Batteries for Our Solar Power System

We have had a major renovation of the roof, with an installation of a 40-panel Solar Power system. There are still some corrections to be done by the installation company later this fall.

We expect to convert some 13 MWh of solar energy to electricity per year.

Some of my friends have asked me if I plan to save the energy for later usage. The short answer is "no - too expensive and too little storage capacity". Generally, it is cheaper to sell electricity to the power grid and buy when needed.

Let's explore what it would mean to save energy and sell when it is more expensive:
Normally, the productive hours are between 10:00 and 16:00. On a particular day (2021-09-16),  the price fell to 1.35 SEK/kWh at 2 PM. If I stored 10 kWh (13.5 SEK) at that time, I would have 9 kWh in the battery, assuming 10% heat loss for the battery.


Later that evening, the price increased to 1.95 SEK per kWh. When selling that energy to the grid, I would get 17.55 SEK. That would make me 4 SEK. 

Higher price differences are quite rare, so up to 5 SEK per day is a realistic target during sunny days.

Luckily, the spot prices are set the day before, and it is possible to get a rough understanding on the weather conditions in advance, so it is possible to make some money. Is that money enough?

What earnings would I need to get an acceptable payoff for the battery?
A 10 kWh battery pack would cost me 125 000 SEK, including a 50% subsidy. That battery pack will last up to 15 years.  

To get the money back, I would need to make 8 300 SEK per year, or 23 SEK per day. Clearly, the price fluctuations won't make a battery pack profitable. The best battery would be selling and buying directly to the power grid.

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