During heating a cover is absolutely recommended. Without a cover the heat losses are highly dependent on the outside temperature. To remember when heating where gas is used, the energy content of 1 kg of gas is approx. 14 kWh. When the heater is running at maximum, gas consumption is 1.6 kg or approx. 22 kWh and about 90% efficiency. To heat 1200 liters of water from 6 -37 degrees will require approx. 50 kWh and will require 2½ hours of heating time.
If there there is frost on the bottle that shows that the gas is getting cold and it is mainly be due to two things if all controls are set to MAX position.