Heat loss of swimming pool with and without pool cover

Now I have accurate temperature sensors in swimming pool and I can find out how much of heat is lost daily.

I took some readings during the evening and night with and without pool cover. I don't have decent readings for the whole day, but now I have some kind of knowledge how much heat is lost in the pool.

The reading with pool cover had higher start temperatures. What is the biggest difference is that with pool cover the top and middle of pool water stays almost the same but the bottom starts to cool down as fast as without cover.

I have three sensor, bottom, middle and top and I estimated the average temperature to be (bottom+2*middle+top)/4.

With cover the heat loss was -0,25C and without cover -0,41C. Interpolated for the whole day, the figures would be -0,35C and -0,59C.

My pool is small, 2,8m * 5,7m * 1,3M which means there is 20748 liters of water. Warming that amount of water +1C takes 24,11kwh energy. So warming up heat loss with cover takes then daily 8,67kWh and without cover 14,23kWh.

My house is heated with geothermal heat pump so I don't use that amount of actual energy. Heat pumps have COP (coefficient of performance ) meaning for instance COP 2 gives me 2kWh of heat energy when electricity is used by pump only 1kWh. COP of heat pumps are typically between 2 and 4 but I don't know what is ours. I'll make an educated guess that it's 2,5, so the energy used is with cover 3,69kWh and without cover 5,69 kWh.

Energy prices in Finland are roughly 0,1e/kWh so daily price would be 0,45e with cover and 0,57e without cover. Yearly those figures will be 126e vs. 207e.

Of course there will be a lot of variables in here. It's now autumn and when it gets colder in winter, the ground will cool pool more. The heat loss from the top is mainly by evaporation. Turning liquid water into gas takes heat energy from the pool. In winter humidity is low and more water evaporates from the pool, cooling it also more. It will be interesting to see what will be the heat loss then.

Using pool cover nearly halves the use costs but the yearly costs are not very high. There are other positive things with pool cover like keeping the humidity at decent levels in pool room. I was thinking about buying a motorized pool cover but with these energy costs it will never pay for itself.