Bidirectional meter – electricity meter for photovoltaic systems


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Companies that rely on renewable energies and operate a photovoltaic system – PV system for short – feed self-generated electricity into the power grid and can also obtain electricity from the public grid. There are different meters for measuring generated and fed-in electricity.

Alternatively, a bidirectional meter can be used directly to measure the current from both sides. This article explains which electricity meter has which task, how a bidirectional meter works and when self-consumption and feed-in are worthwhile.

Photovoltaic electricity meter: These meters exist

In order to measure any electricity produced, fed into the grid or purchased, there are various electricity meters in photovoltaic systems with different tasks:

  • the yield meter measures the total electricity that is produced in the photovoltaic system. The self-generated electricity can be used without financial charges and does not necessarily have to be metered, provided the maximum output of the photovoltaic system does not exceed 10 kWp.
  • the feed-in meter on the other hand, only measures the portion of the electricity produced that is fed into the public power grid. Depending on the amount of feed-in, this is remunerated accordingly. The legislator determines the amount of the feed-in tariff according to the month in which the photovoltaic system is commissioned. The amount is calculated in cents per kilowatt hour.
  • the reference counter is used when electricity is drawn from the power grid and measures the amount drawn by the respective household.

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What is a bidirectional counter?

A bidirectional meter combines the measurements of the feed-in and consumption meters. Although it is only a counter, the values ​​are still determined separately from each other. Since the Federal Network Agency stipulates that all electricity, whether fed in or purchased, must be measured precisely, one meter counts the feed-in into the public power grid and the other the purchased electricity.

By combining two counters, the bidirectional counter saves space. The electricity provider takes care of the maintenance and it is often possible to have the meter read remotely. A fee for renting the meter has to be paid during use, which means that a bidirectional meter is permanently more expensive than two individual meters.

Good to know: In some countries there is the so-called net metering. The bidirectional meter measures the current forwards and backwards and offsets the energy quantities directly with each other. This form does not yet exist in Germany; instead, a balancing meter is preferably used.

Phase problem: Ferraris counter as a solution for three-phase alternating current

A photovoltaic system produces direct current, which first has to be converted into alternating current for self-consumption and feed-in. An inverter is therefore installed in every photovoltaic system, which converts the electricity.

The next challenge is the so-called “phase problem”. The power grid in Germany consists of three phases. Consumers are usually connected to all three phases, the photovoltaic system only feeds in one phase. In this case, the feed-in meter counts the phase of the electricity fed in, the reference meter the phase of the electricity consumed. If these phases are not identical, the electricity produced cannot be used directly and is therefore first fed in and then drawn from the public electricity grid – and both meters count.

Balancing meters, such as the Ferraris meter, can measure and total the current across multiple phases. As a result, the various phases are no longer taken into account and are not negatively noticed by the system operator. Since there are only a few Ferraris meters in circulation today, digital meters are used in which the Ferraris mode is set. This ensures that the electricity produced in-house can be used to the full.

Photovoltaics: Self-consumption of self-generated electricity

Depending on the size of the photovoltaic system and the company’s consumption, the electricity produced may not be fully consumed. With the help of a power storage unit, power that is not required can be stored and is available when less power has been generated due to the weather or when it is needed at different times of the day. The size of the power storage is individual, but should not be able to cover 100 percent of your own needs in order to avoid wasting energy.

Photovoltaics is worthwhile for companies. Once the system has been installed, companies have certainty in their calculations. Large photovoltaic systems are cheaper per installed kilowatt hour and cover a higher proportion of the electricity requirement. With a balancing meter, protection against rising electricity prices is increasingly guaranteed.

Photovoltaics: Calculate your own consumption

In order to calculate the self-consumption of the self-produced electricity, the meter reading of the feed-in meter simply has to be subtracted from the meter reading of the yield meter. The yield counter indicates how much electricity was produced in the photovoltaic system. Any electricity that has not already been used is fed into the public power grid and is therefore recorded by the feed-in meter.

Reading bidirectional meters: Mechanical or electronic electricity meters

Since there are different bidirectional counters, they are also read differently.

  • the mechanical counters has two different electricity meters with relation and feed Marked are. The meter reading continues to rotate and is therefore easy to read.
  • Electronic electricity meters often show only one number. As a rule, the display alternates between the meter readings for electricity feed-in and electricity consumption.

Good to know: Mechanical counters, like Ferraris counters, cannot be read remotely. Digital electricity meters integrated into a smart meter gateway, on the other hand, do. The integration turns the meters into intelligent measuring systems that manage access rights, process measured values ​​and can automatically transmit them to several authorized persons. Several electricity meters can also be connected to a smart meter gateway.

Photovoltaics: feed-in or self-consumption?

Produce electricity and feed it into the public grid: Anyone who passes on electricity that is not used with their photovoltaic system receives a feed-in tariff under the Renewable Energy Sources Act, EEG for short. This is mainly due to the fact that the legislator wants to promote renewable energies. This gives all system owners the opportunity to feed unused electricity into the public grid, in addition to the possibility of remuneration. The feed-in tariff is paid over 20 years and thus offers planning security.

However, using the electricity you generate is more worthwhile than feeding it into the public grid. The reason for this is simple: self-generated electricity from a solar system is cheaper than obtaining electricity from the grid, as there are fewer taxes and surcharges for self-consumption. The feed-in tariff also falls over the years and, in return, photovoltaic modules are becoming cheaper, which means that the costs for self-generated electricity fall and the benefit increases.

Feed-in tariff 2021 for photovoltaic systems

Depending on the number of newly installed photovoltaic systems, the feed-in tariff also varies. The numbers are reported to the Federal Network Agency, which publishes them and adjusts the rates for subsidies accordingly. Due to the fact that the number of photovoltaic systems is constantly increasing, the feed-in tariff is falling at the same time. Since July 1, 2021, this has been 7.47 cents per kilowatt hour for systems up to 10 kilowatt peak, and the remuneration is even lower for larger systems.

As more and more companies rely on photovoltaic power generation, challenges such as the decentralization of energy sources and weather-related fluctuating power generation and feed-in are increasing. This can be counteracted by digitizing the energy transition. The integration of intelligent measuring systems plays an important role.

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