The Core Generator Sizing Formula
To correctly size a backup or prime power generator, you must calculate the apparent power (kVA) required to handle both the continuous running loads and the momentary inrush current of the largest motor starting. The direct answer to how to calculate generator size relies on this unified engineering formula:
Symbol Definition Table
| Symbol | Unit | Definition & Typical Values |
|---|---|---|
| GkVA | kVA | Required Generator Apparent Power capacity. |
| Prun | kW | Total real running power of all simultaneous loads. |
| Psurge | kW | Additional starting surge power (delta) of the single largest motor load above its running power. |
| SF | Dimensionless | Safety/Sizing Factor. Use 1.1 for residential standby, 1.25 for commercial/prime applications to account for future load growth and alternator thermal limits. |
| PF | Dimensionless | Power Factor. Standard generator rating is 0.8 for three-phase and 1.0 for single-phase, but using 0.8 across the board provides a conservative, safe margin for mixed inductive loads. |
When This Formula Applies and Its Assumptions
This formula applies to sizing single-phase residential standby generators (like a Generac Guardian series) and three-phase commercial prime/standby sets. It assumes a standard ambient temperature of 40°C (104°F) and an altitude below 500 feet (150 meters). If your installation exceeds 500 feet, you must derate the final GkVA result by approximately 3% for every 1,000 feet of elevation due to thinner air reducing engine volumetric efficiency. It also assumes you are using a properly rated Automatic Transfer Switch (ATS) that isolates the utility grid.
Realistic Answer Magnitudes
What does a realistic answer look like? For a standard 200-amp residential service panel with central air and an electric range, expect a calculated result between 15 kVA and 22 kVA (roughly 15kW to 22kW at unity PF). For a small commercial workshop with a 3-phase air compressor and welder, expect 30 kVA to 60 kVA. If your math yields 4 kVA for a whole-house build or 500 kVA for a small cabin, your load inventory is flawed.
Rearranged Forms of the Equation
In field diagnostics, you rarely solve for the generator size alone. You often need to back-calculate what a generator can actually support, or verify the power factor of an existing setup. Here are the rearranged forms:
- Solving for Maximum Allowable Load (Prun + Psurge):
(Prun + Psurge) = (GkVA × PF) / SF - Solving for Safety Factor (SF):
SF = (GkVA × PF) / (Prun + Psurge) - Solving for Power Factor (PF):
PF = [ (Prun + Psurge) × SF ] / GkVA
Worked Examples with Unit Tracking
Abstract formulas cause mistakes on the jobsite. Below are two step-by-step calculations tracking units through every operation.
Problem 1: Residential Standby with HVAC and Well Pump
Scenario: Sizing a single-phase standby generator for a home. The simultaneous loads are a Central AC (Run: 4.0 kW, Starting Surge: 12.0 kW), a Well Pump (Run: 1.5 kW, Starting Surge: 4.5 kW), and base loads like lighting and a fridge (Run: 1.5 kW, Surge: 0 kW). We will use a Safety Factor (SF) of 1.1 and a conservative Power Factor (PF) of 0.8 to account for the heavy inductive motors.
- Calculate Total Running Power (Prun):
Prun = 4.0 [kW] (AC) + 1.5 [kW] (Pump) + 1.5 [kW] (Base) = 7.0 [kW] - Calculate the Largest Surge Delta (Psurge):
The AC has the largest surge. The delta is the surge minus its own running power.
Psurge = 12.0 [kW] (AC Surge) - 4.0 [kW] (AC Run) = 8.0 [kW] - Calculate Peak Real Power Demand:
Peak kW = Prun + Psurge = 7.0 [kW] + 8.0 [kW] = 15.0 [kW] - Apply Safety Factor (SF = 1.1):
Adjusted kW = 15.0 [kW] × 1.1 = 16.5 [kW] - Convert to Apparent Power (Divide by PF = 0.8):
GkVA = 16.5 [kW] / 0.8 = 20.625 [kVA]
Conclusion: You need a minimum 20.6 kVA generator. A standard 22kW (22kVA at 1.0 PF single-phase) Kohler 20RESCL or Generac 22kW unit is the correct off-the-shelf selection.
Problem 2: Commercial Workshop with 3-Phase Machinery
Scenario: Sizing a 3-phase diesel prime generator for a fabrication shop. Loads: 3-Phase Air Compressor (Run: 5.0 kW, Surge: 15.0 kW), MIG Welder (Run: 8.0 kW, Surge: 8.0 kW), and LED Bay Lighting (Run: 1.0 kW, Surge: 0 kW). Commercial environments require a stricter Safety Factor (SF) of 1.25. Standard 3-phase PF is 0.8.
- Calculate Total Running Power (Prun):
Prun = 5.0 [kW] + 8.0 [kW] + 1.0 [kW] = 14.0 [kW] - Calculate the Largest Surge Delta (Psurge):
The compressor dictates the surge.
Psurge = 15.0 [kW] - 5.0 [kW] = 10.0 [kW] - Calculate Peak Real Power Demand:
Peak kW = 14.0 [kW] + 10.0 [kW] = 24.0 [kW] - Apply Safety Factor (SF = 1.25):
Adjusted kW = 24.0 [kW] × 1.25 = 30.0 [kW] - Convert to Apparent Power (Divide by PF = 0.8):
GkVA = 30.0 [kW] / 0.8 = 37.5 [kVA]
Conclusion: The mathematical requirement is 37.5 kVA. Accounting for standard commercial frame sizes and alternator thermal limits defined by NFPA 110 standards for Level 1/2 power, you would specify a 40 kVA or 50 kVA 3-phase diesel genset.
Unit Mistakes That Break Your Calculation
If your final generator size looks absurdly high or dangerously low, you likely committed one of these unit errors:
- Confusing Watts and Kilowatts: An HVAC data plate might list 4,500 Watts. If you plug "4500" into the formula instead of "4.5", your resulting kVA will be 1,000 times too large. Always convert base units to kW before calculating.
- Ignoring kVA vs. kW (The PF Trap): Generator alternators are rated in kVA because their copper windings burn out based on current (Amps), not real work (Watts). If you size a generator assuming PF = 1.0 (where kW = kVA) for a facility full of inductive motors, the alternator will overheat and trip its internal breaker even if the engine has plenty of mechanical horsepower.
- Double-Counting Motor Surge: Psurge is the delta (the extra power needed to overcome Locked Rotor Amps). If a motor runs at 2 kW and surges to 6 kW, Psurge is 4 kW, not 6 kW. Adding the full 6 kW on top of the running load artificially inflates the requirement.
- Mixing Horsepower (HP) and kW: 1 HP is approximately 0.746 kW. If a compressor plate says "5 HP", you must multiply 5 × 0.746 = 3.73 kW before entering it into the formula.
Frequently Asked Questions
How to calculate generator size for a 200 amp service panel?
While you can use the formula above for precision, a common rule-of-thumb for a standard 200-amp, 240V split-phase residential service is to calculate the maximum theoretical apparent power: 200 [Amps] × 240 [Volts] = 48,000 VA, or 48 kVA. However, homes rarely run at 100% continuous capacity. For whole-house backup without load shedding, a 22kW to 26kW (approx. 22-26 kVA) air-cooled standby generator is the industry standard. If you have dual 200A panels or heavy continuous electric heating, you must use the load-inventory formula above or upgrade to a 30kW+ liquid-cooled unit.
How to calculate generator size for solar battery charging?
When sizing a generator specifically to feed a solar inverter/charger (like a Victron MultiPlus or Schneider XW Pro) during low-solar winter months, you calculate based on the battery bank's maximum absorption charge rate, not the home's AC loads. Use the formula: GkVA = (Battery Bank Ah × Charge Rate % × Bank Voltage) / (1000 × Inverter Efficiency × PF). For example, charging a 48V, 400Ah LiFePO4 bank at 0.2C (80 Amps) requires roughly 3.84 kW of DC power. Factoring in 90% inverter efficiency and a 1.1 safety margin, you need a generator capable of delivering at least 4.7 kVA (roughly a 5kW unit) just to bulk-charge the batteries while simultaneously supporting baseline AC loads.
How to calculate generator size for a 3-phase industrial motor?
Sizing for a single large 3-phase motor (like a 50 HP irrigation pump) across-the-line requires accounting for massive inrush current, typically 600% of Full Load Amps (FLA). The formula simplifies to: GkVA = (Motor HP × 0.746 × Starting Multiplier) / (Motor Starting PF × Generator Efficiency). A 50 HP motor (37.3 kW) starting across-the-line (Multiplier = 6) at a starting PF of 0.3 requires roughly 746 kVA of generator capacity to prevent severe voltage dip that would trip the motor's contactor. To avoid buying a massive 750 kVA generator, engineers use Variable Frequency Drives (VFDs) or soft-starters, which reduce the starting multiplier from 6 down to 1.5 or 2.0, allowing a much smaller 150 kVA generator to start the same load.
