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Generator Sizing Calculator
Estimates the generator size needed for a set of loads from the total running watts, the starting surge of the largest motor (peak = running total + starting − running of that motor), a safety margin and the power factor, giving the required kW (rounded up to 0.5 kW) and kVA plus the running current at 120 V and 230 V.
When to use
You are choosing a portable or standby generator for a home, site or event and know the running and starting watts of the equipment.
Do not use when: You need branch-circuit or breaker checks (use circuit-load), inverter or battery runtime (use battery-runtime), or three-phase and transfer-switch design; the sequential-start model assumes only one motor starts at a time.
Formula
peak_watts = running_watts_total + (largest_motor_starting_watts − largest_motor_running_watts); required_watts_with_margin = peak_watts × (1 + safety_margin_percent / 100); recommended_kw = ceil(required_watts_with_margin / 500) × 0.5; recommended_kva = recommended_kw / power_factor; running_amps = running_watts_total / (V × power_factor)
Sequential-start method: the generator must carry all running loads plus the extra surge of the largest motor when it starts (motors draw 2–6 × running power for a fraction of a second). Generators are rated in kVA at 0.8 power factor; running them at 50–80 % of rated load gives the longest life and lowest fuel use per kWh.
Inputs
| Parameter | Type | Unit | Required | Description |
|---|---|---|---|---|
running_watts_total | number | W | yes | Sum of the running (continuous) wattage of everything powered at the same time, including the largest motor's running watts. Range: > 0, ≤ 10000000 |
largest_motor_running_watts | number | W | default 0 | Running watts of the single motor-driven load with the highest starting surge (refrigerator, pump, air conditioner); 0 if none. Range: ≥ 0, ≤ 10000000 |
largest_motor_starting_watts | number | W | default 0 | Starting (surge, LRA) watts of that motor, typically 2–3 × its running watts; must be at least its running watts. Range: ≥ 0, ≤ 10000000 |
safety_margin_percent | number | % | default 20 | Reserve added to the peak load so the generator runs below full output (20–25 % typical). Range: ≥ 0, ≤ 100 |
power_factor | number | default 0.8 | Power factor used to convert kW to kVA and watts to amps; generators are rated at 0.8 lagging. Range: > 0, ≤ 1 |
Outputs
| Output | Type | Unit | Description |
|---|---|---|---|
peak_watts | number | W | running_watts_total + (largest_motor_starting_watts − largest_motor_running_watts). |
required_watts_with_margin | number | W | peak_watts × (1 + safety_margin_percent / 100). |
recommended_kw | number | kW | required_watts_with_margin rounded up to the next 0.5 kW. |
recommended_kva | number | kVA | recommended_kw / power_factor. |
running_amps_120v | number | A | running_watts_total / (120 × power_factor). |
running_amps_230v | number | A | running_watts_total / (230 × power_factor). |
load_percent_of_recommended | number | % | running_watts_total / (recommended_kw × 1000) × 100; 50–80 % is the efficient operating band. |
Example
3,000 W running, largest motor 800 W running / 2,200 W starting: {"running_watts_total":3000,"largest_motor_running_watts":800,"largest_motor_starting_watts":2200} → {"peak_watts":4400,"required_watts_with_margin":5280,"recommended_kw":5.5,"recommended_kva":6.88,"running_amps_120v":31.25,"running_amps_230v":16.3}
6,500 W running, well pump 1,500 / 4,500 W, 25 % margin, PF 1: {"running_watts_total":6500,"largest_motor_running_watts":1500,"largest_motor_starting_watts":4500,"safety_margin_percent":25,"power_factor":1} → {"peak_watts":9500,"required_watts_with_margin":11875,"recommended_kw":12,"recommended_kva":12,"running_amps_120v":54.17,"running_amps_230v":28.26}
GET https://tttkmbb.com/api/v1/calculate/generator-sizing?running_watts_total=3000&largest_motor_running_watts=800&largest_motor_starting_watts=2200
Machine access
- API:
GET https://tttkmbb.com/api/v1/calculate/generator-sizing(query parameters) orPOSTwith a JSON body{"inputs": {...}} - Schema: https://tttkmbb.com/api/v1/calculators/generator-sizing · Markdown: https://tttkmbb.com/energy/generator-sizing.md · JSON definition: https://tttkmbb.com/energy/generator-sizing.json
- MCP: server
https://tttkmbb.com/mcp, toolrun_calculator with calculator_id="generator-sizing" - OpenAPI operationId:
estimate_generator_size - Freshness:
static. Authentication: none. Rate limit: fair use (see rate limits).
Sources
- Wikipedia – Engine-generator (reference)
- Wikipedia – Power factor (reference)
- Wikipedia – Inrush current (reference)
FAQ
Where do I find starting watts?
On the appliance nameplate as LRA (locked-rotor amps × voltage) or in the manufacturer's data; if unknown, use 3 × running watts for compressors and pumps, 2 × for fans and power tools.
Why kVA as well as kW?
Generators are rated in apparent power (kVA); the real power they can deliver is kVA × power factor, conventionally 0.8, so a 6.9 kVA set delivers 5.5 kW.
Should I size for all motors starting together?
Normally no: start loads one at a time, largest first. If several motors can start simultaneously (e.g. two compressors on one thermostat), add their surges to the peak.
Related calculators
- Circuit Load Calculator — Breaker check for the circuits the generator feeds.
- Electric Power Calculator — Watts, volts and amps of individual loads.
- Battery Runtime Calculator — Runtime of a battery or inverter for the same loads.