Plan reliable backup power for outages and bills. Adjust depth of discharge and voltage settings. Compare options, then export results as CSV or PDF.
Sample backup plan for an 8‑hour outage. Adjust wattages and hours to match your home.
| Appliance | Watts | Qty | Hours | Energy (kWh) |
|---|---|---|---|---|
| Refrigerator | 150 | 1 | 8 | 1.20 |
| LED lights | 10 | 10 | 6 | 0.60 |
| Wi‑Fi router | 12 | 1 | 8 | 0.10 |
| Ceiling fan | 60 | 2 | 5 | 0.60 |
| Laptop charging | 65 | 1 | 3 | 0.20 |
| Total | 2.70 | |||
The calculator starts with the energy your home needs during backup, then scales it to account for system losses, temperature effects, and a reserve margin.
Backup capacity begins with watt‑hours. A steady 800 W critical load for 8 hours equals 6.4 kWh. If your appliance list totals 2.7 kWh, the smaller figure may reflect staggered runtimes. Use the larger scenario when outages are unpredictable. The calculator compares both methods and keeps inverter sizing tied to peak running watts.
AC power needs conversion. With 92% inverter efficiency and 95% round‑trip battery efficiency, only 0.874 of stored energy reaches loads. Add a 95% temperature factor and delivered fraction becomes 0.830. That means 6.4 kWh of load energy requires about 7.71 kWh usable energy before reserve. These multipliers explain why “usable” is not the same as “nameplate.”
Depth of discharge sets how much of the battery you routinely use. At 80% DoD, the 7.71 kWh usable target becomes 9.64 kWh nominal, before adding any reserve margin. Lead‑acid often uses 50% DoD, which would double the nominal requirement for the same delivered energy. Chemistry selection therefore changes capacity even when loads stay constant.
A 15% reserve supports aging, forecasting error, and partial charging. Applying it to 7.71 kWh yields 8.87 kWh usable, then 11.09 kWh nominal at 80% DoD. For modular systems, a 5 kWh unit would require 3 modules. The reserve margin is also a practical way to reduce deep cycling and extend service life.
Many installers think in amp‑hours for DC buses. The conversion is straightforward: Ah = (kWh × 1000) ÷ V. An 11.09 kWh recommendation at 48 V is roughly 231 Ah. At 24 V, it doubles to about 462 Ah. Voltage does not change energy, but it changes current, cable sizing, and inverter selection. Higher voltage also lowers resistive losses, which can reduce required copper by one or two wire sizes at the same power. When planning surge loads, keep peak watts and surge multiplier aligned with motor starts and compressor cycles in your home.
Nominal capacity is the battery’s nameplate energy. Usable energy is lower because of depth‑of‑discharge limits, inverter losses, battery losses, temperature derating, and your reserve margin.
Use the appliance list when loads run for different durations, because it sums watt‑hours directly. Use total watts and hours when your backup load is relatively steady across the whole outage.
Many LiFePO₄ systems target 70–90% DoD, while lead‑acid is often limited to about 50% for longevity. If unsure, start conservative and refine using measured runtime data.
Stored DC energy must pass through the inverter and battery chemistry losses. Small percentage losses compound, so ignoring them can understate required capacity and shorten real‑world runtime.
Set continuous watts at or above your expected peak running load. For motors and compressors, size surge using the multiplier so startup currents don’t trip protection. Always verify with equipment specifications.
Solar can extend runtime if it can recharge during the outage. However, night hours and cloudy conditions still require stored energy. Size the battery for your worst realistic window, then treat solar as a runtime bonus.
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.