Calculating your generator size needed is straightforward once you know the Generator Size formula and what each input means. This guide explains the method in plain language, walks through a manual calculation, and gives worked examples you can follow — then you can do it instantly with the Generator Size Calculator.
What is Generator Size?
The Generator Size calculation tells you your generator size needed from a few simple inputs. The figure you are solving for here is the generator size needed.
The Generator Size formula
The core formula is:
Generator size needed = Total running watts + Additional starting (surge) wattsHere is what each input means:
- Total running watts — a value measured in W. Example: 3,000 W.
- Additional starting (surge) watts — a value measured in W. Example: 1,500 W.
How to calculate it step by step
- Write down the total running watts (for example, 3,000 W).
- Write down the additional starting (surge) watts (for example, 1,500 W).
- Apply the formula above to get your generator size needed.
- Double-check the result with the Generator Size Calculator.
Worked examples
Example 1
| Input / Output | Value |
|---|---|
| Total running watts | 3,000 W |
| Additional starting (surge) watts | 1,500 W |
| Generator size needed | 4,500 |
| Approx. kVA (PF 0.8) | 5.63 |
With total running watts of 3,000 W and additional starting (surge) watts of 1,500 W, the generator size needed works out to 4,500.
Example 2
With total running watts of 6,000 W and additional starting (surge) watts of 1,500 W, the generator size needed works out to 7,500.
| Result | Value |
|---|---|
| Generator size needed | 7,500 |
| Approx. kVA (PF 0.8) | 9.38 |
Example 3
With total running watts of 1,500 W and additional starting (surge) watts of 1,500 W, the generator size needed works out to 3,000.
| Result | Value |
|---|---|
| Generator size needed | 3,000 |
| Approx. kVA (PF 0.8) | 3.75 |
Tips for an accurate result
- Keep your units consistent — mixing, say, months with years or grams with kilograms is the most common source of error.
- Round only at the very end. Rounding inputs early can shift the final answer noticeably.
- Re-run the numbers whenever an input changes, rather than estimating from an old result.
Prefer not to do the maths by hand? — the Generator Size Calculator does it instantly, for free, with the formula and a worked example built in.
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