Optimally designing a PV installation and storage system

April 2020

Dimensioning the PV installation

The basis for designing your PV installation should always be your annual electricity consumption. This determines the size of the PV installation as well as the battery storage capacity. For PV installations for own consumption, the general rule is that you should generate exactly as much electricity as your household needs. But to optimize your own consumption and also enjoy self-generated electricity in the spring and fall, it’s recommended that the installation be slightly over-dimensioned. This means you need to generate 1.5 times the electricity you actually consume. You also need to take solar radiation values into account, based on your geographic location as well as the pitch and orientation of the roof on which the installation is placed.

As a sample calculation, let’s consider a household of three people and a useful energy consumption of 5,000 kWh per year. We’re planning the installation based on an east-west roof with a 32 degree pitch in southern Germany and a specific annual yield of 1,000 kWh/kWp. Based on these values, a photovoltaic installation with a rated output of 7.5 kWp is recommended. However, many users today choose a system with an output of 9.9 kWp, regardless of their consumption, so that they’ll also be prepared for potential additional consumers in the future, such as an electric car and/or a heat pump. As of a generated amount of 10,000 kWh, 40 percent of the EEG levy (currently €0.02/kWh) has to be paid. Guaranteed 20-year feed-in tariffs also play a major role, because they ensure that in the winter, users will profit from the electricity they fed in during the summer and in this way achieve greater financial autonomy.

Dimensioning the Junelight Smart Battery

Useful energy consumption is also used as the basis for dimensioning the Junelight Smart Battery system. Once again, let’s use the figure of 5,000 kWh per year from the example above for our calculation. Because most customers don’t know the distribution of their energy consumption, we'll use an approximate consumption distribution. The first step is to calculate average daily consumption: 5,000 kWh/365 = 13.7 kWh/day

Because we’re assuming that our night-time energy consumption is the same as our daytime consumption, we arrive at a night-time energy requirement of 9.6 kWh. This should also represent the minimum capacity in our example. To calculate more precise values, we recommend that you read your electricity meter at 5:00 pm and 7:00 am for at least seven days. The difference will give you your approximate night-time requirement. For an exact measurement, use an energy meter.

In our example, we use a Junelight Smart Battery with a 9.9 kWh capacity. One advantage for you is that the Junelight Smart Battery can be upgraded in 3.3 kWh increments (up to max. 19.8 kWh) at any time. This gives you additional flexibility if your night-time consumption increases due to additional consumers, such as an electric car.

Summary

With an optimally designed installation adapted to your useful energy consumption, you can easily achieve a degree of autonomy of 60 to 70 percent. By adapting your consumption and reducing the load on your heating system – for example, using a domestic hot water heat pump that can be controlled via a smart grid-ready interface integrated in the Junelight Smart Battery – you can easily achieve autonomy of over 80 percent. By carefully over-dimensioning your photovoltaic installation (given the available roof space), you can already cover the remaining energy required in winter during the summer, based on the guaranteed 20-year feed-in tariff.

Released by Alexander Völker