It takes about one joule to heat a cubic meter of air by one degree.
So you need around 22,500/3,600x(50-10) = 250 W.
But you need to consider many factors, mainly insulation for avoid heat loss and air humidity. . gas. Therefore, it is more reliable to use 3000 watts of heating.
The hot water flow requires knowing the temperature difference of the hot water. Since the efficiency of the heat sink is difficult to calculate, the temperature difference is not easy to calculate accurately. But it doesn't have to be completely precise. I guess it would be better to be more relaxed.
The key to your energy efficiency is insulation.