Inverter Size Calculator

Q: How do I size an inverter?

Add up all running watts of your loads. Identify the load with the highest startup surge (typically motors: 3–7× running). The inverter must handle both the continuous running load and the peak surge. Add 20–25% safety margin and divide by inverter efficiency.

Q: What is the difference between running watts and surge watts?

Running (continuous) watts is the steady power consumption during normal operation. Surge (peak) watts is the brief spike at startup — motors, compressors, and pumps can draw 3–7× their running watts for 1–3 seconds at start.

Q: What efficiency should I assume for an inverter?

Most quality sine wave inverters operate at 85–95% efficiency at full load. Use 85–90% for conservative sizing. Inverter efficiency drops at very light loads (below 10–20% of capacity).

Find the minimum inverter rating (VA or kVA) to power your loads. The calculator factors in running power, motor surge, inverter efficiency, and a safety margin.

This calculator provides estimates only. Consult the equipment manufacturer and a qualified electrician before installing any inverter system.

Calculate Inverter Size

Total Running Load (Watts)

Largest Motor Surge (Watts)Peak startup power of your highest-surge appliance (0 if no motors)

Inverter Efficiency (%)

Safety Margin (%)

Load Power Factor

Recommended Inverter

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Minimum VA Rating

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Minimum kVA Rating

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Peak Surge Handled (W)

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DC Input Required (W)

Formula

Effective load (W) = max(running load, surge load) × (1 + margin/100)
VA rating = Effective load ÷ Power Factor
DC input required = Effective load ÷ (Efficiency/100)

The surge load is the total running watts minus the largest motor's running watts, plus that motor's surge watts — all other loads continue running while the motor starts.

Typical Surge Multipliers

Refrigerator / freezer: 3–5× running watts at compressor start. A 150W fridge may surge to 600–750W.
Air conditioner: 3–7× running watts. A 1000W AC unit can surge to 3,000–7,000W — the most demanding common appliance.
Water pump / sump pump: 3–5× running watts. A 500W pump may surge to 1,500–2,500W.
Power tools (circular saw, drill): 2–3× running watts on startup.
LED lights, TVs, computers: Minimal surge — essentially no startup inrush to worry about.

Modified Sine Wave vs Pure Sine Wave

Modified sine wave inverters are cheaper but cause problems with some loads:

AC motors run hotter and less efficiently — size up by 20% minimum.
Some electronics (medical equipment, certain power tools) may not work correctly.
Laser printers and microwaves may hum or overheat.

Pure sine wave inverters work with all AC equipment and are recommended for any installation with motors, sensitive electronics, or medical devices.

Frequently Asked Questions

How do I size an inverter?

Step 1: Add all continuous running watts. Step 2: Find the appliance with the highest startup surge (motors, compressors) — look at the nameplate or use 3–5× running watts. Step 3: The inverter must handle the total running load while also absorbing that surge. Add 20% safety margin. Divide by efficiency and power factor to get VA rating.

What is the difference between running watts and surge watts?

Running watts is steady-state consumption — what the appliance draws once it's up and operating. Surge (peak) watts is the brief spike at startup, lasting 1–3 seconds. An air conditioner might run at 900W but surge to 5,000W at start. An inverter must handle both — it needs the surge capacity even if only briefly.

What efficiency should I assume for an inverter?

Use 85–90% for quality pure sine wave units. Cheap or modified sine wave units may be 80–85%. Efficiency also drops at very light loads — most inverters are most efficient at 60–80% of rated capacity. The 90% default in this calculator is appropriate for quality units at moderate load.

Do I need to match inverter voltage to battery voltage?

Yes. The inverter's DC input voltage must match your battery bank voltage: 12V, 24V, or 48V systems. Higher voltage systems (24V, 48V) have lower DC current for the same power, which means thinner, cheaper DC cables. For systems above 1 kW, 24V or 48V banks are strongly preferred.