# Question regarding PV-Investment and feed-in Tariffs

Hello everyone,

I do have a question regarding the optimisation of solar power systems and feed-in compensation.

If I define one sink as “excess energy” and define the variable cost negative, OEMOF does take the negative cost into account for the optimisation. In my scenario I took the hourly energy consumption of a three-person household spread out over a year. If I let OEMOF run the optimisation and defined the PV-sink (invest=1300, lifetime=20, wacc=0.038) as well as defining the “excess energy” (nominal_value = 567, variable_cost=-0.0816) , OEMOF wants a PV with a power of 567 kWp.

Which is definitely to much, a test with online tools suggested around 6 – 10 kWp. Is there anyway to get a solution around this power without completely removing the nominal_value of the “excess energy”?

If you just want to optimise your house system you could set the variable costs of the excess energy to zero. In that case the system will feed into the grid only if there is energy “left”. You can calculate the feed-in gain afterwards. Another variant is to use a “normal” component and not an invest component and vary the the nominal value from e.g. 5 to 15.

Hey thanks for the replay.

if i put the variable costs of the excess energy to zero. OEMOF calculates the power to 0.6 kWp which feels a bit too low.

If 0.6 kWp is enough to solve your problem there is no reason to install more because it is the cheapest solution. If you can sell you electricity with 8.16 ct the cheapest solution is to install as much as possible.
I do not know your exact research question but a solph model is a mathematical model and it will find the optimal mathematical solution. So you have to design the model so that the best mathematical solution is also the best solution for your research question.

You may have to set additional borders or constraints. Solph will help you to define the mathematical model but it will not help to design the model.

2 Likes