Generator size calculator

1. The running load is the total of running watts × quantity for everything that runs at the same time.
2. The largest starting surge is the biggest extra draw of any single appliance when it starts. Motors, pumps and compressors briefly draw more than their running watts. The calculator assumes one motor starts at a time while everything else is already on.
3. The peak is the running load plus that largest surge.
4. kVA = kW ÷ power factor. Generators are usually rated at a power factor of 0.8.
5. The estimate is whichever is larger: the running kVA plus your headroom, or the peak kVA. The indicative size is the first two common ratings at or above that figure.

Small home backup. A fridge (150 W running, starting at about three times that), ten 10 W LED lights, a 100 W television, a 15 W router and a 60 W laptop. Running load: 425 W. Largest surge: the fridge, 300 W extra. Peak: 725 W, or 0.91 kVA at a power factor of 0.8. The running load plus 25% headroom is only 0.66 kVA, so the peak decides it. Indicative size: 1–2 kVA.

House with a borehole pump. A 750 W pump (three times surge), 200 W of lights, the same fridge and a 100 W television. Running load: 1,200 W. Largest surge: the pump, 1,500 W extra. Peak: 2,700 W, or 3.38 kVA. Indicative size: 3.5–5 kVA. The pump start, not the running load, sets the size.

These figures use typical appliance values. Your own rating plates may differ.

• Preset appliance figures are typical examples, not the values for your appliances. Read the rating plate.
• Starting current varies a lot. Soft starters and variable-speed drives reduce it; some older motors draw far more than three times their running current.
• Engine output falls with altitude and heat. Johannesburg sits at roughly 1,750 m and Pretoria at roughly 1,350 m above sea level. Manufacturers publish derating figures, which this calculator does not apply.
• Three-phase sets need the load balanced across phases, and each phase supplies only about a third of the rating. Phase balance and motor starting need a professional assessment.
• Inverter generators and conventional sets handle surges differently. Check the surge rating in the manual.
• The common ratings used for the range are 1 kVA to 2000 kVA. They are not a product list.

A generator that feeds a building must be connected through a changeover switch, so it can never back-feed the mains. That connection is work for a registered electrician, who can issue a Certificate of Compliance for the installation. Never connect a generator to a plug socket. Final sizing needs the actual loads, the manufacturer specifications and a qualified assessment.