Can't get a straight answer Immersion heater

tomsolomon111

I have read dozens of sites and still can't get a straight answer on this. How much does it cost to run an immersion heater full time?

Let's say for one minute money is no object, I have heard it costs 50p per hour to run one of these with a 125l tank and a 3kwh heater element. Others say it switches on every 12 minutes for 6 minutes which would mean it was on for at least twelve hours a day. With both of these scenarios the usage would be, with one bath a day, approximately for the 50p hr £84 per week and for the on 12 out of 24 at my current tariff of 13.37p ( taking into account it supposedly takes 1h to reheat the water after a bath) 13h x 3kwh x 7d = £36.50pw

I know I'm not using £36pw and definitely using less than £84pw.

Everybody has advice on how to use these things, but never have I found any actual figures on how they work and how much they are using. Is there some conspiracy to stop people know the real fact about these things, or do people really not know. If you reply to this post with why don't you just turn it off when not using it. I will ignore you! :rotfl:

WobblyDog

According to table 13 in this document, a hot water water cylinder should not lose more than a particular amount of energy in 24 hours:
http://www.planningportal.gov.uk/uploads/br/BR_PDF_PTL_DOMHEAT.pdf

According to my calculations, assuming the price of a KWh is 13.37p:

If you don't use any hot water, the number of KWh needed to keep a 125 litre tank warm is:
1.28 x (0.2 + 0.051 x (125)^(2/3)) = 1.89 KWh per day
giving a weekly cost of £1.77

If you take one 100 litre bath per day, and the hot water is at 65C, and the imcoming water is at 15C, and the specific heat capacity of the water is 4200 joules per kilogram per degree C, the number of KWh needed for the baths is:
100 * 4200 * (65-15) / 3600000 = 5.83 KWh per day
giving a weekly cost of £5.46

Combining the cost of baths and heat loss from the tank,
the total weekly cost is £7.23

The worst case scenario is having a permanent queue of dirty people wanting baths. With a 3KW heater, the weekly cost would be £67.38.

I don't think immersion heaters will switch on more than a few times per day if no one is using hot water. There is some "hysteresis" built into the thermostat.

The energy required to keep the tank warm might be a slight underestimate, because it doesn't take into account loses through the pipes attached to the tank.

CashStrapped

tomsolomon111 wrote: »

Is there some conspiracy to stop people know the real fact about these things, or do people really not know. If you reply to this post with why don't you just turn it off when not using it. I will ignore you! :rotfl:

Why does everything have to be a conspiracy these days. If we have a conspiracy about immersion heaters then we really are at rock bottom.

What you need to understand is that there are so many variables, everyone will give a slightly different answer.

How much hot water do you use? How well insulated is the tank? What is the temperature of the mains incoming water? What setting is on the immersion thermostat? What is the power of the immersion? What does your electricity cost per kwh?

All these things can affect the cost!

So the cost can range from as little as £9 per month to £54 a month if not higher. It all depends on use. The basic calculation is

3 (power of the immersion in kw) x 60 (hours immersion used in a month)

= 180 (this is the total in kwh)

180kwh x 0.1 (cost of electricity in £) = £18 (per month cost)

I used £0.1 as the cost of electric in this calculation, it can vary a lot.

You can use whatever figures (based on the hours you use it) you wish and get a different result.

That is why you will never get an exact or similar answer from each person you ask!

If you still do not trust our answers ...watch this video, it may help

https://www.youtube.com/watch?v=obWkCGFhuus

tomsolomon111

Awesome answer, thanks wobblydog. people would have us believe that these things literally eat electricity. This is quite clearly not the case. It is nice to get some approximate real figures on this so that it can be figured into calculating usage.

"The energy required to keep the tank warm might be a slight underestimate, because it doesn't take into account loses through the pipes attached to the tank."

I would imagine the loss to be negligible, short of say, a water leak or bad insulation on the tank.

matelodave

You can reduce the amount of heat lost in the pipework around the tank by putting insulation over them.

What wastes energy is running the cold out of the hot tap until it runs hot - not only have you wassed a gallon or more of water but you have a gallon or more of hot water sitting in the plumbing getting cold until someone does it again. Multiple short run-offs for rinsing your hands or other stuff wastes more hot water than actually filling a sink, basin or bath.

Reduce your energy consumption by using all the hot water in your tank rather than keeping it hot all day - the hotter the water the more heat is lost so there's no advantage in keeping you tank hot all night when you are asleep or even during the day when you are out.

Richie-from-the-Boro

tomsolomon111 wrote: »

Awesome answer, thanks wobblydog. people would have us believe that these things literally eat electricity. This is quite clearly not the case. It is nice to get some approximate real figures on this so that it can be figured into calculating usage.

"The energy required to keep the tank warm might be a slight underestimate, because it doesn't take into account loses through the pipes attached to the tank."

I would imagine the loss to be negligible, short of say, a water leak or bad insulation on the tank.

How much it looses is a function of insulation. Example a PartL cylinder will lose 1/2 as much as a non PartL cylinder, ditto the pipe run ≥ insulation.

Nellybee

i have a gravity fed central heating system the way its set up is i have to have hot water and central heating on at the same time in winter.In the summer i have the gas heating the hot water for 1 hour a day we dont use a lot of hot water .So would using the immersion in the summer be any cheaper than gas , i also have solar so this would offset the cost a little, i have looked into solar immersion but cost wise and getting back the outlay was to me not worth it.:beer:

Gloomendoom

Nellybee wrote: »

i have a gravity fed central heating system the way its set up is i have to have hot water and central heating on at the same time in winter.In the summer i have the gas heating the hot water for 1 hour a day we dont use a lot of hot water .So would using the immersion in the summer be any cheaper than gas , i also have solar so this would offset the cost a little, i have looked into solar immersion but cost wise and getting back the outlay was to me not worth it.:beer:

Too many variables to make a call. You will need to do what I did and take readings one year using gas and the next using the electric immersion heater.

In my case the immersion heater cost less than half the price of gas. We do have E7 though.

Cardew

The efficiency of an immersion heater is 100%, and that is about all that can be stated to answer your question with any certainty.

If the water in your tank is cold it will initially use electricity at the rate of 3kW(40p an hour at the price you pay) for however long it takes to heat the water to the temperature you set on the thermostat.

When it reaches that temperature, the only consideration is the heat loss from your tank and pipes, and of course how much hot water you use.

The loss from a modern well insulated hot water tank is surprisingly low - often in the region of 1kWh every 24 hours.

Cardew

Gloomendoom wrote: »

In my case the immersion heater cost less than half the price of gas. We do have E7 though.

Shouldn't that be the other way around? i.e. gas costs less than half the price of an immersion heater using E7.

Gas costs around 2.5p to 3p/kWh.

E7 electricity costs 5p to 7p/kWh

Admittedly you have to factor in boiler efficiency, but gas will always be cheaper than E7 electricity. and with an E7 tariff you pay more for daytime electricity.

prosaver

my parents use too go mental if the immersion heater was left on, like if it was costting about 20 pound a hour ..ha ha