Guides · Choosing a generator
How to Work Out the Generator Size You Need
Add up everything that must run at the same time, allow for the surge motors draw as they start, and convert the total from kilowatts to kVA with some headroom. For most homes the first decision is essential circuits or the whole house, because a geyser, stove or pool pump can double the answer. Too small and the set trips; far too big and a diesel runs too lightly.
Technical guide · Last reviewed 16 October 2026
kW, kVA and power factor
Appliances are rated in watts (W) or kilowatts (kW): the real power they use. Generators are usually rated in kilovolt-amperes (kVA): the output voltage multiplied by the most current the alternator can carry, whether or not all of that current does work. The two differ by the power factor, which reflects how much of the current does useful work.
Diesel sets are normally rated at a power factor of 0.8, so a 100 kVA set delivers about 80 kW: the kW figure is what the engine can supply, the kVA figure what the alternator can carry. To convert your load, divide the kW by 0.8: a 4 kW load needs at least 5 kVA. Kettles and geyser elements have a power factor near 1, so they use the engine’s kilowatts to the full; motors draw extra current that does no work, so a building full of them can pull the real figure below 0.8 and need more kVA than the sum suggests. Small inverter and petrol units are often rated in watts, so check whether the figure is continuous or peak.
Essential circuits or the whole house?
Most home generators in Gauteng exist for load shedding and municipal outages. An essential-circuits installation puts lights, the fridge and TV plugs, Wi-Fi, the alarm, the gate motor and perhaps the borehole pump on a separate board fed through the changeover, while the geyser, stove, oven and pool pump wait for the mains. A whole-house installation must cover whichever heavy loads could switch on together.
For outages of a few hours, essential circuits usually win: the geyser stays hot, cooking fits around the schedule, and a smaller set runs closer to its rating, which suits a diesel. Whole-house backup suits long outages and boards that are hard to split, but needs a much larger set or load management. Splitting a board and wiring a changeover is work for a qualified electrician or technician; see generator installation.
Method
Five steps to size a generator
- 01
Choose the essential loads
Decide what must run during an outage. Leaving out the geyser and oven can halve the answer.
- 02
Find each running load
Read the rating plate or manual, in watts or kW, and add up everything that could run at once.
- 03
Allow for starting surges
Add the extra current the largest motor draws as it starts.
- 04
Convert to kVA
Divide the total kW by 0.8 for a diesel set, or compare kW directly with an inverter unit’s continuous rating.
- 05
Leave headroom
Headroom covers future loads, hot days, altitude and loads starting together.
Example running loads for a home
| Appliance | Typical running power | Starting surge? |
|---|---|---|
| LED lights (10 bulbs) | Typical running powerAbout 100 W | Starting surge?No |
| Fridge-freezer | Typical running power150–300 W | Starting surge?Yes, several times running power |
| TV and decoder | Typical running power100–200 W | Starting surge?No |
| Wi-Fi router and laptop | Typical running powerAbout 80 W | Starting surge?No |
| Alarm system and gate motor | Typical running powerTens of watts for battery-backed chargers; a few hundred watts for a mains-driven motor | Starting surge?Mains-driven gate motor: yes |
| Borehole pump | Typical running power750–1,500 W | Starting surge?Yes, large |
| Pool pump | Typical running power550–1,100 W | Starting surge?Yes |
| Split air conditioner | Typical running power800–1,500 W | Starting surge?Yes, unless an inverter model |
| Kettle | Typical running power2,000–2,200 W | Starting surge?No |
| Geyser element | Typical running power2,000–4,000 W | Starting surge?No |
A worked example
Suppose the essentials are lights (100 W), a fridge-freezer (300 W), TV and decoder (200 W), Wi-Fi and a laptop (80 W), the alarm and gate motor (allow 300 W) and a borehole pump (1,100 W). The running total is about 2,080 W, or 2.1 kW.
A common rule of thumb allows about three times a pump’s running power while it starts. That adds about 2.2 kW while everything else runs: a brief peak of about 4.3 kW. The running load converts to 2.1 ÷ 0.8 ≈ 2.6 kVA; the peak to 4.3 ÷ 0.8 ≈ 5.4 kVA. A set of around 6 kVA carries this comfortably; a 3 kVA unit would run the house but would likely trip when the pump starts. Starting the pump first, or fitting a soft starter, lowers the peak, which can be cheaper than a bigger generator.
Motor starting: why the surge often decides the size
Motors started direct-on-line, as many borehole pumps, pool pumps and compressors are, commonly draw three to seven times their running current for the moment it takes to reach speed. That current has a low power factor, so the alternator sees a large kVA demand even though few kilowatts are used, and the rule of thumb in the example above is a minimum, not a ceiling.
The voltage dips until the AVR raises the excitation, and the engine slows until the governor adds fuel. A well-sized set rides through with a flicker; an undersized one dips until contactors drop out, the motor stalls or the set trips. Several motors starting together, as when the changeover moves a house of fridges and pumps onto the set, is a common cause of trips as an outage begins. Generator and alternator data sheets often quote motor-starting capability separately; for large pumps, that figure should decide the set.
Reading the nameplate
Standby and prime ratings
Many diesel sets carry two figures, like the 250 kVA / 275 kVA on this canopy. The lower is usually the prime rating, for unlimited hours at a varying load; the higher is usually the standby rating, for limited hours a year during mains failures.
A set that runs several hours every day through load shedding is doing prime-power work, so size the everyday load against the prime figure. The exact definitions are in the manufacturer’s data sheet.
Getting it right
Things that change the answer
Altitude and heat
Engines lose power in thin, hot air. Johannesburg sits at about 1,750 m and Pretoria at about 1,340 m, so check the derated output in the manufacturer’s data for your altitude and for a canopy in summer sun.
Single or three phase: Single vs three phase
A three-phase set offers about a third of its rating on each phase, so one overloaded phase can trip it.
Sensitive electronics: Voltage and frequency basics
Computers and solar inverters need stable voltage and frequency from the set.
Oversizing has a cost: Wet stacking
A diesel at a small fraction of its rating never gets properly hot, and unburnt fuel and soot collect in the exhaust: wet stacking.
Future loads
Planning air conditioning, a second fridge or a workshop? Include them now or leave room.
Businesses and sites: Commercial generator services
For offices, shops and plants, readings taken under real working load beat estimates from rating plates.
What to send us for a sizing check
Want a technician to check your sizing, or to find out why a set trips when loads start? Send:
- A list of what must run during an outage
- Photos of the rating plates on pumps (or a borehole pump’s control box), compressors and air conditioners
- Whether the supply is single-phase or three-phase
- The make, model and kVA of your generator, if you have one
Photographing plates is safe; leave opening the distribution board to a technician. Contact us to arrange a visit.
FAQ
Frequently asked questions
What size generator do I need to run my whole house?
It depends on what “whole house” includes. Without the geyser, stove and oven, many homes manage on a set in the 5–10 kVA range; with them, the size rises sharply. List your loads as above, or ask us to assess them.
Is a bigger generator always the safer choice?
No. A diesel that normally runs very lightly suffers from wet stacking. Common practice is to size the set so the everyday load sits somewhere around half to three-quarters of its prime rating, with the largest starting surge still covered.
Will a generator charge my solar inverter batteries?
Usually, if it covers the inverter’s charging current plus the house load and its output is stable enough for the inverter to accept. See voltage and frequency basics.
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