Use this online inverter sizing calculator to estimate inverter wattage, surge handling, and battery capacity for backup power, UPS systems, solar systems, and off-grid loads.
Use the result as a sizing guide. Real system design should also consider inverter waveform, startup characteristics, battery chemistry, and wiring losses.
Inverter sizing starts with the real load you want to power. The inverter must comfortably carry the continuous running load and also survive the temporary surge load from motors, compressors, and switch-mode equipment. If backup time matters, the battery bank must also store enough usable energy to cover that load.
This inverter size calculator is useful when selecting a home inverter, UPS inverter, solar inverter, or backup power system. It combines running load, surge load, battery voltage, efficiency, and backup time so you can estimate both inverter rating and battery bank size in one place.
That makes it easier to compare backup options and avoid selecting an inverter that is too small for startup demand or a battery bank that cannot support the required runtime.
| Variable | Meaning | Typical Unit |
|---|---|---|
| Prun | Total continuous running load | W |
| t | Required backup time | h |
| eta | Inverter efficiency | Unitless or % |
| Vbatt | Battery bank voltage | V |
| Ah | Battery capacity needed | Ah |
Enter running load and surge load in watts, backup time in hours, battery voltage in volts, and efficiency and usable battery capacity as percentages.
Results are shown in watts for inverter size, watt-hours for stored energy, and amp-hours for battery capacity, which are the most common units used in inverter and battery sizing.
Running load = 800 W, surge load = 1500 W, backup time = 4 h, battery = 24 V, efficiency = 90%, usable battery = 80%.
Recommended inverter rating = 1.25 x 800 = 1000 W
Battery energy needed = 800 x 4 / 0.90 = 3555.56 Wh
Battery capacity = 3555.56 / (24 x 0.80) = 185.19 Ah
Running load = 1800 W, surge load = 2500 W, backup time = 2 h, battery = 48 V, efficiency = 92%, usable battery = 80%.
Recommended inverter rating = 1.25 x 1800 = 2250 W
Battery energy needed = 1800 x 2 / 0.92 = 3913.04 Wh
Battery capacity = 3913.04 / (48 x 0.80) = 101.90 Ah
When choosing an inverter, pay attention to load type as well as total wattage. Resistive loads such as lamps and heaters are usually simple to supply, but motors, compressors, pumps, refrigerators, and tools can create high startup demand. That is why this inverter sizing calculator shows both continuous and surge needs. It is also wise to leave extra capacity for future expansion, longer battery life, and better inverter performance under real operating conditions.