# 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.

- Calculator id: `generator-sizing` · Category: Home Energy & Electrical (`energy`) · Tool name: `estimate_generator_size`
- Canonical page: https://tttkmbb.com/energy/generator-sizing · This document: https://tttkmbb.com/energy/generator-sizing.md · JSON definition: https://tttkmbb.com/energy/generator-sizing.json

## Purpose

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.

**Use when:** 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.

## Input

| Parameter | Type | Unit | Required | Description |
| --- | --- | --- | --- | --- |
| `running_watts_total` | number | W | required | Sum of the running (continuous) wattage of everything powered at the same time, including the largest motor's running watts. (> 0, max 10000000) |
| `largest_motor_running_watts` | number | W | optional, default 0 | Running watts of the single motor-driven load with the highest starting surge (refrigerator, pump, air conditioner); 0 if none. (min 0, max 10000000) |
| `largest_motor_starting_watts` | number | W | optional, default 0 | Starting (surge, LRA) watts of that motor, typically 2–3 × its running watts; must be at least its running watts. (min 0, max 10000000) |
| `safety_margin_percent` | number | % | optional, default 20 | Reserve added to the peak load so the generator runs below full output (20–25 % typical). (min 0, max 100) |
| `power_factor` | number |  | optional, default 0.8 | Power factor used to convert kW to kVA and watts to amps; generators are rated at 0.8 lagging. (> 0, max 1) |

## Output

| Field | 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. |

## 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.

## Data Sources

- Wikipedia – Engine-generator — https://en.wikipedia.org/wiki/Engine-generator (reference, retrieved 2026-09-24)
- Wikipedia – Power factor — https://en.wikipedia.org/wiki/Power_factor (reference, retrieved 2026-09-24)
- Wikipedia – Inrush current — https://en.wikipedia.org/wiki/Inrush_current (reference, retrieved 2026-09-24)

Data freshness: `static`. Deterministic formula with fixed constants; results never go stale. Inputs supplied by the caller determine the output.

## API

- `GET https://tttkmbb.com/api/v1/calculate/generator-sizing?running_watts_total=…`
- `POST https://tttkmbb.com/api/v1/calculate/generator-sizing` with JSON body `{"inputs": {…}}`
- Response: unified envelope (`success`, `request`, `result.values`, `result.units`, `sources`, `freshness`, `timestamp`, `next_actions`, `links`); see https://tttkmbb.com/docs/response-format.md
- Schema: https://tttkmbb.com/api/v1/calculators/generator-sizing · OpenAPI operationId `estimate_generator_size` in https://tttkmbb.com/openapi.json
- Authentication: none. Rate limit: fair use, see https://tttkmbb.com/docs/rate-limits.md.

## MCP

- Server: `https://tttkmbb.com/mcp` (Streamable HTTP, JSON-RPC 2.0, no auth)
- Tool:  `run_calculator` with `{"calculator_id": "generator-sizing", "inputs": {…}}`

## Example

- 3,000 W running, largest motor 800 W running / 2,200 W starting: inputs `{"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: inputs `{"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
```

## Limitations

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. 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. All values are computed from the formula above; no measurement or live data is involved.

## 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

- [Circuit Load Calculator](https://tttkmbb.com/energy/circuit-load.md) — Breaker check for the circuits the generator feeds.
- [Electric Power Calculator](https://tttkmbb.com/physics/electric-power.md) — Watts, volts and amps of individual loads.
- [Battery Runtime Calculator](https://tttkmbb.com/engineering/battery-runtime.md) — Runtime of a battery or inverter for the same loads.