How it’s calculated, step by step
- What your appliances use during the outage (Wh) = watts × hours × % of time running × quantity, added up for all of them. It’s the same maths as the electricity usage calculator, but with the hours of the outage.
- What the battery has to deliver = the above ÷ the inverter’s efficiency. Converting the battery’s current into the kind your outlets use loses some energy: according to the US NREL lab, an inverter is over 90% efficient near its maximum and less at low loads; we use 85% to be on the safe side.
- The minimum battery = the above ÷ the share of the battery you’ll use. It’s best not to drain it completely: according to Australia’s national science agency (CSIRO), a lithium battery drained fully every time lasted 180 cycles, and drained halfway, 1,300.
- The inverter must handle everything you switch on at once (continuous power) and, on top of that, the start-up kick of the motor that draws the most (surge power). It’s the method used, for example, by the public utility EWEB in the United States to size a generator.
With what the calculator loads when you open it (fridge, 4 LED bulbs, a fan, 2 chargers and the TV), the appliances use 1,874 Wh. The battery has to deliver 1,874 ÷ 0.85 = 2,205 Wh, and if it’s lithium and you use 80% of it, you need at least 2,205 ÷ 0.8 = 2,756 Wh. The inverter must supply 426 W continuously and handle 1,026 W for an instant when the fridge starts.
Wh and Ah: how to read a battery’s capacity
Power stations state their capacity in Wh, which is what the calculator gives you. Many standalone batteries state it in Ah: to convert to Wh, multiply by its volts (a 12 V, 100 Ah battery is 1,200 Wh). If you’re not sure what each one means, our beginner’s guide explains it with examples.
Which battery type: lithium, LiFePO4, gel, AGM or lead-acid
| Type | Cycle life | Energy per kilo | What you should know |
|---|---|---|---|
| LiFePO4 lithium (LFP) | 3,000–6,000 | 90–130 Wh/kg | Cheap, abundant materials and very good life through charging and discharging. |
| Other lithium (NMC) | 3,000–6,000 | 100–205 Wh/kg | Lighter per Wh, but contains cobalt, which is expensive and scarce. |
| Flooded lead-acid (liquid) | 800–3,000 | 30–50 Wh/kg | Handles deep discharges, but needs topping up with distilled water and gives off gas: it needs ventilation. |
| Gel lead-acid | 800–3,000 | 30–50 Wh/kg | Almost maintenance-free and doesn’t spill acid, but needs a carefully controlled charger, and many don’t handle deep discharges well: check the maker sells it as deep-cycle. |
| AGM lead-acid | 800–3,000 | 30–50 Wh/kg | Can be deeply discharged without losing life and handles a wider temperature range, but costs more and also needs a controlled charger. |
| Car battery | Wears out fast if drained often | Built to deliver a kick of current to start an engine, not to run an inverter. | |
Cycle life and energy per kilo come from CSIRO’s table; the details of each lead-acid type come from PVEducation, the solar energy course from Arizona State University and UNSW. Why the calculator suggests using 80% of a lithium battery and 50% of a lead one: draining a battery less extends its life (CSIRO), and deep-cycle lead batteries can take discharges beyond 50% (PVEducation), so 50% is a cautious choice. You can change it.
What to look for in a power station
- Capacity in Wh: at least the “minimum battery” the calculator gives you.
- Inverter output in W (continuous): at least the “continuous power”.
- Surge or peak power: at least the “surge power”, if you’ll plug in a fridge or a pump.
- Battery chemistry: if it’s LiFePO4 (LFP), it lasts many more cycles.
Safety
- Flooded lead-acid batteries give off gas when charging: keep them somewhere ventilated.
- Keep tools and metal objects away from the terminals: a short circuit can destroy the battery.
- Connecting an inverter or a battery to your home’s wiring (rather than plugging an appliance into it) is a job for an electrician.
- A dead battery doesn’t go in the trash: take it to a recycling point.
Sources
- CSIRO (Australian Government), 2021. Batteries for Homes: rechargeable batteries for domestic energy storage.
- C. Honsberg and S. Bowden, PVEducation. Lead Acid Batteries.
- EWEB (Eugene Water & Electric Board). Generator Sizing Considerations.
- National Renewable Energy Laboratory (NREL), November 2000. Renewable Energy for Rural Schools (an inverter is over 90% efficient near its maximum and less at low loads; the 85% is our own cautious figure).
- Ente Nacional Regulador del Gas y la Electricidad (Argentina). Consumo básico de electrodomésticos (appliance wattages, in Spanish).