Cheap rate electricity immersion heating v gas central heating water, heating

davidkerr1956

Hi
We are in a quandary and we are hoping for some advice from someone far more knowledgeable than I, on a way forward 👨🏼‍🦳.My wife and I live in a 4 Bedroom, 5 year old detached house and run two fully electric cars. We currently are on Loyal Octopus 12m Fixed for the Gas element of the tariff. (8.75p per KWh standing charge 26.10p per day) We are on Octopus Go for our electricity ( 7.5p per KWh 00.30 - 04.30 & 30.83p per KWh all other hours, with a standing charge of 24.01p per day). Our current gas consumption is estimated 11680kwh whilst our electricity consumption is estimated at 2685 KWh, without the cars. We estimate the cars will add a further maximum 100kwh per month each (2400 KWh total, all of which will be incremental to the above estimate, but will be charged at 2400Kwh x 7.5p = £180). We currently have changed our ways of electricity consumption, by timing our washing machine and dishwasher to ‘do their bit’ during the off peak tariff times. We are aware of the fire issues and have all the necessary precautions in place.
My question is:
We have a thermostatically controlled immersion heater, so would it be cheaper to heat our water via the immersion heater if we time it to come on during the ‘off peak electric tariff’ or should we carry on using the combo boiler to heat water?

Please note: We currently have the gas central heating on, with water, but when the weather breaks, will turn the central heating off and either use the immersion or gas to heat water, depending on which is the cheapest way.
Sorry it’s a bit long winded, but I thought it best to lay out all the facts for one of you genius’s to give me the benefit of their knowledge
Thank yo in advance
David

MWT

Given the relative efficiency of gas vs electric then it will be slightly cheaper to use the immersion in the cheap period over night rather than the gas, but if you need to top up the hot water during the day then the gas is obviously better at that time.

With your estimate of 100kWh per car per month, that suggests very low mileage of around 3-400 miles a month for each of them, does that sound right?

davidkerr1956

Many thanks for your response, which is much appreciated. Yes, the mileage is about right. I may have misinformed you re the type of boiler that we have. It is an Ideal Condensing Boiler and not a combi boiler. One of my concerns about the water, is relating to the boiler which is say, already heating the radiators, does that mean that some of the energy is transferred to water heating, therefore the additional consumption is only incremental. 

MWT

davidkerr1956 said:

One of my concerns about the water, is relating to the boiler which is say, already heating the radiators, does that mean that some of the energy is transferred to water heating, therefore the additional consumption is only incremental. 

If your system has been plumbed in a conventional manner, the central heating water flow will bypass the hot water cylinder unless the cylinder stat is calling for heat.

So you do not gain hot water just because your heating is running.

As long as your tank is well insulated and is large enough to meet your daily needs without reheating then the electricity has the edge.

The only advantage you have with the gas is that you can use it any time of day at the same price, but that price is higher than your cheap rate electricity and the gap is widened once you allow for the inefficiencies in gas water heating...

lohr500

As advised by MWT, I would switch to using the off peak electricity to heat your water.

Even if your gas and electricity costs were the same per kWh, directly heating the tank with the immersion heater will be more efficient than indirectly heating using the gas boiler. Your off-peak electricity cost per kWh is less than your gas cost per kWh, so go for it.

Two things to watch though.

Make sure the time is set correctly on the immersion heater timer as well as the on and off times, and check the time regularly. If the clock time or on/off timers gets out of sync with your Octopus Go off peak time, you will then be using the expensive peak rate to heat the water 

I'm not sure if the Octopus Go times shift with the GMT/BST daylight saving hours time change, or if they remain on GMT. You will need to keep an eye on this when the clocks go forward by an hour at the end of this month and make sure that the immersion timer and the off peak period remain aligned.

davidkerr1956

Hi MWT & lohr500
Many thanks for you advice, which I have taken on board fully. It is so refreshing to receive some impartial and sound advice. If you wish to know about cars, I am your man but, energy related issues, you are the experts.
I thank you both so much
Regards
David

lohr500

I'm no expert but I am looking at a similar scenario myself now that heating oil prices have increased significantly.

I could use our August 2023 fixed off peak EDF GoElectric tariff to heat our hot water instead of the oil boiler. I did some calculations for another reply a few days ago and worked out that if oil cost me more than £0.85 per litre then it would be more cost effective to use our off peak electricity rate. Today's oil price is above £1 per litre. But I have 2/3 of a tank of oil remaining which I paid £0.56 per litre for.

So my conundrum is this : Do I keep burning the oil I have at £0.56 per litre to heat water as well as run the central heating, or do I save the oil for the heating and switch to more expensive (+50%) electricity for the hot water?  We have enough oil to see us through well into the summer.

My gut feel is that oil prices will drop below £0.85 per litre by the summer so I have decided to keep using our oil for both water and heating. Only time will tell if I made the right call !!  

MWT

lohr500 said:

I'm not sure if the Octopus Go times shift with the GMT/BST daylight saving hours time change, or if they remain on GMT. You will need to keep an eye on this when the clocks go forward by an hour at the end of this month and make sure that the immersion timer and the off peak period remain aligned.

'Go' and other Smart ToU tariffs from Octopus all adjust for the change between GMT/BST, so if the timeslot is 00:30 - 04:30 GMT then it will shift to 00:30-04:30 BST when the clocks change. 

lohr500

Good info MWT. Thanks.
OP just needs to make sure then that he adjusts the time on the immersion heater timer when the clocks move if the timer doesn't automatically adjust for GMT/BST.