It takes about 4.5kWh to heat 100 litres of tapwater
So your cylinder should take around 15kWh to go from cold to hot in one go. Less on subsequent times when it is already partially hot.
Did you use any hot water during the day?
Did you heat it in one go then turn off?
How many radiators were on?
Is water running out of the Megaflow relief valve?
It is much easier to measure HW usage in summer when no heating is on.
It would be very unusual to use 300 litres of hot water in one day unless you like American style showers or have multiple hot baths. Not impossible.
Central heating can use much more gas.
So your cylinder should take around 15kWh to go from cold to hot in one go. Less on subsequent times when it is already partially hot.
Did you use any hot water during the day?
Did you heat it in one go then turn off?
How many radiators were on?
Is water running out of the Megaflow relief valve?
It is much easier to measure HW usage in summer when no heating is on.
It would be very unusual to use 300 litres of hot water in one day unless you like American style showers or have multiple hot baths. Not impossible.
Central heating can use much more gas.