What Size Generator Do You Need? How to Calculate KVA Step by Step
Add up your running loads in kW, divide by 0.8 to get KVA, then allow for motor starting current and a 20–25% safety margin. Includes a worked example for a villa.
To choose the right generator size: add up the power of every appliance that will run at the same time in kilowatts (kW), divide the total by 0.8 to convert it to kilovolt-amperes (KVA), then add an allowance for the starting current of air conditioners and pumps, plus a safety margin of 20% to 25%. The result is the rating to look for. This method saves you from a generator that is too small and trips at peak load, or one that is too big and costs you more to buy and to fuel for no benefit.
What is the difference between KVA and kW?
- kW (kilowatt): the real power your appliances consume and turn into work — cooling, lighting or motion.
- KVA (kilovolt-ampere): the apparent power the generator must supply, which includes the real power plus the share that motors and coils need.
- Power factor: the ratio between the two. Generators are usually rated at a power factor of 0.8, so kW = KVA × 0.8 and KVA = kW ÷ 0.8.
Example: a 100 KVA generator delivers about 80 kW of real power.
Step 1: List your loads
Write down every appliance that will run on the generator, its power from the rating plate (in watts or kilowatts), how many you have, and whether it runs continuously or cycles on and off. Focus on the big loads:
- Split and central air conditioning — the largest summer load in the UAE.
- Water and pool pumps.
- Water heaters and electric ovens.
- Lifts, compressors and industrial equipment.
- Lighting, sockets and small appliances.
If the rating plate only shows amps, the approximate power of a single-phase appliance is voltage × current: 230 V × amps ÷ 1000 = kW, roughly.
Step 2: Allow for starting current
Electric motors — AC compressors, pumps and lifts — draw a much higher current when they start than when they run, sometimes several times higher for a moment. If several air conditioners start together when power comes back, the voltage can dip or the generator can trip. The fixes:
- Size for your largest motor with extra capacity for its starting current.
- Start the big loads in sequence rather than all at once — some control panels and ATS switches do this for you.
- Use soft starters or inverter air conditioners where possible.
Step 3: Safety margin and duty type
Do not run a generator at 100% of its rating all the time. The usual rule is a normal load of 70% to 80% of the generator's capacity, so add a margin of 20% to 25% on top of your calculation. Check the rating type as well:
- Standby: for running during power cuts only, for limited hours.
- Prime: for continuous daily running, such as sites not connected to the grid. The prime rating of a given generator is lower than its standby rating.
Send us your appliance list and we will size your generator for free
Worked example: a mid-size villa
Assume a villa with the following loads running together in summer (approximate values for illustration only — base your own calculation on your appliances' rating plates):
| Load | Qty | Approx. power each | Total |
|---|---|---|---|
| Split AC | 8 | 2 kW | 16 kW |
| Water heaters | 3 | 2 kW | 6 kW |
| Kitchen (oven and appliances) | — | — | 5 kW |
| Lighting and sockets | — | — | 4 kW |
| Water pump | 1 | 1.5 kW | 1.5 kW |
| Total | 32.5 kW |
- Conversion: 32.5 ÷ 0.8 ≈ 41 KVA.
- 25% safety margin: 41 × 1.25 ≈ 51 KVA.
- With AC starting current: a generator of around 60 KVA fits best, or 50 KVA if the air conditioners are set to start in sequence.
A residential building (lifts, pumps and central AC) or a factory (motors and compressors) is sized the same way, but there it is better to have a technician measure the actual loads on site with a meter.
Common sizing mistakes
- Going by the size of the house instead of listing the actual loads.
- Forgetting the starting current of air conditioners and pumps.
- Buying an oversized generator "just in case" and then running it at a very low load, which harms the engine and burns more fuel.
- Mixing up KVA and kW when comparing quotes.
- Not planning for future additions, such as an annex, a pool or a new production line.
The bottom line
The formula is simple: total load in kW ÷ 0.8, then an allowance for starting current, then 20–25% for safety. At Halab Al Shahba in Sharjah we have generators up to 100 KVA for villas and shops, such as the Perkins 100 KVA generator with a soundproof canopy and the FPT 75 KVA generator; generators from 101 to 400 KVA for buildings and facilities, such as the brand-new Cummins 250 KVA generator; and larger units for projects, such as the 2024 Cummins 625 KVA generator — with delivery and installation across the UAE.
Send us your loads and get a recommendation for the right size
