Electrical and solar
Solar and backup power calculator
This calculator sizes a solar and backup power system from what you actually run. List your appliances, how many of each, their wattage and how many hours a day they run, and it works out your daily energy use, the inverter size for the load you run at once, the battery capacity to carry you through the evening or a load-shedding block, and the solar panels to recharge it all.
What you need
This calculator works out the figures in your browser, which needs JavaScript switched on. The method below shows how to work it out by hand.
Estimates for planning and budgeting. Real quantities depend on the site and the workmanship, so check with your builder before you order.
Harare gets about 5.5 peak sun hours a day on average across the year, which is the figure the panel count is based on; winter is lower and summer higher. Lithium (LiFePO4) batteries can be used to about 90% of their capacity; lead-acid batteries only to about 50% if they are to last, so they need almost twice the nominal size.
Leave electric geysers, stoves and kettles off a solar system unless it is sized for them: they use more power than everything else combined. A solar geyser and gas for cooking cut the system size dramatically.
How it is worked out
- Daily energy = the sum of quantity × watts × hours for every appliance, in kWh.
- Inverter = the total of everything that could run at once × 1.25 for start-up surges, rounded up to a standard size (3, 5, 8, 10, 12 or 16 kW).
- Battery = daily energy × night share ÷ usable fraction, in kWh, and in 5.12 kWh lithium modules.
- Solar array = daily energy ÷ (peak sun hours × efficiency), in kW, and in panels of the chosen wattage.
What it assumes
- Fridge and freezer hours are compressor running time, not hours switched on.
- Motors and compressors can surge to three times their running power; large pumps may need a bigger inverter.
Common questions
What size inverter do I need for a house in Zimbabwe?
For lights, fridge, TV, WiFi and occasional microwave or iron, a 5 kW inverter is common. Add a borehole pump and air conditioning, and 8 kW or more.
How many solar panels to run a fridge?
A typical fridge uses about 1 to 1.5 kWh a day. In Harare that needs about 300 to 400 W of panels on its own, so one 550 W panel with some margin.
Lithium or lead-acid batteries?
Lithium costs more up front but can be used more deeply and lasts many more cycles, so it is usually cheaper over the life of the system.
Want exact quantities for your plan?
A calculator works one wall or one slab at a time. A bill of quantities works from your actual drawings and counts everything, which is what builders price against and what stops you buying twice.