Generator Sizing Calculator
Size a generator for a house, cottage or construction site: account for starting currents of motors and pumps, choose petrol or diesel, calculate fuel consumption and required runtime.
What the calculator computes
- Total working power of all electrical appliances
- Peak power with starting currents (motors, pumps, refrigerators)
- Recommended generator power with a 20–30% margin
- Single-phase or three-phase generator
- Fuel consumption (petrol/diesel/gas) and runtime on one tank
- ATS (automatic transfer switch) recommendation
How to size a generator
Rule 1: the generator must cover not only working but also starting power. Electric motors (pumps, refrigerators, compressors) draw 3–5 times their nominal current at start. The generator's peak power is usually listed in specifications.
Rule 2: a 20–30% power margin — the generator should run at 50–80% load: at 100% the service life drops, below 25% petrol engines foul the plugs and diesels coke up.
Rule 3: a three-phase generator up to 10 kW should not feed single-phase load with imbalance over 30% — the neutral is overloaded. For houses up to 10 kW, a single-phase generator is usually chosen.
Rule 4: for emergency power of boilers and pumps, not only power but sine wave quality matters — inverter generators (electronic) are safe for electronics, classic (synchronous) ones are cheaper and more powerful.
Starting coefficients table
| Appliance | Working power | Starting coefficient |
|---|---|---|
| Refrigerator | 150–300 W | ×3–5 |
| Freezer | 200–400 W | ×3–5 |
| Submersible pump 1 kW | 1000 W | ×4–6 |
| Circulation pump | 60–100 W | ×2–3 |
| Compressor 2 hp | 1500 W | ×5 |
| Air conditioner 9 BTU | 800–1000 W | ×3–5 |
| Angle grinder / circular saw | 1200–2000 W | ×1.5–2 |
| Lamps, heaters, kettle | per nameplate | ×1.0 |
| TV, PC | 100–300 W | ×1.2–1.5 |
Calculation formulas
Peak power: P_peak = P_working + (P_start_max − P_nom_max), i.e. the excess of the largest motor's starting current is added to the sum of working powers.
Generator power: P_gen = P_peak × 1.25. For single-phase consumers cos φ 0.8–1.0.
For three-phase consumers: P = √3 × U × I × cos φ = 692 × I × 0.8 at 400 V.
Fuel consumption: petrol generator — 0.4–0.5 L/kWh at 75% load; diesel — 0.25–0.35 L/kWh; gas — 0.4 m³/kWh. Runtime on one tank: t = V_tank / consumption.
Petrol or diesel? Comparison
| Criterion | Petrol | Diesel |
|---|---|---|
| Typical power | 0.8–7.5 kW | 5–500 kW |
| Service life | 1000–2000 h | 10,000–30,000 h |
| Fuel consumption | 30–40% higher | more economical |
| Noise | 68–75 dB | 72–85 dB (quieter at low rpm) |
| Start at -20 °C | easier | harder (heater needed) |
| Fuel storage | petrol spoils in 3 months | diesel stores 1–2 years |
| Recommendation | cottage, construction, up to 5–7 kW | permanent backup, >8 kW |
How to use the calculator
- Check the appliances from the list (or enter powers manually).
- Specify starting coefficients of motors automatically or manually.
- Select the fuel type for consumption and runtime calculation.
- Press «Calculate» — working and peak power, recommended generator, fuel consumption, ATS.
Common mistakes
- Sizing by working power without starting currents — the pump «won't turn», the generator stalls.
- Running the generator at 100% load — reduced service life and winding overheating.
- Two-phase load on a three-phase generator — imbalance and ATS failure.
- Connecting the house «to a socket» via a return cord — danger of electrocution for linemen and generator burnout. Only via ATS with interlock!
- Petrol in the tank for more than 3 months — the generator won't start when needed.
Generator FAQ
Which generator is needed for a 150 m² house?
The minimum set during a power outage: boiler (0.5–1 kW), pump (1 kW), refrigerator (0.3 kW), lighting and TV (0.5 kW) — working load 2.5–3 kW, peak with pump 5–7 kW. Recommendation: a petrol inverter generator 4–6 kW or diesel 5–8 kW with ATS.
Why doesn't the generator «pull» the pump although the power fits?
Because only working power was accounted. A 1.1 kW submersible pump draws up to 5–6 kW for 1–3 seconds at start. A classic generator must have peak power no less. Solution: a larger inverter generator or a soft starter for the pump.
What is ATS and do I need it?
ATS is an automatic transfer switch: a relay/controller that starts the generator when the grid fails, switches the house to it, and switches back when power returns. Mandatory for permanent backup; connected only through a switch with an interlock that excludes generator and grid working together.
How much fuel does a generator burn per day?
A diesel 5 kW at 50% load — 0.25 L/kWh × 2.5 kW × 24 h ≈ 15–20 L/day. A petrol 5 kW — 25–30 L/day. A gas generator on mains gas is the cheapest option for permanent operation (0.4 m³/kWh).
Can a three-phase house be powered from a single-phase generator?
Only if three-phase consumers are switched off and the board is re-commutated to a single-phase scheme via ATS — this requires a project. Three-phase loads (machines, 380 V boilers) are powered only from a three-phase generator with balanced load.