Calculator
Standby Generator Sizing Calculator
Build the list of circuits that have to stay alive during an outage and get the standby generator that covers them, total running watts, the peak when the biggest motor starts last, required kW and kVA at the set’s power factor, the next standard genset rating, and the transfer switch amps. Free, no signup.
Free
Loads on the generatorEverything that can run at the same time during an outage. Running watts is the steady draw; the surge multiplier is how many times that a motor pulls at the instant it starts.
| Load | Qty | RunningSteady-state watts for one unit. For a motor row this becomes the NEC table full-load amps at your voltage. | SurgeStarting watts ÷ running watts for one unit. 1× is resistive (heat, lights). Motor rows use the NEC 430.7(B) locked-rotor code letter instead. | Remove |
|---|---|---|---|---|
Sizing margin, power factor & load management
Recommended standby generator16 kW
11.7 kW peak starting · 6.99 kW running · air-cooled class
Healthy, 20%+ spare capacity
- Running: 6,990 W
- Starting surge: 4,712 W
- Spare capacity: 4,298 W
Total running load
6,990 W6.99 kW continuous across 8 loads
Peak starting load
11,702 WLargest motor started last: Central A/C, 3 ton adds 4,712 W
Required capacity (+20%)
14.04 kW14.04 kVA at PF 1
Set rating in kVA
16 kVA16 kW ÷ 1 power factor
Transfer switch
100 A66.7 A rated, next standard ATS size
Spare capacity
56.3%Headroom above the continuous running load
Every load at a glance
| Load | Running | Starting |
|---|---|---|
| Central A/C, 3 ton8,512 W starting (2.24×) | 3,800 W | +4,712 W |
| Refrigerator / freezer1,200 W starting (6×) | 200 W | +1,000 W |
| Furnace blower, ½ HP2,352 W starting (2.94×) | 800 W | +1,552 W |
| Sump pump, ⅓ HP1,304 W starting (1.63×) | 800 W | +504 W |
| LED bulb (each) × 15Resistive, no inrush | 150 W | - |
| Wi-Fi router + modemResistive, no inrush | 40 W | - |
| TV, 55" LEDResistive, no inrush | 200 W | - |
| MicrowaveResistive, no inrush | 1,000 W | - |
Starting is the extra draw when that one unit kicks in on top of everything already running. Only the single largest value is added to the peak, motors are assumed not to start simultaneously.
Industry benchmarks
- Essential circuits only7.5-12 kWFridge, furnace blower, sump/well pump, lights and outlets.
- Most 2,000 sq ft homes14-22 kWEssentials plus one central A/C and a kitchen. Air-cooled.
- Large home, everything on22-48 kWTwo A/C systems, electric water heat, range, dryer. Liquid-cooled above 26 kW.
- Motor inrush across the line5-7× FLANEC 430.7(B) code letters F-H (5.0-7.09 locked-rotor kVA/HP) for Design B motors.
Surge multipliers and the sizing margin are manufacturer-practice heuristics, not NEC requirements. NEC Article 702 covers optional standby systems, 702.4(B) covers load management, and 220.87 lets an existing building be sized from a year of maximum-demand data. Confirm the final model with the manufacturer's sizing software and the AHJ.
What you'll get
List the loads that run at once
Start from a backup scope, essential circuits, essentials plus central A/C, or whole home, then add, remove, or edit rows. Each load carries published running watts and a starting (surge) multiplier, or you can size a motor straight from horsepower and its NEC locked-rotor code letter.
See running vs peak starting demand
The tool adds up every running watt, then adds the single largest starting surge on top, the largest motor started last. Only one motor is assumed to start at a time, so the answer never depends on the order of your list.
Get the set, the kVA, and the switch
Apply a sizing margin and read off the recommended standard standby kW rating, the required kVA at your power factor, the transfer switch ampere size, and how much spare capacity is left over the continuous load.
How it works
- 1
List the loads that run at once
Start from a backup scope, essential circuits, essentials plus central A/C, or whole home, then add, remove, or edit rows. Each load carries published running watts and a starting (surge) multiplier, or you can size a motor straight from horsepower and its NEC locked-rotor code letter.
- 2
See running vs peak starting demand
The tool adds up every running watt, then adds the single largest starting surge on top, the largest motor started last. Only one motor is assumed to start at a time, so the answer never depends on the order of your list.
- 3
Get the set, the kVA, and the switch
Apply a sizing margin and read off the recommended standard standby kW rating, the required kVA at your power factor, the transfer switch ampere size, and how much spare capacity is left over the continuous load.
Running watts vs starting watts, why generators get undersized
A generator has two limits, and only one of them is on the sticker. The rated (running) kW is the continuous power it will make all night. The surge capability is the much larger, very brief output it can hold while a motor pulls locked-rotor current. An across-the-line induction motor draws roughly five to seven times its full-load amps at the instant it starts, that is exactly what NEC Table 430.7(B) code letters F through H describe, at 5.0 to 7.09 locked-rotor kVA per horsepower. Size a set on running watts alone and the well pump or the A/C condenser will drop the voltage far enough to stall the motor, trip the breaker, or shut the set down. The standard fix is the "largest motor started last" method: add every running watt, then add the biggest single starting surge on top, because motors do not all start at the same moment.
This calculator runs that method end to end. It totals running watts across your load list, finds the largest starting delta, applies a sizing margin, and rounds up to the next standard standby rating from 7.5 kW air-cooled through 150 kW liquid-cooled. It reports both kW and kVA, kVA = kW ÷ power factor, and single-phase air-cooled home standby sets are rated at 1.0 power factor while three-phase standby sets are rated at 0.8, plus the transfer switch amps and the spare capacity left over your continuous load. Motor rows read full-load amps straight from NEC Tables 430.248 and 430.250 and locked-rotor kVA from Table 430.7(B). If a load-management module will shed circuits, tick them and the set is sized to what actually runs together, the allowance NEC 702.4(B) makes for optional standby systems.
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Generator Sizing Calculator FAQs
It depends on what you want running, not on square footage. Essential circuits only, refrigerator, furnace blower, sump or well pump, lights and outlets, typically land between 7.5 and 12 kW. Add one central A/C and a working kitchen and most 2,000 sq ft homes need 14 to 22 kW. A large home with two A/C systems, electric water heating, a range, and a dryer runs 22 to 48 kW, and anything above 26 kW is a liquid-cooled set. Build the actual load list rather than guessing from house size.
Running watts is the steady draw once something is up to speed. Starting watts is the brief surge a motor pulls at the instant it energises, when the rotor is effectively stalled, commonly two to six times the running figure, and for an across-the-line motor roughly five to seven times its full-load amps. Resistive loads like water heaters, ranges, and incandescent lights have no inrush at all, so their starting watts equal their running watts.
kVA = kW ÷ power factor. Residential single-phase air-cooled standby sets are rated at 1.0 power factor, so their kVA equals their kW; three-phase standby sets are rated at 0.8, so a 100 kW set is 125 kVA. Amps are kW × 1000 ÷ (V × PF) for single-phase and kW × 1000 ÷ (√3 × V × PF) for three-phase. A 22 kW set at 240 V single-phase is 91.7 A, which takes the next standard transfer switch, 100 A. Common ATS sizes are 100, 150, 200, and 400 A.
This is a sizing estimate, not a specification. The surge multipliers and the sizing margin are heuristics drawn from manufacturer sizing guides, the NEC does not publish appliance surge watts or a required generator safety factor. The final selection follows the manufacturer’s own sizing software (which accounts for that alternator’s voltage-dip and step-load limits, altitude, temperature, and fuel derates) and the authority having jurisdiction. NEC Article 702 governs optional standby systems, 702.4(B) covers load management, and 220.87 lets an existing building be sized from a year of maximum-demand data.
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