What does self-consumption mean?

Self-consumption of local renewable energy such as solar energy is the economic model in which the building uses the electricity produced by the local sources for its own electrical needs, thus acting as both producer and consumer, or prosumer. Self-consumption is becoming in the 2020s the preferred economic model for several reasons:

• Self-consumption offers, or will soon offer, greater economic benefits and better control of energy bills
• Self-consumption enables buildings to consume their own decarbonized energy
• Self-consumption promises greater independence from the grid and future electricity rate variations

Why does self-consumption impact the building electrical installation?

Fig. K38 – A photovoltaic system can be connected directly to the electrical distribution network ([a]) or connected to the building electrical installation and used for self-consumption ([b]).

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  [a] PV production exported to the distribution network

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  [b] PV production used for self-consumption

WHEN EXPORTING TO THE DISTRIBUTION NETWORK, YOUR PHOTOVOLTAIC SYSTEM AND YOUR BUILDING INSTALLATION ARE SEPARATED AND INDEPENDENT, see Fig. K38 - [a]

When the PV production is entirely exported to the distribution network, the PV installation is connected to the electrical distribution network without any connection to the building electrical system, see Fig. K38 - [a] (even if the points of connection can be electrically very close). Although part of the same physical infrastructure, the PV system and the building installation are two independent and autonomous electrical units. The PV energy injected to the distribution network and the energy consumed by the building are measured by two independent meters. The PV installation requires a minimum of control functions, usually handled by the PV inverters, and has no impact on the building control. PV installation rules are set by IEC 60364-7-712 and distribution network connection rules.

WHEN SELF-CONSUMING, YOUR PV INSTALLATION IS PART OF YOUR BUILDING INSTALLATION, see Fig. K38 - [b]

In this second case (see Fig. K38 - [b]), the connection of the PV installation to the building electrical installation is done downward the utility meter. The PV installation is part of the building installation, and thus, its sizing, earthing system, and protection equipment are dependent on the building electrical installation. The integration of the PV installation may also require modifications in the other parts of the building electrical installation.

WHEN SELF-CONSUMING, ADDITIONAL SOURCES OR ENERGY STORAGE EQUIPMENT CAN BE ADDED, OPENING THE DOOR TO ISLANDED OPERATION, see Fig. K39 below

Fig. K39 – PV production and energy storage used for self-consumption in islanded mode (= disconnected from the distribution network)

What is changing when integrating solar production for self-consumption?

There are three fundamental changes that occur when the choice is made to self-consume the solar energy produced by PV panels:

• The electrical installation is no longer supplied by a single source, but by two or more sources operating in parallel to the grid supply
• Each local source will produce energy (or not, depending on conditions), which means that the installation has multiple operating modes, according to energy source combinations
• The photovoltaic panels produce a direct current (DC) output and use power inverters to convert it to AC

Fig. K40 – Installation integrating PV (renewables) for self-consumption

Consequently, photovoltaic self-consumption rises important technical considerations when designing the building electrical installation, such as:

• Where to connect the photovoltaic production?
• How to calculate the system parameters of the installation?
• How to size a building installation with solar production?
• How to protect a building electrical system with integrated solar production?
• How to manage the photovoltaic system and the building loads?

The energy produced by local sources (solar ...) is supplying the loads in parallel with the grid

During the day, while the sunlight and solar panels are producing electricity, the electrical installation is powered both by the grid and the photovoltaic system. These two sources operate in parallel, with no transfer of supply from one source to the other.

To facilitate this, inverters convert the DC electricity produced by the solar panels into AC electricity. The PV inverters synchronize their output power with the grid voltage and frequency to avoid mismatch and stability issues.

Fig. K41 – The photovoltaic output is synchronized with the grid voltage and frequency to avoid mismatch and stability issues

Self-consumption of local energy produced happens naturally

Self-consumption of local energy produced does not require any specific equipment to manage the flow of electrons – this happens naturally, the locally produced power goes to the loads, as the electricity takes the path of least resistance.

The path to the loads, made of cables and busbars, has a much lower resistance than the path to the transformer and the grid. Therefore, the loads will consume the available local production, and will pull some additional energy from the grid, as needed.

Fig. K42 – The local produced power (such as solar) goes to the loads, as the path to the loads has a much lower resistance than the path to the transformer and the grid

If the local production exceeds the loads consumption — on weekends, for example — the excess electricity will naturally go to the grid. Depending on the distribution network operator, injecting locally produced energy into the grid might be allowed or not, and should be managed. See Excess local production - how to manage.

At night, photovoltaics systems do not produce, but may consume energy

At night, the solar panels do not produce electricity. Even though the photovoltaic inverters are on standby, they may consume a little bit of electrical energy. Usually, the nighttime consumption of the photovoltaic inverters is lower than 1 watt / hour.

Standby power losses can be avoided by disconnecting the photovoltaic system during the night. However, this is rarely done because disconnection requires the use of additional equipment and a daily switching operation.

Fig. K43 – Nighttime power consumption of photovoltaic inverter – extract from technical specification

What is the difference with a classical back-up generator?

Usage of local sources such as back up generator is common in electrical installation for safety services for example.

Fig. K44 – Example of Installation integrating back-up generator

The main difference of such installation with a Prosumer is that such an installation is not intended to produce energy locally when connected to the grid, so there are only two operating modes:

• Connected to the grid with no local production
• Disconnected from the grid with local production.

Type of system earthing in disconnected mode is easily managed, additionally back-up generator based on synchronous generator are able to provide overcurrent in case of short-circuit including earth fault allowing easily automatic disconnection of the supply as a protective measure against line to earth fault. Complex architectures with several generators rising stability and control issues, and/or close transition with the distribution network is restricted to limited application such as hospital or datacenter where PV and battery energy storage is applicable to any type of building including residential premises.

Note: Such installations are covered by IEC 60364-5-55.