For any standalone photovoltaic system, a battery is essential, because electricity is most needed when the sun is not shining. With all the many battery types around, lead acid type batteries still appear to be the best choice for a low-tech system in a harsh environment like a garden house.

First, we need to understand that the cycle life of lead acid batteries strongly depends on the degree of discharge (DOD) per cycle. I.e., a battery rated as “120 Ah” would be only discharged by x% of its total capacity, and last much longer than with a full discharge every cycle.

This is the battery we are talking about – quite impressive packaging, no wonder, because there is liquid sulfuric acid inside!

It is a N120S NRG Solar 12 V, 120 Ah, “Deep Cycle” battery. About EUR 75 if you buy it a the right spot.

Some average life cycle data vs. DOD% for lead acid batteries.

With cost, DOD%, etc, you can easily calculate the total energy stored over the cycle life of the battery. Time-wise, the battery can last about 8 years, at least according to the manufacturer’s data. So, with about 100 cycles maximum per year, one cycle per day (unless earth rotation will pick up dramatically…), no particular useful life beyond 800 cycles.

With these data, it is quite clear that for the type of battery brand I am using (which is not the best most expensive super grade solar battery), running at 20~40% DOD will give quite OK life time expectancy, and cost even lower than domestic/grid power here in Germany, at least with the paid-off solar cells and charger.

Sure, measures will need to be taken to avoid full discharge of the battery in winter, e.g., by disconnecting all major consumers, and just allow for some LED or fluorescent lights, and maybe, a few cellphone chargers.