Step 1: List running loads
Every load the generator must carry simultaneously, in kW: lighting, HVAC, pumps, equipment. Nameplate amps × volts ÷ 1000 = kW per load (three-phase: × 1.732). Be honest about simultaneity — not everything runs at once.
Size it once, size it right — the method pros use, plus the interactive calculator.
Undersized generators stall on motor starting. Oversized ones wet-stack, waste fuel, and cost double. Sizing is the single highest-leverage decision in backup power — and it is arithmetic, not art. This guide gives you the method; the calculator below does the math.
kW, fuel, hours, nameplate photos — that’s the listing. Get today’s price →
Every load the generator must carry simultaneously, in kW: lighting, HVAC, pumps, equipment. Nameplate amps × volts ÷ 1000 = kW per load (three-phase: × 1.732). Be honest about simultaneity — not everything runs at once.
Motors draw 5–7x running current at startup (locked-rotor). The generator must survive the largest motor’s starting kVA on top of everything else already running. This step — not the running total — is what sizes most generators.
Add ~25% above the calculated need: future loads, voltage dip margin, and the fact that generators should not run at 100% continuously. Round up to the next standard size (20, 30, 45, 60, 80, 100, 125, 150, 200, 250… kW).
Enter your loads below. This is a planning estimate — final sizing for commercial work belongs to a licensed electrician or engineer.
Method: starting kVA of the largest motor (HP × 0.746 × surge ÷ power factor 0.8) added to running load, plus 25% headroom, divided by derate, rounded up to standard sizes.
kW vs kVA: kW = kVA × power factor (0.8 standard) — a “100 kVA” set is an 80 kW set. Listings mix them; always confirm which. Single-phase derate: many three-phase sets are reconfigured or derated for single-phase — check the nameplate, not the listing title. Do not size to the service: size to the loads you will actually back up (NEC 702 optional standby lets you pick). The 20–50 kW band is the most liquid used market — small commercial, farms, contractors — which is why it holds value best.
Voltage dip on motor starting — motors stall, contactors chatter, electronics reboot. Chronic undersizing kills both the generator and the loads.
Light loading causes wet stacking (unburned fuel in the exhaust), carbon buildup, and wasted fuel. Diesel sets want to run above ~30% load — see the load bank guide.
Both: kVA is the alternator’s current limit, kW is the engine’s power limit. At 0.8 power factor they bind together; at low power factor, kVA binds first.
Within its rating, yes — paralleling switchgear lets multiple sets share load. Buying one oversized set ‘for growth’ is usually worse than buying right-sized now.
Residential: the load-calculation method above plus the manufacturer’s sizing tool is standard practice. Commercial: get the engineer — code and liability require it.
Nameplate (make, model, kW, serial), hour meter, and overall condition. The nameplate is the entire specification.
Call or text 312-880-8174 or use the quote form. Tell us the kW, fuel, hours, and condition.
A real offer based on your photos and today’s market — not a calculator, not a guess.
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Size it right with the interactive calculator.
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Diesel or natural gas, running or parts — send photos for today’s price.