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Generator Size Calculator

Running watts, starting surge, kW/kVA and amps — the load tally worked through step by step
Size a generator for a fridge (700 W), furnace blower (800 W), sump pump (800 W run / 2400 W start), lights (600 W) and a microwave (1000 W)
How many amps is 5500 W at 240 V single phase?
What current does a 100 kVA three-phase generator deliver at 480 V, and what is its kW rating at 0.8 power factor?
Size a standby generator for a house with 6000 W of general load and a 3-ton air conditioner

Running Watts, Starting Watts, and the Peak

Sizing a generator is an arithmetic problem with one twist: motors draw far more current for the first second or two than they do while running.

  1. List every load you intend to run at once, with its running watts and, for anything with a motor or compressor, its starting (surge) watts.
  2. Add the running watts to get the continuous demand PrunP_{run}.
  3. Find the single largest surge increment, max⁡(Pstart−Prun)\max(P_{start} - P_{run}) over all motor loads — motors are not assumed to start together.
  4. Peak demand =Prun+= P_{run} + that increment.
  5. Add headroom, typically 20–25%, so the set is not run at its limit; the continuous rating should exceed 1.25×Prun1.25 \times P_{run}.
  6. Round up to a standard size — generators are sold in fixed increments such as 7.5, 10, 14, 22 kW.

Starting surges are large: a fridge compressor draws roughly 2–3× its running watts, a well pump or air conditioner more. A soft starter cuts an air conditioner's inrush substantially and can change the size of set required.

Watts, kVA, Amps and the Home Standby Case

The electrical conversions

Single phase: I=PV,PkW=kVA×PF\text{Single phase: } I = \frac{P}{V}, \qquad P_{kW} = kVA \times PF

Three phase: I=kVA×10003×VLL\text{Three phase: } I = \frac{kVA \times 1000}{\sqrt{3} \times V_{LL}}

Generators are rated in kVA (apparent power) while loads are quoted in kW (real power). At the usual PF=0.8PF = 0.8, a 100 kVA set is an 80 kW set — sizing a generator against its kVA number when your tally is in kW overstates capacity by 25%.

Portable versus whole-house

A portable set is sized to a chosen circuit list: fridge, furnace blower, sump pump, lighting, and one large appliance at a time. Peaks of 5–7.5 kW are typical.

A whole-house standby must instead cover the calculated service load, or a managed subset of it. Load management (a transfer switch that sheds the air conditioner while the range is on) is often cheaper than the next size up.

Where the real numbers come from. A tally like the one here teaches the method; the binding calculation is the NEC Article 220 load calculation, with NEC Article 702 governing optional standby systems, plus the manufacturer's motor-starting kVA tables, and it must be performed by a licensed electrician for a real installation. Nothing on this page is an approval to install or connect a generator.

Common Mistakes to Avoid

  • Summing every appliance's starting watts. Motors do not start together; add only the largest single starting increment on top of the running total.
  • Sizing to the peak surge alone. The surge is momentary; the continuous rating must cover the running load with headroom, and both checks apply.
  • Confusing kVA with kW. Multiply kVA by the power factor to get kW. A 22 kVA set at PF=0.8PF = 0.8 delivers 17.6 kW.
  • Confusing peak and running ratings. "7500 W" on a portable is often the surge figure; its running rating may be 6,000 W.
  • Forgetting the well pump, sump pump or heat pump. The loads that matter most in an outage are exactly the ones with the largest inrush.
  • Ignoring derating. Altitude and high ambient temperature reduce engine output; use the manufacturer's derating curve.
  • Skipping the conductor, breaker and transfer-switch sizing. The generator is one part of an installation the NEC governs end to end.

Examples

Step 1: Running total: 700+800+800+600+1,000=3,900 W700 + 800 + 800 + 600 + 1{,}000 = 3{,}900\ \text{W}
Step 2: Largest starting increment: the sump pump, 2,400−800=1,600 W2{,}400 - 800 = 1{,}600\ \text{W}
Step 3: Peak demand: 3,900+1,600=5,500 W3{,}900 + 1{,}600 = 5{,}500\ \text{W}
Step 4: Continuous rating with 25% headroom: 1.25×3,900=4,875 W1.25 \times 3{,}900 = 4{,}875\ \text{W} — less than the peak, so the surge governs
Step 5: Apply 20% headroom to the peak: 1.20×5,500=6,600 W1.20 \times 5{,}500 = 6{,}600\ \text{W}
Step 6: Round up to the next standard size: a 7,000–7,500 W running-rated set
Answer: Prun=3.9P_{run} = 3.9 kW, peak =5.5= 5.5 kW, target ≈6.6\approx 6.6 kW — select a set with a running rating of about 7–7.5 kW, confirmed against the manufacturer's data

Step 1: Single phase: I=P/V=5,500 W÷240 V=22.9 AI = P/V = 5{,}500\ \text{W} \div 240\ \text{V} = 22.9\ \text{A}
Step 2: Three phase: I=(kVA×1000)/(3×VLL)I = (kVA \times 1000)/(\sqrt{3} \times V_{LL})
Step 3: 3×480=1.732×480=831.4\sqrt{3} \times 480 = 1.732 \times 480 = 831.4
Step 4: I=100,000÷831.4=120.3 AI = 100{,}000 \div 831.4 = 120.3\ \text{A}
Step 5: Real power: PkW=kVA×PF=100×0.8=80 kWP_{kW} = kVA \times PF = 100 \times 0.8 = 80\ \text{kW}
Answer: 22.922.9 A at 240 V single phase; the 100 kVA set delivers 120.3120.3 A at 480 V three phase and 8080 kW of real power

Step 1: Running total: 6,000+3,500=9,500 W6{,}000 + 3{,}500 = 9{,}500\ \text{W}
Step 2: Continuous rating with 25% headroom: 1.25×9,500=11,875 W≈12 kW1.25 \times 9{,}500 = 11{,}875\ \text{W} \approx 12\ \text{kW}
Step 3: Air-conditioner starting increment: 12,000−3,500=8,500 W12{,}000 - 3{,}500 = 8{,}500\ \text{W}
Step 4: Momentary peak: 9,500+8,500=18,000 W9{,}500 + 8{,}500 = 18{,}000\ \text{W}
Step 5: The 12 kW continuous requirement is met by a 14 kW set, but its published motor-starting capability must cover the 18 kW momentary demand — which typically pushes the selection to a 20–22 kW unit, or keeps the 14 kW set with a soft starter or load-shedding transfer switch
Step 6: Check the kVA rating too: at PF=0.8PF = 0.8, a 22 kVA nameplate is only 17.6 kW
Answer: Prun=9.5P_{run} = 9.5 kW needs â‰Ĩ12\ge 12 kW continuous, and the 18 kW momentary start typically drives selection to a 20–22 kW standby set — the final size must come from an NEC Article 220 load calculation and the manufacturer's motor-starting tables

Frequently Asked Questions

Add the running watts of everything you will run at once, then add the largest single starting increment (starting watts minus running watts) from your motor loads. That total is the peak demand; add 20–25% headroom and round up to the next standard size. Confirm the result with an NEC Article 220 load calculation before any installation.

It depends on the load list, not the square footage. A portable covering a fridge, furnace blower, sump pump, lights and one appliance typically lands around 5–7.5 kW. A whole-house standby covering an air conditioner usually starts near 14 kW and rises to 22 kW or more, driven mainly by the compressor's starting surge. A licensed electrician's load calculation is what determines the actual size.

For a three-phase set, I = (kVA × 1000) / (√3 × line-to-line volts). A 100 kVA generator at 480 V gives 100,000 / (1.732 × 480) = 120.3 A. For single phase, drop the √3: I = watts / volts, so 5,500 W at 240 V is 22.9 A.

Running watts is the continuous draw; starting watts is the momentary surge a motor pulls as it comes up to speed, often two to three times higher and more for compressors. A generator must cover the running total continuously and the largest single surge momentarily — the manufacturer's motor-starting kVA table is the authority on the second.

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