Solar immersion vs gas water heating

CatRoberts

I have solar panels and the option of gas or electric to heat my water tank. If immersion is on in the dark (ie zero solar), the smart meter shows it costs 96p/hr to heat my water, approx 90 mins if we've used all the water up. 

I have a 4 month old viessman 38kw system boiler but can't get a smart meter for gas so have no idea how much I'm using hour-by -hour. 

Does anyone know how much on today's prices my boiler would cost vs immersion to heat the water tank? I am trying to work out the tipping point sunshine-wise when it makes sense to switch between the two. Eg today, my smart meter said 8p an hour to heat water, so in this sun clearly better to use electricity.

theoretica

If you don't use all the electricity you generate do you get paid for it?  How much per kWh compared to what you pay for a kWh of gas?

[Deleted User]

We use about 5kWh a day on gas to heat our cylinder, and about 3kWh on electricity. 

The IHD will only show an accurate cost if the correct tariff has been sent to your electricity meter. I am now reheating my cylinder using gas as I can get 25p+p/kWh for exported solar.

Rodders53

The per hour cost is meaningless in this comparison you are trying to do.

Your 'per hour' immersion cost tells me you have a 3kW immersion element and your unit rate is around 32p per kWh (EPG subsidised).

The crucial info is it takes around 4.5kWh to reheat and costs 32*4.5 = 144 pence 

As gas boilers are a bit inefficient and have some pipework heat losses then let's say you'll need 5or 6kWh of gas (at xx p/kWh on your tariff - lets say 10p/kWh = 50-60 pence).  If your unit rate is lower its cheaper still.

The gas boiler will reheat the tank in a fraction of the time the immersion heater takes.  IF all 38kW can be transferred by the coil in the HW tank it'll take 5/38 hours = 8 minutes.  (Also assuming both tank thermostats are the same).

Solar: cost is 'free' to heat when sun shines (on a deemed export tariff).  If you are fully metered for export... it depends on the export rate vs using it in the home (NB you'd need a proper solar diverter to heat HW at input rates of less than 3kW or you'll import electric if a cloud appears etc.,. if doing it manually - unless you have a very large solar array).

CatRoberts

Fantastic, this was exactly what I needed. So if my solar levels are at less than 35- 40p/hr, I'm probably good on electricity, as that's approx 50-60p a tank. If the smart meter is showing more than that, I'm better off turning off the immersion and switching on the gas. 
Of course, I could drive myself mad over 20p here and there, so I'll use rule of thumb set immersion timer april-oct and switch it off for winter.  

markin

If you can export at 15p on octopus , gas will always be cheaper.

ispookie666

@Rodders53
I doubt the gas boiler will be running at full capacity for all the time. It will probably take that much time to heat the water in the pipes to 60Cand then indirectly heating whilst modulation all along. 

CatRoberts

markin said:

If you can export at 15p on octopus , gas will always be cheaper.

I'm on the original feed-in tariff where they assume you use/export 50/50 and don't actually measure it, as it used to be such a negligible sum when the tariff was set up. 

[Deleted User]

CatRoberts said:

markin said:

If you can export at 15p on octopus , gas will always be cheaper.

I'm on the original feed-in tariff where they assume you use/export 50/50 and don't actually measure it, as it used to be such a negligible sum when the tariff was set up. 

In  that case using excess solar to heat your cylinder makes a lot of sense. Whether the cost of an EDDI or similar makes financial sense is a separate discussion.

Qyburn

CatRoberts said:

markin said:

If you can export at 15p on octopus , gas will always be cheaper.

I'm on the original feed-in tariff where they assume you use/export 50/50 and don't actually measure it, as it used to be such a negligible sum when the tariff was set up. 

On that scheme your use of your solar power is free.  Your FIT and export payments will be the same whether you use the electricity or whether you export it.