Make an outage load list
Start with what must keep running during the outage. Whole-building average consumption includes loads you may be willing to switch off. For each essential item, record running watts, likely hours and whether a motor or compressor has a starting surge.
| Illustrative essential load | Running power | Outage operation | Energy |
|---|---|---|---|
| Six LED lamps | 60 W total | 4 h | 0.24 kWh |
| Three fans | 150 W total | 4 h | 0.60 kWh |
| Router | 15 W | 4 h | 0.06 kWh |
| Refrigerator | 120 W while on | 2 h equivalent on-time | 0.24 kWh |
| Total | 345 W if all run | 4 h outage | 1.14 kWh |
The refrigerator duty cycle is an assumption. Hot weather or a frequently opened door changes it. Its start current is not represented by its 120 W running input. Record appliance data and measurements before selecting an inverter.
Move from delivered AC energy to battery nameplate
Assume 90% battery-to-load discharge efficiency, an 80% usable SOC window, 80% capacity retention at the design end of life, and 10% extra energy margin. For this simplified example, auxiliaries are excluded and should be added if material.
1.14 × 1.10 ÷ (0.80 × 0.80 × 0.90) = 2.177 kWh nominal. Rounding up to available equipment is necessary, but this number alone does not qualify a product. Check allowed parallel operation, BMS communication and inverter compatibility.
Do not divide by efficiency again if the supplier’s quoted “usable energy” is already AC energy at the same operating conditions. Always state where energy is measured: cell, battery DC terminals or AC load.
Size the inverter separately
A 2.2 kWh battery could deliver a low load for hours, but may be unable to start a pump. The inverter must support simultaneous running load, apparent power, motor surge and the required overload duration. The battery and BMS must also supply the corresponding DC current.
For example, 1 kW AC at 90% efficiency and a 48 V operating voltage draws about 1,000 ÷ (0.90 × 48) = 23.1 A. Current rises as voltage falls. Do not use nominal voltage alone to select wiring or protection, and do not infer a continuous current rating from battery energy.
Check the starting SOC
The energy calculation assumes the full intended SOC window is available when the outage begins. A battery with a 20% minimum SOC that starts an outage at 50% has only 30 percentage points of nameplate capacity available before reaching the floor.
For a 5.12 kWh battery, 80% retained capacity and 90% discharge efficiency, that 30% window yields 5.12 × 0.30 × 0.80 × 0.90 = 1.106 kWh AC. That is less than the 1.14 kWh example load, even before auxiliaries. A reserve policy can matter more than buying a slightly larger nominal pack.
Take these checks to your installer
- Confirm that the inverter supports backup operation and the actual battery model.
- List critical circuits and ensure safe changeover and isolation.
- Obtain continuous and peak power/current ratings across the operating temperature range.
- Confirm usable SOC limits, ageing assumptions and what the warranty promises.
- Check the time and source available to recharge between outages.
Use the backup notebook for a simple energy comparison. The BESS studio supports a more detailed series/parallel topology, power margins, auxiliary fraction and voltage checks. Its word “module” means a full-voltage string, which may differ from a commercial product’s terminology.
Sources and scope
- U.S. DOE: Solar energy and storage basics ↗
Distinction between power and stored energy, and losses. Load list, margins and numerical examples are original teaching scenarios.
Reference links provide technical context; the worked examples and interpretation are specific to this guide. No external source certifies these tools.