A separate kWh meter in the consumer unit: what for and how it should be done

Who this article is for Background and uses, not DIY instructions. Fitting a meter in a consumer unit is work for someone who is competent under NEN 3140.

Do not confuse it with the grid operator's meter

The meter your energy bill is based on belongs to the grid operator and is sealed in the meter cupboard. You leave that alone. Full stop.

A kWh meter in the consumer unit is something else: a sub-meter. It belongs to you, sits after the main switch and only measures what passes through it.

What you would want one for

1. Recharging costs to a tenant. If you let a flat, a room or a commercial unit without its own connection, you measure the consumption with a sub-meter. Here MID certification is required if you charge money based on that measurement.

2. Metering the EV charger separately. For business mileage records or to claim home charging costs from your employer. Here too the measurement must be reliable and traceable, so MID. See EV charger at home: single-phase or three-phase.

3. Owners' association (VvE) with shared facilities. Lift installation, communal lighting, shared EV chargers. To divide costs fairly you want to know what goes where.

4. Insight into your own consumption. What does the heat pump really use, what goes to the garage. For this any meter will do, even a cheap one without certification.

5. Solar panels and battery. To know how much you generate, use and feed back. This is often already in the inverter or available via the P1 port, so check first whether you really need that meter. See solar panels and your consumer unit.

MID: when is it required

MID stands for the European Measuring Instruments Directive. An MID certified meter is certified for accuracy and may be used as the basis for billing.

MID needed:

• You recharge consumption to a tenant
• You invoice electricity to a third party
• You claim charging costs from an employer or lease company
• You divide costs within an owners' association (VvE) based on metering

MID not needed:

• Purely for your own insight
• For monitoring and automation

An MID meter costs more than an ordinary one, and the difference in price is small compared with an argument with a tenant about the bill. If in doubt, choose MID.

Where it goes in the consumer unit

A sub-meter always sits after the protection of the circuit it measures, so after the breaker. It then measures exactly the consumption of that circuit.

Important points when fitting it:

• Line and neutral must both pass through the meter. A meter that only sees the line cannot measure.
• The cross-section of the connecting wire follows the breaker, not the meter. See wiring in the consumer unit.
• On three-phase, a three-phase meter. A single-phase meter on one phase of a three-phase circuit only measures a third.
• For larger currents, a meter with current transformers. Above roughly 63A the current no longer goes directly through the meter and you measure via CTs. Then the transformer ratio is critical: a wrongly set ratio gives an invoice that is wrong by a factor of ten.
• Modules: allow 2 to 4 modules per meter. That adds up in a full consumer unit.

Reading and recording

Modern DIN rail meters often have a Modbus or pulse output. With that you can read the consumption automatically into an energy management system or into your own records.

For recharging to tenants that is practical: you read the meter monthly instead of someone having to stand in the meter cupboard with a phone. For an owners' association (VvE) with several meters it is almost a precondition for keeping it workable.

Make sure you record the meter readings at the start and end of a tenancy. Without a starting reading you have no case.