What does self-consumption mean?

As solar electricity becomes more cost effective and a commoditized alternative for electricity production, many building owners are seeking to incorporate PV systems for self-consumption—consuming the energy as it is being produced. This chapter explains the remarkably simple operation of an electrical installation with a photovoltaic system for self-consumption.

Solar electricity is supplied 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. P28 – The photovoltaic output is synchronized with the grid voltage and frequency to avoid mismatch and stability issues

Solar self-consumption happens naturally

Solar self-consumption does not require any specific equipment to manage the flow of electrons – this happens naturally, the photovoltaic-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 photovoltaic production, and will pull some additional energy from the grid, as needed.

Fig. P29 – The photovoltaic produced power goes to the load, as the path to the loads has a much lower resistance than the path to the transformer and the grid

If the solar production exceeds the load consumption—on weekends, for example—the excess electricity goes to the grid, for which financial compensation may also be possible. If, however, the contract with the energy supplier does not allow injection of electricity to the grid, solar production should be curtailed at these times, managed by using storage or by shifting the loads to the period of photovoltaic production.

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. Anyway, this is rarely done because disconnection requires the use of additional equipment and a daily switching operation.

Fig. P30 – Nighttime power consumption of photovoltaic inverter – extract from technical specification

Grid-tie photovoltaic inverters disconnect in case of grid outage

When there is a loss of electrical power from the grid side, and if no other local source can provide backup power, the grid-tie PV inverters disconnect rapidly, and the electrical installation is no longer supplied.

Although this may at first seem counterintuitive, it addresses a serious safety concern. In case of utility supply loss, installations with local generation are required to guarantee that they do not inject power into the grid to keep utility workers safe.

Fig. P31 – Grid-tie photovoltaic inverters disconnect rapidly in case of grid outage

Off grid solar operation is possible – but requires electrical storage or a generator

Photovoltaic installations cannot ensure standalone operation of the electrical installation in off-grid mode because the solar energy produced is volatile, predictable but unplannable, and has limited control capabilities. To provide an off-grid operation, photovoltaic installations must be associated with another major and stable source such as storage or a generator.

Operation of such an electrical installation in both grid-connected and off-grid mode is more complex, and will require a specific electrical installation design, an additional control system, and an effective protection plan for each operating mode.