The self-consumption bonus

Information and details regarding the self-consumption of solar energy

July 2010

Solar power plant operators do it every single day, most notably around noon time: They consume the electricity their plants generate. The result: self-consumption. The reason: physics. These facts didn’t change, the only new thing is that self-consumption is both favored and promoted by lawmakers.

Whereas in the past compensation for feeding-in solar energy has been completely detached from the energy consumption, the coordination of consumption and generation is now becoming increasingly important, especially following the German Bundestag's amendments to the Renewable Energy Sources Act (EEG) in early May. Through a combination of reduced monetary compensation, payable sales tax and saved power purchasing costs, operators should end up receiving up to 9.5 cents more for each kilowatt hour of solar energy they consume instead of feeding it to the public grid (depending on the amount of self-consumption and with an assumed electricity price of 23,8 cents incl. VAT).

Self-consumption is consequently becoming a key issue when discussing the profitability of solar power plants. The big questions are therefore: How much self-consumption can an average home achieve under normal circumstances, i.e., without taking any additional measures? And what solutions are capable of increasing the amount of self-consumption?

"Natural" self-consumption in a private home

To determine self-consumption, the average consumption of solar energy must be compared with the amount of energy generated by the solar power plants. Consumption depends on how many and what kind of electrical appliances are used as well as the number of people living in the household and what their consumer habits are. The amount of generated energy, on the other hand, depends on the power and location of the PV plant as well as the prevailing weather conditions.

Figure 1 illustrates the generation and consumption of energy from a 5 kWp solar plant and a four-person household on a typical summer day. The generation capacity curve (gray) generally takes the shape of a bell curve on clear days. The consumption load curve (blue) indicates typical load peaks at noon time with other consumption peaks in the morning and evening hours. The amount of solar energy directly consumed corresponds to the portion of consumed power (blue areas) that is "inside" the generated PV energy (light gray areas). That amount is depicted as dark blue in the figure.

SMA has performed extensive analyses to determine average self-consumption rate. Result: In a four-person household, self-consumption as a share of the total amount of generated energy is about 20 to 40 percent per year (Fig. 2).

The limits of "natural" self-consumption

This subject raises the question regarding the 60 to 80 percent of energy for which the time of generation and consumption are not congruent. They result from seasonal effects on the one hand and the daytime availability of solar power on the other. For example, in winter the demand for electric energy generally increases while the generation capacity decreases and in summer the opposite holds true. And each day brings energy requirements that cannot be met with solar power alone - whether for cooking or watching television in the evening of making breakfast at 7 am in the morning.

Increasing self-consumption

It is nevertheless possible to increase the amount of self-consumption that is automatically present in private households. The easiest way to do that is to change consumption behavior: Whoever shows initiative and only uses major electrical appliances during hours of strong irradiation can increase their share of self-consumption by up to ten percentage points.

The number of appliances that can be switched on at the same time is of course limited by the current power of the PV plant - switching on too many appliances will result in "wasted" potential for self-consumption. Major electrical appliances should therefore be started successively rather than simultaneously. Specific example: Do not switch on the washing machine at the same time as the dishwasher or oven. Instead, switch them on one after the other or let them run in a time-delayed manner. To this end, a convenient monitoring solution for displaying the current solar power such as SMA's Sunny Beam remote display is extremely helpful.

Combining the power monitoring device of a PV plant with an electric switching apparatus allows for the implementation of automated solutions for increasing self-consumption. The same basic rules apply here as well: Only switch on appliances if power is available in sufficient quantities and is not being used for other purposes - otherwise switch them on later or one after the other. To this end, such systems should not only be able to determine their power generation, but also their current rate of power consumption. Otherwise there is a risk that appliances which are currently running and which consume all or just some of the available PV power will be ignored. In the worst case scenario, an aditional peak load would be generated and exceed the amount of energy supplied by the PV plant - the connected appliance would then have to draw at least a part of its energy from the grid.
An effective technical solution would therefore not only record PV power, but also monitor the feed-in counter. After all, the feed-in counter measures the amount of generated energy that is not consumed in the household itself. If energy is fed into the grid, then PV power clearly exceeds consumption - additional appliances can be switched on. A product solution based on this functional principle will be introduced by SMA at the upcoming Intersolar trade fair.

The intermediate storage of PV power with battery systems will also offer an appealing choice in the medium term. For whoever is able to select any given time when PV power is to be consumed, can also significantly increase their self-consumption. An advanced technical solution for this purpose is already available with the Sunny Backup System by SMA. With a software update and an additional meter interface it can increase the self-consumption rate significantly — aside from the actual function of the fail-proof energy supply (Fig. 3). Another advantage: Almost any PV plant can be retrofitted with a Sunny Backup set provided the system is equipped with PV inverters from SMA.

Prospects: Self-consumption for commercial consumers

In addition to the circumstances evaluated here with respect to private households, self-consumption can also be an appealing choice for commercial consumers. One reason can be found in the power limit for exercising the self-consumption option - the limit of 30 kWp was increased to up to 500 kWp during the amendment of the EEG. It should however be noted that commercial consumers are difficult to categorize since their individual consumption behaviors can be vastly different. If there is a high baseload demand, which refers to a consistently high energy demand throughout the day, a very high self-consumption rate is conceivable. SMA has addressed this topic in much greater detail in a different article.