Generator & ATS Sizing Calculator
List what must run in an outage; get a generator size (kVA) that copes with motor starts and UPS loads, plus ATS rating and fuel per hour.
Loads the generator must carry
Only what must run during an outage, not the whole building.
Total running load: 8.6 kW
25% suits most offices. Go higher if more air-cons or freezers are planned.
Fuel
Not everything runs at once. 50–70% of the running total is common.
Results
Recommended generator (standard size)
20 kVA
≈ 16 kW
Running kVA (incl. UPS factor)
11.4kVA
Minimum incl. headroom
14.2kVA
kVA while the largest motor starts
18.0kVA
Size needed to handle that start
17.4kVA
Rated current per phase
29A
Minimum ATS rating
40A
Fuel use (approx.) Estimate
≈ 2.3 litres/hour
A full tank lasts: ≈ 43.0 hours
How we calculate
1. Running kVA = (ordinary load W ÷ 0.8) + (UPS load W ÷ 0.9 × UPS factor).
2. Minimum = running kVA × (1 + headroom), × 1.1 more for a prime rating.
3. Largest motor start: start kVA = W ÷ 0.8 × multiplier (direct start 6, soft starter 3, inverter 1.5). We assume the set can take about twice its rating for a few seconds with roughly a 20% voltage dip, so the size needed = (other loads kVA) + (start kVA ÷ 2).
4. Pick the next standard size above both 2 and 3. Current per phase = kVA × 1000 ÷ 230 (single-phase) or ÷ (1.732 × 400) (three-phase); the ATS is the next standard rating above it.
5. Fuel ≈ 0.3 litres per kWh at 50–75% load, and more per kWh at lighter loads. Real figures come from the maker's fuel table.
These results are estimates to help with planning and budgeting. Our engineers will verify everything on site before final design and equipment proposal.
The same rules of thumb our engineers use for a first design. Good enough for budgeting; the final numbers come from a site survey.
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Open calculatorSend us a floor plan or a few photos of the site
We’ll come back with a few questions, then a first design. The first consultation is free, in Lao, Thai or English, on WhatsApp or our Facebook page.