Solar and inverter sizing basics
6 min read · updated 9/16/2026
Most oversized solar systems are sold to people who never measured their load, and most undersized ones to people who guessed. Twenty minutes of arithmetic saves hundreds of dollars.
Start with your load, not the panels
List the appliances you want running during an outage and their wattage. Add them up — that peak figure sets the inverter size.
Multiply each appliance's wattage by the hours you need it, and add those together. That daily total, in watt-hours, sets the battery size.
Add roughly 30% headroom to both figures for losses and future additions.
Panels and batteries
Panels are sized to refill the battery during usable daylight — in most of Southern Africa that is around five productive hours a day.
Lithium batteries can be discharged much deeper than lead-acid, so a smaller nominal lithium bank often delivers more usable energy.
Geysers and electric stoves are usually cheaper to solve separately (solar geyser, gas hob) than to carry on the inverter.
What a good installation includes
Correctly rated cable and breakers, a proper earth, and a changeover that cannot backfeed the grid.
Labelled DB board, a written commissioning report and a warranty on both parts and workmanship.
Use an installer who will show the load calculation they based the quote on.