Solar PV Power Immersion Heater - Have I Lost The Plot?

[R500POP]

We are having a 2.5kw solar system fitted under the rent you roof scheme through British gas as it seems we have nothing to loose & a small amount to gain (typically over a year the system will provide 50-65% of your electricity requirements).

Anyway, this got me thinking about the manner in which we heat our water. Currently we use gas, set twice a day, once in the early morning, once mid afternoon.

I was thinking that if we did the early morning water heating using the gas & the mid afternoon using the solar electricity, the problem being the immersion is about 2.6kw, using more than the cells will produce on a perfect day.

So my question is - can you get l"low power immersions (say 500-1000w) that would enable me to heat the water using the immersion whilst leaving some power in hand?

[R500POP]

FlossyThePig said:

If you want the sun to heat your water do it directly. Fit a solar powered water heater on the roof at the same time as the PV array, it will be much more efficient. Even more savings if you can fit a much larger tank.

Have you checked out all the options with PV arrays. Most companies are renting roof space to put up their arrays and I believe are getting all the money from the government for all the power generated, not just any surplus that goes into the grid that you get a percentage of.

We are going through British gas, who do get all of the monetry income yes, but only from power we are not using that's going "back to grid". We get to use as much (or as British gas hope as little) electricity as we like, any surplus gets sold by British gas & they get the money. Hence the reason we are looking into heating the water during the day, when the house is empty, using electricity.

The cells will produce up to 2,52kw of electricity, if I can get a 1000W (1kw) immersion it means we have some power in hand and could hopefully heat the water for our evening usage for free.

Obviously all of the above is pending me studying the power the PV cells produce during a "normal" (read dull) day & ensuring that our household comsuption (fridge/freezer etc) during the days the house is empty is at least 1kw less than the power the cells are producing.

Then, using 2 7 day timers (one on heating control, one on immersion control) I can sync everything so at the weekends when we are in we use gas to heat the water as normal, and during the weekdays when we are at work we use electricity to heat the water.

[R500POP]

FlossyThePig said:

I don't know the state of funding at the moment but:

heatmyhome said:

This government incentive pays you 41p per Kwh produced. This will be paid even if you use the energy yourself, giving a guaranteed income and savings on your bill.

You are going to have to be clever with your power distribution to make sure fridge, freezers, etc. get power when needed rather than your immersion heater.
Heating water directly by solar power is more efficient than your proposal and independant.

Yep, that's the fun part.

If on a normal day with the house empty we have 1kw "in hand" then it's all good.

[R500POP]

cpas said:

Let us all know how you get on. I'm considering such a system as you can get them installed FOC as long as the provider gets all the surplus energy. I'm sure you can get lower wattage immersion heaters or even external pump fed heaters, but I would also agree that a direct system of heating the water would be better. Definitely worth looking into.

More efficient yes, but when the electricity is free & I'd have to pay for solar heating, using the energy from the PV cells is cheaper since British Gas are getting the income from the PV cells I figure I may as well use as much of it as I can.

[R500POP]

Herbs said:

Don't bother doing it. Heating water via electricity is one of the most expensive ways. What happens on a very overcast day when it isn't producing much electric and you've got your immersion heater running non-stop?? - it'll be a false economy

If I have 1kw spare on a dull day when the house is on tickover where's the expense?????

[R500POP]

Herbs said:

Not to mention that any excess electricity is sold back to the energy company for 3p per unit (same cost as gas) then it will cost you more to use an immersion.

But that 3p per unit is going to British Gas, not us, so where is it costing us money?

Did a test last night, with the house dormant, fridge/freezer on, it draws about 500w, so I just have to wait & see how much output we get from the cell of an overcast day now.

[R500POP]

Herbs said:

Eg: your missus puts the dishwasher or electric oven on. This then uses any spare electric that was being used to power the immersion. You are now paying for having the immersion on. Having it on for 1 hour as just cost you the same as having the boiler on for 4 hours.

That's the thing, the house is empty between 8am & 5pm, so that's pretty unlikely.

[R500POP]

http://www.howden-electroheating.com/ make a 1kw immersion. For about £65 they will even make a bespoke unit.

[R500POP]

caziques said:

I'm not up to speed on these feed in tariffs, but it would seem a bit stupid to use electricity which you get paid some stupid amount for - to replace gas which is vastly cheaper.

Try to keep up chap!!!!!

I'm using the rent a roof scheme, so i so I get first refusal on any power generated, the "overspill" goes to the grid & British Gas get the FIT. Hence why it makes sense to use as much of the power generated as possible.

[R500POP]

The panels went in OK yesterday, all finished just as the sun was starting to set.

When I left the house at 6:55am this morning we were seeing 0.1kw being produced, so at least they are working.

I'm now going to keep an eye on how things progress.

[R500POP]

That's the thing, the wife is at home 2/3 days a week, plus weekends, so we'll get the benefit for 4/5 days a week.

Things like dishwasher, washing m/c etc will be on during the day rather than at night.