Minimum continuous inverter--
Minimum surge rating--
Inverter Size Calculator
Estimate inverter size from appliance watts, surge load, and safety margin.
Practical intro
Use this inverter size calculator when matching an inverter to selected AC loads from a battery, vehicle, RV, solar setup, or backup power plan. It separates continuous running watts from surge watts because motors, compressors, pumps, and power tools may need brief startup power above their normal running load.
When to use this calculator
- Sizing an inverter for a small battery backup or off-grid load list.
- Checking whether a portable inverter can start a refrigerator, pump, or tool.
- Comparing pure sine wave inverter ratings before buying.
- Understanding the difference between continuous output, peak output, and battery-side current.
Input explanation
- Running watts is the steady load the inverter must carry during normal operation.
- Surge watts is the highest startup or momentary load expected.
- Safety margin adds room for measurement error, heat, altitude, inverter derating, and future small loads.
- The calculator estimates AC output size, not DC cable size or battery capacity.
- Battery voltage and inverter efficiency are needed separately to size fuses, cables, and battery draw.
Formula or estimation method
Minimum continuous inverter = running watts x (1 + margin percentage).
Minimum surge rating = surge watts x (1 + margin percentage).
Approximate DC current = AC watts / battery voltage / efficiency.
Runtime depends on battery watt-hours, not inverter size alone.
Step-by-step worked example
If the planned load is 800 running watts with a 1,200 watt surge and a 20% margin, the inverter should be planned around at least 960 continuous watts and 1,440 surge watts. The battery and cables must also handle the current. At 12 volts, a 960 watt AC load can draw roughly 94 amps at 85% efficiency, so wiring and fusing are critical.
Common mistakes
- Buying an inverter based only on peak watts and ignoring continuous rating.
- Forgetting motor surge from refrigerators, pumps, microwaves, or tools.
- Using undersized DC cables that overheat or cause voltage drop.
- Ignoring pure sine wave requirements for sensitive electronics or motors.
- Assuming a bigger inverter creates more battery runtime.
Planning notes and limitations
- A larger inverter can start bigger loads, but it may have higher idle draw.
- Battery voltage matters: high-watt loads on 12V systems can require very high current.
- Check inverter waveform, grounding, transfer behavior, temperature rating, and installation clearance.
- This calculator does not size battery cables, fuses, disconnects, charging, or code-required installation details.
Backup power safety checklist
Before relying on the estimate, confirm the equipment manual, fuel or battery limits, ventilation needs, weather protection, cords, transfer method, and local requirements. Fuel-burning generators belong outdoors away from openings because carbon monoxide can be deadly. Electrical connections, transfer switches, high-current batteries, and permanent wiring should be handled under applicable code by qualified people.
Safety disclaimer
This calculator is for planning only. Follow inverter manufacturer instructions, battery safety guidance, fuse and cable requirements, local electrical code, and licensed installer advice for connected or high-current systems.
FAQ
Inverter Size Calculator questions
What is the difference between continuous and surge watts?
Continuous watts are what the inverter can supply steadily. Surge watts are brief startup power for loads such as motors or compressors.
Do I need a pure sine wave inverter?
Many electronics, motors, chargers, and appliances perform better or require pure sine wave power. Check the device manual.
Can I run a refrigerator?
Possibly, if the inverter handles compressor surge and the battery has enough usable energy for the duty cycle.
Does inverter size determine runtime?
No. Runtime mainly depends on battery capacity, load watts, efficiency, and usable depth of discharge.
Why are DC cables important?
High inverter loads can draw large battery-side current. Undersized cables or missing fuses can create fire and equipment hazards.
Before you buy equipment
Use this result as a screening estimate, then compare it with rated running watts, surge watts, usable watt-hours, fuel availability, recharge time, maintenance, and installation requirements. A backup plan is safer when it starts with fewer essential loads and leaves reserve instead of assuming every rating can be used at the same time.