Heating your water: False economy tariffs?

Can someone help me with some maths?

Out of curiosity, I rang EDF to find out if heating your water ALL NIGHT on the cheaper rate was a false economy.

  • Day rate: 13 centimes per unit

  • Night rate: 9 centimes per unit

One unit is 1kw.

It takes 2.5 kw per hour to heat the tank.

So, how much do you spend if you:

a) leave the water on for two hours on the day tariff

b) leave the water on for 8 hours on the night tariff

By my calculation I have three red apples and two green ones.

Can anyone help me with this please?


Indeed it does have a thermostat.

Interested in how much it costs to heat 300 litres from cold.

I like your test idea... will certainly give that a try!

I stand to be corrected, but the water surely is not heating all night? You must have a thermostat? Our water cylinder holds the water under pressure, so takes very little to heat, depending of course on how much is used each day. What I did was take a meter reading before the cheap hours kick in, and after we have turned everything else off for the night, then read the meter again the next morning. Aside from the fridge, that tells us how much we used on heating water. Hope this helps.

They didn't say it's a false economy. I am trying to work out for myself if in fact it would be cheaper to put the water on to heat for two hours in the day time - rather than for 8 hours at night time.

But i'm curios to know why they said it's a false economy and why you would need to heat your boiler longer during the night then during daytime to achieve the same output temperature. BTW, the day-night system is only cheaper if you really and systematically run all your "heavy household machinery" (washing drying, dishwasher, boiler, ironing but also these heavy, heat-storage system to warm up your house) during the night. And even then the savings made are in the area of 50-75 Euros p/a. and your house is resounding from al that machinery in operation..... :-)

O. So there's much more to it than before... good grief!

How many liters are we talking here? What is the actual temp to be achieved and maintained in the heater? What is the temp of the water you'll start with?

You need about 4.19 joules to heat one gram of water 1 degree. 1 kw/h equals 3,600 kjoules. so with 1 kwH you can heat approx 895.188 liters of water 1 degree, or 29.6 liters of water from ambient (21C) to 50 C or 22.6 liters of water from 12C (tapwater) to 50C.

So you can do the rest of the math (I'll go for a green one please)

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