Heating hot water cylinder -gas or economy7?

tired_dad

We have a traditional low pressure system with cold water tank in loft.

we get cheap electricity 2330-0530 on octopus energy.

im trying to work out if its cheaper to use gas to heat the hot water tank ? Or is it cheaper to use the cheap overnight tariff? Has anyone worked out the calculations?

MWT

Depends on the exact numbers, but roughly speaking...

If you are on the IOG tariff with Octopus and getting your low-rate electricity for 7p/kWh then allowing for the losses on a gas boiler you are probably going to be better using electricity, but not by much.

If you are paying more than 7p/kWh for your electricity then odds are you are better using gas..

.

QrizB

Agree withe above.

You're getting a six hour cheap rate period which isn't Economy 7 (E7 gives 7 hours of cheap electricity, more if you're in Scotland). Hence the suggestion that you're on Intelligent Octopus Go, which is a six hour EV tariff.

Also, if your gas tariff is Octopus Tracker, you're currently paying around 5p/kWh which means there's even less chance of saving money with electric hot water heating.

FreeBear

QrizB said: Also, if your gas tariff is Octopus Tracker, you're currently paying around 5p/kWh which means there's even less chance of saving money with electric hot water heating.

But... You also need to factor in the energy that the boiler uses to heat up the water circulating around the system before it gets to the water tank. If the boiler is just firing up to top up the temperature rather than heating a full tank from cold, electric could well work out cheaper.

matelodave

FreeBear said:

QrizB said: Also, if your gas tariff is Octopus Tracker, you're currently paying around 5p/kWh which means there's even less chance of saving money with electric hot water heating.

But... You also need to factor in the energy that the boiler uses to heat up the water circulating around the system before it gets to the water tank. If the boiler is just firing up to top up the temperature rather than heating a full tank from cold, electric could well work out cheaper.

I'm guessing that just heating a hot water tank from a gas boiler is probably only around 50-60% efficient by the time that overall system losses are taken in account (boiler start-up, pump running, plumbing heat losses etc) v 100% efficiency for an immersion. Boilers are very inefficient when they start up - most of the heat goes out the flue until they have warmed up and settled down, thats why combi boilers aren't everso good for short draw offs of hot water.

I'd suggest that without doing complex sums, the break even point is probably around 10-11p/kwh for leccy compared to using a gas boiler at around 7p/kwh+50% = 10.5p/kwh.

debitcardmayhem

matelodave said:

FreeBear said:

QrizB said: Also, if your gas tariff is Octopus Tracker, you're currently paying around 5p/kWh which means there's even less chance of saving money with electric hot water heating.

But... You also need to factor in the energy that the boiler uses to heat up the water circulating around the system before it gets to the water tank. If the boiler is just firing up to top up the temperature rather than heating a full tank from cold, electric could well work out cheaper.

I'm guessing that just heating a hot water tank from a gas boiler is probably only around 50-60% efficient by the time that overall system losses are taken in account (boiler start-up, pump running, plumbing heat losses etc) v 100% efficiency for an immersion. Boilers are very inefficient when they start up - most of the heat goes out the flue until they have warmed up and settled down, thats why combi boilers aren't everso good for short draw offs of hot water.

I'd suggest that without doing complex sums, the break even point is probably around 10-11p/kwh for leccy compared to using a gas boiler at around 7p/kwh+50% = 10.5p/kwh.

But if on Octopus Tracker Dec24 my average gas is 5.33p for the last 3 months = 8p/kWh

matelodave

That's what's been said - you either guess or do the sums. It's down to the price that YOU pay for gas or leccy. It's nothing to do with the cost that I pay or anyone else on this thread pays.

If the OP gets his leccy for tuppence a kilowatt hour then he's obviously better off using electricity but if it costs more per kwh more than the cost of gas (taking account some of the inefficiencies) then use with gas.

If you get cheap gas then you need really cheap leccy. Its not really rocket science.

tired_dad

Hiya; great thoughts from everyone.

apologies im on Octopus Intelligent Go getting 7p per kWh from 2330-0530.

i also have solar. Was generating 5kwh today during the day.

while I get 15p/kwh feed in, I’m wondering about optimising my gas usage.

if break even is 11p/kWh seems better to sell to grid rather than divert to immersion heater

QrizB

Sell solar to grid for 15p/kWh.

Then decide whether to use off-peak electricity at 7p/kWh or gas at whatever-rate-youre-paying (you haven't told us that part yet).

matelodave

why not heat your water overnight with leccy at 7p/kwh and flog the solar at 15p/kwh during the day unless you are getting gas at around 4-5pkwh.

MWT

tired_dad said:

if break even is 11p/kWh seems better to sell to grid rather than divert to immersion heater

I certainly wouldn't divert solar to heat water unless you were on an old FiT with deemed export.