Our Tesla Powerwall and Octopus Tesla Tarrif experience and a question

Matblack

Hi everyone 

I posted back in February about our plans to have solar and a Tesla Powerwall installed. 

We are now up and running, signed up to the Tesla synchronous tariff, where we pay just over 11p/kWh for import from the grid and the same for our export.

This is quite frankly an amazing deal for us, it has its drawbacks if you don't want Elon playing Big Brother with your Powerwall and using it to make money balancing the grid with it from time to time, but the convenience outweighs that for us.

Because of the good weather recently we've been completly self sufficient and made a few quid back for cloudier times.

The Powerwall powers the house overnight and until the sun rises when it's down at about 50% before the solar overtakes our needs and and stats to refill it.

Bottom line is we love it. 

But I have a question which I'm experimenting with at the moment.

Given the price of electricity on the Tesla plan is it worth changing over to heating water with the immersion heater rather than the boiler?

Traditionally the answer seems to have been no give the difference between kWh prices of gas and electricity BUT given the much lower difference on the Tesla tariff would one be better than the other?

On an average day we've been using 8kWh of gas. Octopus very kindly tell you in cubic meters and kWh so comparison is easier. 

I believe there is a disparity in efficiency between a gas boiler and an immersion heater too. 

Do I use my solar to heat water, thus reducing my income or paying in winter at the low level of 11p/kWh or use gas at about 7p/kWh? There is an environmental argument for switching to solar too. 

Right now I have a CT clamp on the immersion to get a clear picture of additional power draw and gave turned the gas hot water off. I've been very frivolous and switched the immersion on 24/7 😲 just to start to get a picture. The tank is one of the closed system ones and pretty well insulated but obviously its not economical and we'd get a timer fitted or find a way to get the Hive system to control the immersion.

Id be really grateful for and insight or experience shared on this topic 

Thanks in advance 

QrizB

Matblack said:

Do I use my solar to heat water, thus reducing my income or paying in winter at the low level of 11p/kWh or use gas at about 7p/kWh?

Assuming a relatively modern boiler at 85% efficiency, there's not a lot in it right now.

• 8kWh of heat @ 85% efficiency = 9.4kWh of gas @ 7p/kWh = 66p.
• 8kWh of heat from electricity @ 11p/kWh  = 88p.

That's an extra 22p/day at current prices.

If you're on a variable tariff, there's an expectation that gas will rise to 11p/kWh from October at which time electricity will work out cheaper for you. (It'll be cheaper for space heating, too, which will be odd. Are there any limits on how many kWh you get at 11p?)

EcoScruples

I can't give you an answer on this but I can share my experience and what I plan to do.
I use my oil fired boiler to heat my water during the spring/summer time so about 180 days and worked out i use about 1 litre of oil a day.
If I use my Eddi (which isn't installed yet so I'm turning my immersion on for a few hours a day) I will save around £200 per year at current oil prices.
Makes no difference in the autumn/winter as I run the heating any way.

That's about a 2.5 year payback with the extra benefit of having more oil available for heating and less hassle in topping up at the wrong time of year.
Also saving the environment somewhat from more fossil fuel being burnt.

I worked out that I could just sell my export for 7.5p to Octopus and make enough to cover the standing charge which would amount to an average of 5kw export a day.

I'm going for the saving of oil route until I know a full years export and Eddi usage.

Gas of course is much cheaper than oil so you maybe able to stay that way but you also need to know how much you export in a year to make that decision I guess.

BUFF

Matblack said:

Do I use my solar to heat water, thus reducing my income or paying in winter at the low level of 11p/kWh or use gas at about 7p/kWh?

Bear in mind that come October (depending upon what tariff you are on) gas is probably going to be of the order of 11p/kWh...

gefnew

When you had the system installed did you not consider a diverter to feed the immersion heater to use spare solar power.

Matblack

It's a definite possibility going forward, you aren't supposed to use a diverter with the Tesla tarrif but frankly I don't know how they would know if one did.

I'm not quite sure what the advantage would be, with a synchronous tariff wouldn't one might as well export the excess to the grid and buy it back later if the Powerwall is exhausted.

arty688

gefnew said:

When you had the system installed did you not consider a diverter to feed the immersion heater to use spare solar power.

I think the general consensus ATM is on purely financial terms  a diverter is a waste of money for most people. The spare solar can also be sold so you have to compare export price vs gas price . Things could change in October , but gas prices probably will go down next year but who knows.

QrizB

gefnew said:

When you had the system installed did you not consider a diverter to feed the immersion heater to use spare solar power.

Diverters are pointless and a complete waste of money if you've got a Powerwall and are on the Tesla tariff.

(There might be a very niche case for one if you've got a big PV system and a DNO limitation on your export.)

[Deleted User]

In my experience, it is not a simple gas versus electricity efficiency question. A gas boiler has to transmit heat via a pipe so there are pipe heat losses to take into account. FWiW, when I re-heat my cylinder with gas I use over 7kWh a day compared to less than 4kWh a day on electricity. Pipe loss doesn’t matter as much in the Winter as any lost heat contributes to home heating.

Matblack

Well that's a turn up

A whole 24 hours on immersion being on constantly, including a bath and a couple of showers and the result? ~5.5kWh.

On gas with two 1 hour stints early am and afternoon the average gas usage is around 7kWh.

This isn't a very accurate experiment there's minimal data and a lot of variables but it would seem that the costs are pretty comparable with gas at 7p and electric at 11p

We need to bear in mind that today was a very hot day and the differential between the water and the ambient tempreture is much smaller than in winter so the heat loss is reduced and the thermostat will be triggering less. 

I've ordered a fused timer to replace the standard fused spur so we'll be able to set a couple of stints to power the immersion. During these times the draw will be more because of increased cooling time in between  stints and temp drop.

Logic and received wisdom says the timer will reduce the running costs, time will tell!

Dollar, I like your thinking, unfortunately our boiler and tank are in the loft such means net contribution to the heat of the house is minimal, although all the pipes are well insulated.