA

Inverters are sold in standard VA ratings; buy this size or the next one up

Pure Sine Wave Recommendation

Always choose a Pure Sine Wave inverter for residential backup, refrigerators, power tools, and electronic appliances to prevent humming, harmonic overheating, and equipment damage.

Step-by-Step Engineering Calculation

1Total Continuous Load: $P_{\text{load}} = 800\text{ W}$
2Safety Headroom (20%): $P_{\text{cont}} = 800\text{ W} \times 1.20 = \mathbf{960\text{ W}}$
3Peak Surge Requirement: $P_{\text{surge}} = \mathbf{1800\text{ W}}$
4Standard size: inverters come in standard VA ratings, so round up to the next standard unit: $\mathbf{1000\text{ VA}}$ continuous (confirm its surge rating covers 1,800 W).
5Recommendation: Choose an inverter with at least 960 W continuous output and 1800 W peak surge rating (Pure Sine Wave recommended for sensitive electronics & inductive motor loads).

Formula & Theory

Continuous (W) = Total Running Watts × 1.20 | Surge (W) = Running Watts + Motor Surge

Calculates the minimum recommended continuous output rating and surge capability for DC-to-AC power inverters.

Variable Definitions

P_cont:: Continuous Load (Watts (W)) – Total simultaneously running appliances
P_surge:: Surge Load (Watts (W)) – Peak inrush power during motor/compressor startup

Worked Engineering Example

Cabin Inverter Setup

Running a 300W refrigerator (1200W starting surge), 100W TV, and 50W lights simultaneously.

Inputs:

Continuous Load: 450 W
Peak Surge: 1350 W
Safety Margin: 20%

Result: Recommended Continuous: 540 W | Recommended Surge: 1350 W

Key Assumptions

•Includes standard 20% continuous safety headroom.

Limitations & Safety Scope

•Inductive motor loads (air compressors, well pumps, air conditioners) can produce starting surges 3 to 6 times their nameplate running wattage.

Frequently Asked Questions

Should I choose Pure Sine Wave or Modified Sine Wave?

Pure Sine Wave is strongly recommended for almost all modern electronics, refrigerators, CPAP machines, audio equipment, and AC motors.

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Educational & Reference Notice: This calculator provides technical estimates based on standard physical formulas. Actual electrical installation, protective device sizing, and conductor selection must be verified against the applicable electrical code (NEC, IEC, CEC) by a licensed electrician or qualified professional engineer.