I'm a little rusty as i've not done this since Uni about 20 years ago, so sorry if it seems a dumb question. I have a solar thermal panel heating a hot water cylinder and i've always wondered how much energy it actually puts out in kW. The fluid (water) being pumped from the solar panel header to the coil in the tank is flowing at 600-800 litrers per hour or 0.22 Kg per second. the temperature of the water entering the heating coil in the tank is 55C and leaving the coil is 46C so a dT of 9C. CP of water is 4.2 so using Q=m Cp dt i get 0.22x4.2x9 = 8.316 KJ per second (as the flow rate was in kg per second) is this correct 8watts seems way to low as after about 3 hours its heated enought water for a couple of good showers. If left on all day on a sunny day the water is steaming and too hot to hold your hand under? I was thinking that i might need to multiply the answer by 3600 to convert to kW but that makes 29,000 which seems way to high, have i missed something obvious i was assuming the answer to be around 2000-3000W as a normal immersion heater is rated at 3kw and can heat a hot water tank in a couple of hours. Any help much appriciated.

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Hi welcome to the forum,

Your calculations are indeed correct. Here we haven't considered the efficiency of heat transfer and other losses in solar water heater. The heat from Sun is considered "low grade energy" compared to the electric heaters as electric heaters are more than 95% efficient they convert the electric energy to heat almost entirely. On the other hand solar water heaters have less than 60% efficiency.

According to Thermodynamics, heat is low grade energy and electricity is high grade energy. The efficiency of converting the low grade energy to high grade energy is always very low. (consider example of gasoline engines, steam power plants, etc)

On the other hand converting electricity to heat is almost 100% efficient process.