I have an 80 litre tank and even during the winter in less than three hours it's maxed out the temperature. I'm surprised you can't heat a 50 litre tank overnight.

There are heat pump hotwater heaters now. Expensive IMHO but if you're using a lot of hotwater they supposedly pay for themselves in three or four years.

If you have the space a solar tank would use almost no electricity.

 

The power of the heater (watts) does not impact the cost of operations. It only affects the time required to heat the contents of the tank. So, The main issues really come down to: How much hot water do you use in a day? and How long can the tank maintain the water at a temperature hot enough for your needs?

However, there may be a third issue. A tank designed to heat and store is typically designed so that water entering the tank as the hot water is drawn down does not significantly lower the temperature of the water in the tank. This is usually accomplished by having the "tank" actually be composed of long coils of tubing rather than actually being a single tank. Regular tank water heaters address this issue by being thermostatically controlled and re-heating the water periodically as needed.

So, if your family only needs 80 liters per day, then the problem is either because: 1) the tank is either not sufficiently well insulated, or 2) it is allowing incoming cold water to mix with the hot water and lower the temperature. My guess is the second case is the cause of your problem, you have the wrong tank for your application and have hooked it up to a time-controlled switch preventing it from operating as intended.

If your needs are really well below 200 liters per day, then you need to figure out how much energy you are burning to heat the extra water. That information (watts per liter) should be available from the vendor. Then, you can decide if the savings merits changing to a smaller storage tank, possibly of another technology as NickZ noted. My guess is you are best off to just leave things as they are and keep the 200-liter tank.

 

Disconnect the cold water feed to the smaller tank and replace it with a hot water feed from the large tank. You now have a 250 litre hot water supply each morning. You could switch off the smaller tank heater as it really shouldn't be needed. As someone else said a solar tank would do much the same thing and installing a solar hot water panel on the roof would just about eliminate your electricity bill. Final option is to install solar photo voltaic and get rid of the electricity bill that way.

 

Question-What sort of tank is it? Is it a properly insulated french ballon or is it nothing more than a copper tank with an immersion heater?

 

Scaling down to 50l from 200l sounds pretty drastic to me!

If the 50 litre boiler is to be installed on heures creuses (night tarif) then there'll be hot water for the early riser(s) and a cat-lick for others.

A 200 litre boiler is designed for a family of 4. You could downsize to 150 or 100 litres before going the whole way to 50l

When our 200l went kaput, I bought (around 400€) and installed an ultra flat 80 litre Atlantic Linéo. I fixed it horizontally over the washbasin right up against the ceiling. It's connected permanently - no day/night tarif switch - and you can program it for one, two three or four showers - put it on sleep mode when you're away and even wake it up with wi-fi on the way back home. No noise (buzzing, bubbling, hissing) and the space liberated by the 200 litre cumulus was more than welcome in the tiny 6m² bathroom.

 

Is this an upmarket instant hot water system? If so it certainly beats heating up a tank full only for it to go cold and for you to heat it up again

 

No, it's not an instant system.

It exists in 30, 50, 80 or 100 liter capacities with two heating elements in two separate boilers.
We programmed it to heat water for two shower capacity when just us, or for full four shower capacity when the house was full.

The English version is called Vertigo
https://www.atlantic-comfort.com/Our-products/Water-heating/Electric-water-heaters/Vertigo