About this calculator
Size an inverter and lithium battery bank to run the essentials through load shedding: the inverter’s continuous and surge ratings, and how many battery modules you need for the hours you want.
How it’s calculated
Add up the watts of everything you want running at the same time: lights, router, TV, fridge, computer. The inverter’s continuous rating must at least match that total, and its peak (surge) rating must cover the moment a fridge or pump motor starts, which can be several times its running power.
Energy needed = watts × hours. Each battery module gives its rated capacity times its usable depth of discharge: Pylontech’s US3000C, for example, holds 3.55 kWh and may be discharged to 95%, about 3.37 kWh. Modules = energy ÷ usable energy per module, rounded up.
To size solar panels that recharge the batteries, pick the nearest city. Panel output comes from the European Commission’s PVGIS solar database (satellite data for 2005 to 2023), for panels facing north at 26° with 14% losses in cables and the inverter. One kilowatt-peak (kWp) of panels gives about 4.0 kWh a day in Durban and Pietermaritzburg, 4.7 to 5.0 kWh on the Highveld and up to 5.2 kWh in Upington. Panels needed = the outage energy ÷ the city’s output per kWp in its worst month, so the batteries still fill up in winter. Cape Town’s worst month (June) gives only 2.9 kWh per kWp a day.
Worked example
With these inputs:
- Load during an outage (watts, everything on at once)
- 1000
- Hours of backup
- 4
- Start-up surge of the biggest motor (watts, optional)
- 0
- Battery module size (kWh)
- 3.55
- Usable depth of discharge
- 95%
- Solar panels to recharge the batteries (nearest city)
- No solar panels
the result is:
- Battery modules
- 2 × 3.55 kWh
- Energy for the outage
- 4 kWh
- Usable per module
- 3.37 kWh at 95% depth of discharge
- Backup time with these modules
- 6.7 hours
- Inverter surge rating at least
- 1,000 W
Tips from the trade
- Leave the geyser, kettle, oven and heaters off the backup circuit; they would need a far bigger system.
- Put the backup on its own distribution board circuit so only the essentials run from the inverter.
- Check each battery’s depth of discharge on its datasheet; it sets how much of the rated capacity you can actually use.
Common questions
What size battery do I need for 4 hours of load shedding?
Multiply the watts you run by 4. For 1,000 W that is 4 kWh, so two 3.55 kWh modules at 95% depth of discharge (about 6.7 kWh usable).
How many solar panels do I need to recharge the batteries?
Divide the energy you use in an outage by your city’s worst-month output per kWp. In Johannesburg 4 kWh ÷ 4.30 is about 0.93 kWp, or 930 W of panels; in Cape Town 4 kWh ÷ 2.90 is about 1.38 kWp because of short winter days.
Results are estimates for planning and pricing. Check quantities against your drawings, the manufacturer’s data and SANS 10400 (the National Building Regulations) and your local municipality before ordering.