This depends on the metering system used. Basically, there are three different metering systems for landlord-to-tenant electricity: the double busbar model, the totaliser model using conventional metering technology, and the totaliser model using smart meters.
The double busbar model physically separates landlord-to-tenant electricity users from those using third-party electricity: a second busbar is installed alongside the usual single busbar. This model is rarely used, not least because having an electrician reconnect individual apartments when a tenant switches from landlord-to-tenant electricity to third-party electricity (and vice versa) costs a lot of money.
In practice, many landlords opt for the totaliser model with virtual meters. This model can be implemented using either conventional or smart meters. It is also possible to combine the use of conventional and smart meters. In the totaliser model, all the providers and consumers are connected to one busbar. A bidirectional meter is installed as a totaliser at the grid connection point. The amount of solar electricity generated is registered by a generation meter. The consumption of all the solar electricity generated is attributed arithmetically to the landlord-to-tenant electricity customers. Final consumers who do not receive their electricity from the solar installation (i.e. who are not landlord-to-tenant electricity customers) are supplied with electricity by an external provider.
In order to exclude the quantities consumed by those final consumers not taking part in the landlord-to-tenant electricity model, their meters are moved virtually to the grid connection point. The quantities consumed by these final consumers (measured at the respective sub-meter) are fully subtracted from the total amount of electricity purchased from the grid and measured at the totaliser. Where it is positive, the remaining difference is allocated to the consumers of landlord-to-tenant electricity as their purchase of additional electricity. If, on the other hand, the difference is negative (i.e. if the amount of electricity consumed by the final consumers not taking part in the landlord-to-tenant electricity model exceeds the amount purchased from the grid as measured by the totaliser), it is still assumed that the final consumers not taking part in the landlord-to-tenant electricity model were fully supplied by their external provider. At the same time, it is assumed that the afore-mentioned difference (which can only come from the solar installation) was fed into the grid. This quantity of electricity, which is only fed in “arithmetically”, is added to the quantity of electricity measured at the grid feed meter. This arithmetically fed in electricity is also eligible for funding under the Renewable Energy Sources Act.
In order to ascertain the quantity of landlord-to-tenant electricity, not only the amount of the solar power measured at the grid feed meter which is fed into the grid, but also the amount of arithmetically fed in electricity must be subtracted from the quantity of electricity measured at the generation meter. The remainder is the quantity of landlord-to-tenant electricity. It can never be greater than the total consumption of those consuming the landlord-to-tenant electricity.
The totaliser model makes it possible to ensure that the electricity consumers are free to choose their supplier behind the grid connection point with relatively little inconvenience. This is because no installation work is required when a customer switches from landlord-to-tenant electricity to third-party electricity (and vice versa). It is merely necessary to note the figures on the meter.
The legal framework established by the Landlord-to-Tenant Electricity Act permits the totaliser model using conventional metering technology. When the rates of the landlord-to-tenant electricity premium were calculated, the costs of providing the metering infrastructure to implement the totaliser model were included.
However, particularly when conventional metering technology is used, the totaliser model does not precisely reflect the physical distribution of the electricity generated locally in the building. This is because, as already stated, it is also possible for apartments which are not participating in the landlord-to-tenant electricity model to physically consume the locally generated electricity. This imprecision, which affects the billing, can be significantly reduced by metering and calculating every 15 minutes. Smart meters are capable of delivering this quarter-hourly metering and calculation. For this reason, in future the metering concept for landlord-to-tenant electricity should move towards the use of smart meters. The general rules governing the switch to smart meters can be found in the Metering Point Operation Act (find out more here).