Generator Sizing Guide: What kVA Do You Need?

A generator that is undersized trips out under load or struggles to start motors. One that is wildly oversized burns fuel inefficiently and costs more than it needs to. Getting the size right is a short calculation built on three things: what you are running, what it takes to start, and a sensible safety margin.

Step 1: add up the running load

List every device the generator must power and note its running power. Convert the total to kVA using the power factor — generators are rated in kVA because that is the apparent power they actually have to deliver, not just the useful kW. For a typical mixed site load the power factor sits around 0.8, so kVA is a bit higher than the kW total.

This running total is the floor, not the answer. Most jobs need more because of how equipment starts.

Step 2: allow for starting surges

Motors — pumps, compressors, lifts — draw far more current at the moment they start than when they run, sometimes several times their running figure. If everything could start at once, the generator has to cope with that peak without stalling.

Identify the largest motor and add its starting surge on top of the running total. If multiple large motors can start together, account for that. This step is where motor-heavy sites end up needing a noticeably bigger machine than the steady-state numbers imply.

Step 3: add headroom

Finish with a margin — commonly around 20-25% — for safety, voltage stability, and any load you might add later. That cushion keeps the generator off its limit, where efficiency and reliability both suffer.

The sizing in short

1. Sum the running power of all loads.
2. Convert to kVA using the power factor.
3. Add the largest motor's starting surge.
4. Add ~20-25% headroom.
5. Choose the next standard generator rating above that figure.

Avoid oversizing too

Bigger is not automatically safer. A large generator running lightly loaded wastes fuel and risks wet stacking, where unburnt fuel builds up in the engine. Aim to match the rating to real demand plus reasonable headroom — not to buy the biggest unit available.

Rent the size the job needs

Power demand changes from site to site and season to season, which makes generators a strong rental fit — you bring in the exact kVA each job needs rather than owning one compromise unit. For the cost trade-off between renting, leasing and owning, see rent, lease or finance equipment, and model your own numbers in the rent vs lease vs buy calculator.