Plan dependable backup power for home or shop. Compare voltages, batteries, and efficiency assumptions clearly. Get right-sized inverter, battery count, and confidence every time.
| Appliance | Running W | Surge W | Duty % | Notes |
|---|---|---|---|---|
| Refrigerator | 150 | 900 | 60 | Compressor start surge |
| LED Lights (10) | 120 | 120 | 80 | Low surge |
| Ceiling Fan | 70 | 140 | 70 | Moderate start |
| Wi‑Fi Router | 15 | 15 | 100 | Always on |
| TV | 110 | 110 | 40 | Varies by size |
Accurate inverter sizing starts with a complete appliance list and realistic running watts. Many homes underestimate small loads like routers, chargers, and lighting. If your baseline is 420 W and you add a 15% growth margin, planning load becomes 483 W. This margin protects budgets because undersized inverters trigger nuisance shutdowns and force early upgrades.
Motors and compressors can demand 3× to 7× their running watts for a few seconds. A refrigerator rated 150 W may surge near 900 W. The calculator estimates surge by combining the largest surge item with other running loads, then adds a surge margin for headroom. This approach reduces the risk of dimming lights, relay chatter, or inverter fault codes at startup.
Energy must pass through conversion stages. With 90% inverter efficiency and 3% cable loss, every 1000 Wh of AC demand can require about 1143 Wh from the battery side. Small percentage losses compound across long backup windows. Use conservative values when cables are long, ambient temperatures are high, or the system operates near maximum output.
Backup hours are driven by average demand, not peak demand. Duty cycle helps estimate average use; for example, a fan at 70 W with 70% duty averages 49 W. Required battery energy is increased by depth of discharge and battery efficiency. If DoD is set to 80% and battery efficiency is 90%, only 72% of stored energy is effectively available for AC loads.
Use the continuous watt recommendation to select a standard inverter size, and verify the surge rating exceeds the calculated surge requirement. Review estimated DC current; higher current increases cable heating and voltage drop, so 24 V or 48 V systems often improve reliability. Compare battery series and parallel counts to confirm the bank matches your chosen battery model. For financial planning, multiply inverter and battery costs by expected replacements, and include maintenance, fuel alternatives, and total downtime risk.
Running watts determine continuous sizing, while the highest realistic surge watt value drives starting capacity. Add a growth margin if you plan new appliances, and use conservative efficiency and cable loss values when wiring runs are long.
Check the appliance label or manual for starting watts or locked-rotor amps. If you only know running watts, use typical multipliers: 3× for fans, 5× for pumps, and up to 7× for compressors.
Duty cycle estimates average energy use over time. A device that runs 30% of the time consumes roughly 30% of its running-watt energy over the backup window, which can significantly reduce required battery capacity.
Often, yes. Higher voltage lowers DC current for the same power, which can reduce cable size, voltage drop, and heating. It may require more batteries in series, so compare total battery counts and wiring complexity.
They limit usable stored energy. At 80% DoD and 90% battery efficiency, only about 72% of rated energy is available for loads, so the calculator increases required battery capacity accordingly.
Use them as a planning estimate. Confirm nameplate ratings, surge behavior, inverter surge duration, charging method, and safety limits. A qualified installer should verify wiring, protection devices, ventilation, and local electrical codes.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.