Choosing a Home Generator: Petrol vs Diesel
Home generator selection guide: power calculation, petrol vs diesel, inverter technology, Automatic Transfer Switches (ATS), and type recommendations.
For a typical home without electric heating, a 5–7 kW generator is usually sufficient: sum the power ratings of all appliances operating simultaneously, multiply the rating of equipment with electric motors by a starting factor of 2–5, and add a 20–30% safety margin. Petrol generators are more affordable and easier to start in winter, making them suitable for occasional outages; diesel generators are more expensive but more economical and offer a longer service life, making them the better choice for frequent or prolonged outages. You can calculate the required power for your specific appliance set using the online calculator provided in this article.
For a single-phase house, select a single-phase generator. Connect it only via a changeover switch or an Automatic Transfer Switch (ATS). When checking capacity, always refer to the continuous (rated) power rather than the maximum peak figure on the rating plate. For gas boilers and sensitive electronics, you will need an inverter generator or a station that provides a pure sine wave.
Why do you need a generator?
A generator (power station) provides autonomous power supply during central grid outages. The importance of this is clearly demonstrated by heating: a gas boiler requires only 100–200 W to power its pump and control electronics, but without them, the boiler and the entire heating system will stop working in winter. The same applies to water supply from a borehole pump and your refrigerator.
Calculating the required power
Sum the power ratings of all appliances that must operate at the same time. For appliances with electric motors (pumps, compressors), multiply the rated power by a starting factor (2–5).
| Appliance | Power, W | Starting factor |
|---|---|---|
| LED lighting | 100–300 | 1 |
| Refrigerator | 150–300 | 3–5 |
| Borehole pump | 500–1500 | 3–5 |
| Gas boiler | 100–200 | 1 |
| Washing machine | 1500–2500 | 2–3 |
| Electric hob | 3000–7000 | 1 |
Add a 20–30% safety margin to the calculated power. For a typical home without electric heating, a 5–7 kW generator is sufficient.
In the generator's technical documentation, look for the continuous (rated) power, not the maximum power: the maximum power is only available for a short duration (usually to cover starting currents), whereas the large figure on the rating plate is almost always the maximum. You should compare models and calculate loads based on the continuous power.
You can calculate the required generator power for your specific appliance set using the calculator below.
Single-phase or three-phase
A common mistake is purchasing a three-phase generator (400 V) for a home with single-phase wiring. The capacity of a three-phase unit is divided across three phases; a single-phase load can only utilise one of them, meaning you can only access approximately one-third of the rated power, effectively wasting two-thirds of the capacity. A three-phase generator is only justified where there are three-phase consumers or a 400 V connection with a load distributed across phases; for a standard home with a 230 V connection, choose a single-phase model.
Petrol vs Diesel
| Parameter | Petrol | Diesel |
|---|---|---|
| Cost | Lower | Higher |
| Fuel consumption | Higher | Lower |
| Service life | Shorter | Longer |
| Noise level | Lower than an open diesel unit | Open 3000 rpm models are noisier than petrol; a 1500 rpm diesel in a soundproof canopy is quieter than petrol |
| Winter starting | Easier | More difficult (may require heating) |
| Best suited for | Occasional outages | Frequent or prolonged outages |
Inverter Generators
These deliver a stable sine wave with low total harmonic distortion (THD), making them safe for sensitive electronics such as boilers, computers, and medical equipment. They are more expensive but operate more quietly and are more efficient under partial loads.
A specific technical nuance concerns phase-dependent gas boilers (those using ionisation flame sensing): they require correct phase-to-neutral orientation and a neutral connection that is bonded to earth. In many generators, particularly inverter models, the neutral is "floating," which may cause the boiler to fail to ignite or trigger an error code. The solution is to verify the boiler manufacturer's requirements and, if necessary, bond the generator's neutral to the earth according to the station's manual.
Connecting to the house: via changeover switch or ATS only
A generator must never be connected via a standard socket or placed in parallel with the mains supply. Running a domestic generator in parallel with the utility grid is prohibited and dangerous: if the mains voltage is restored, unsynchronised connection will damage the generator, and back-feeding voltage from the generator into a disconnected line is lethal for technicians working on the network. Therefore, it is mandatory to use a changeover switch (transfer switch) or an Automatic Transfer Switch (ATS) unit, which physically prevents the simultaneous connection of both power sources.
An ATS unit automatically starts the generator when mains voltage is lost and switches the load. When the mains power is restored, it switches back and shuts down the generator. This is highly recommended for primary residences.
Recommendations
- Summer house (seasonal): petrol 2–3 kW
- Primary residence (permanent): petrol 5–7 kW or diesel 5–10 kW with an ATS
- Commercial premises: diesel 10+ kW in a soundproof canopy