1. Defining Basic Requirements

Before choosing an emergency generator, you should determine which devices must continue to run in an emergency. This prevents over-dimensioning and saves costs. Typical household consumers have the following power requirements:

• Heating and Hot Water:
  Gas heating: approx. 300 W
  Oil heating including hot water: approx. 600–3000 W
  Heat pump (air/water, ground source or water source heat pump): approx. 5000–10000 W
• Water heater: Electric boiler approx. 600–1000 W
• Refrigeration: Refrigerator and freezer each approx. 300 W
• Kitchen:
  Standard hot plate: approx. 800 W
  Quick hot plate: approx. 2200 W
  Microwave: depending on size 600–1500 W
• Communication and Security: Telephone, router, TV, radio, alarm system: total under 100 W
• Lighting: LED lights approx. 15–100 W per lamp
• Water pump or domestic water system: Power requirement depends on the model

Not all devices need to be supplied in an emergency. Electric stoves and dryers have high power consumption and can usually be replaced for the outage period, for example, with a gas cooker or grill. A list of priorities helps determine the appropriate power class of the generator.

2. Technical Details for Selection

Power Reserve and Starting Current

Inductive loads such as pumps and motors require significantly higher current to start than during operation. Therefore, plan for a reserve of around 20–30% in the generator's power output. This protects against voltage drops when several consumers start simultaneously.

Single-phase or three-phase?

Most household appliances run on 230 V, i.e., single-phase. Three-phase generators distribute their power across three phases; if these are unevenly loaded, the voltage can fluctuate. If no three-phase devices are present, a single-phase generator is often the better choice. For heat pumps or large pumps, a three-phase device is necessary; it should then be adequately dimensioned to handle unbalanced loads.

Fuel Choice and Runtime

Diesel generators are considered robust and designed for longer runtimes. Petrol generators are more suitable for shorter operations and are often cheaper. For gas appliances, fuel storage is uncomplicated, but they are less common. Remember to store fuels in secure, break-proof containers and only keep legally permitted quantities.

3. Correct Connection to the House Grid

A frequently mentioned myth is that an emergency generator can simply be fed into the house grid via any socket. This is dangerous and prohibited: Connecting via a socket can lead to damage to the house installation and carries the risk of electric shock. Furthermore, there is a risk of uncontrolled back-feeding into the public grid, which endangers both grid operators and other households.

To avoid this, the generator must always be connected via a professionally installed grid isolation switch (changeover system) or an ATS. This ensures that the house system is completely disconnected from the public grid during emergency power operation.

The installation of such a changeover system may only be carried out by a qualified electrician. In addition, RCDs (residual current devices) and proper grounding of the generator are mandatory. Without these safety mechanisms, life-threatening situations and, in the worst case, the destruction of your generator by uncontrolled voltages and frequencies are threatened.

4. Safety and Operating Instructions

An emergency generator must never be simply fed into the house grid via a normal socket. According to the grid operators' guidelines, the installation must be designed to ensure a safe and all-pole disconnection between the public grid and the emergency power supply. The most important points:

• Grid Isolation Switch: A changeover switch completely disconnects the house grid from the public grid before the generator is connected. Parallel operation is only permitted for short periods for synchronization and is generally not required for simple house systems.
• Prevent Back-feeding: No energy from the generator may flow back into the public grid, as this can lead to dangerous voltages and damage to the installation. An automatic switching system ensures that the grid is only reconnected when the public supply returns stably.
• RCD and Grounding: A residual current device (RCD) must be present to prevent dangerous contact currents. The generator must be properly grounded; for models with insulation monitoring, the grounding obligation is omitted if this function is provided.
• Professional Electrical Installation: Planning, connection, and maintenance should be carried out by a qualified electrician. This ensures that the correct cable type is used, the changeover device functions correctly, and all regulations are complied with.
• Regular Tests: Regularly test run the generator to ensure it starts immediately in an emergency. Also, observe maintenance intervals for oil and filter changes.
• Location: Always place the generator in a well-ventilated area so that exhaust gases are safely discharged and there is no risk of carbon monoxide poisoning. It must never be operated in enclosed spaces.