Generator sizing is where a lot of money gets wasted in both directions. Here is the method we use.
Start with what must run, not everything
List the loads that genuinely need to operate during an outage. In most homes this is lighting, refrigeration, water pump, internet and a few sockets — not the entire house. Separating critical circuits onto a dedicated sub-board dramatically reduces the generator size required.
Account for starting current
Motors — pumps, compressors, air conditioning — draw several times their running current at startup. A 1 kW pump can demand 4 to 6 kW for a moment. The generator must handle the largest starting surge while already carrying the other running loads.
Do not oversize
A diesel generator running below roughly 40 percent of rated load for extended periods suffers wet-stacking: unburnt fuel accumulates in the exhaust, glazing cylinder bores and shortening engine life. A generator sized for a load it never sees is actively damaging itself.
Add a sensible margin, then stop
We typically size for the calculated peak plus around 20 percent headroom for future additions. Beyond that you are paying more upfront, burning more fuel, and running the set inefficiently for its entire life.
The calculation takes about an hour on site with a clamp meter. It is worth doing properly before anyone quotes you a number.
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