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?

[FlossyThePig]

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.

[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.

[FlossyThePig]

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.

[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.

[cpas]

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.

[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.

[jeff m]

I thought domestic water heating was usually by transfer using a glycol closed loop.
Although swimming pools use direct transfer with thermostat controlled valves.
Could be different in UK as I'm in the US.

[Herbs]

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

Either accept the small benefits that come with renting your roof out or get a loan and pay for a solar system and then receive all the benefits IMO.

[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?????

[Herbs]

R500POP said:

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?????

Okay assuming that you have a gravity system heated by gas then

Average electric price is 12.5 per unit
Average gas price is 3p per unit

Therefore heating via electric is over 4 times more expensive. Now so long as you are always producing more electric than you are using on 3 out of 4 days then you will break even.

Factor in how often it will be overcast, using excess electricity so there is no spare to power the immersion and how often you will actually use all of the hot water that you have heated via immersion then you are giving yourself a very narrow window of opportunity to actually save any money.

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.

Good idea in prinicple but not in practice unfortunately

[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.

[Paul Drawmer]

This is a 3.88kW array plus a solar thermal panel.


I haven't fixed up my proper monitoring system off the inverter yet, but looking at the generation meter; in the 34 days it's been working, a total of 268kWh has been produced.

That's would run a 3kW immersion heater for 2.6 hours per day in October.

I will get paid £110 for that electricity. Which means that if I produce 12X that in a year the return on capital will be about 10%. It's only 1 month, which has been a very sunny October, so that's not a figure to base an investment on.

[Paul Drawmer]

Further thought. What you need is some way of only having the immersion on when there's a certain amount of power being produced, so that you only use the PV power.

It may be possible to have a link to the inverter to measure output and so switch the immersion, but as it isn't your inverter that might not be on.

Alternatively, you could have a small separate solar panel and use the varying DC output from that to switch the immersion via a suitable circuit. You'd need to do some experimentation to find the ideal switching point, but it would ensure that you got the most benefit from the PV power.

Edited by Paul Drawmer on Friday 12th November 08:28

[Herbs]

R500POP said:

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.

As I said it's a good idea in principle but in reality it will be too hard to put into practice.

The Electric doesn't get pooled - it's either used instantly to power your home or sent back to the national grid. The amount of electricity you are producing can vary instantly as can additional drains on your electric.

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.

If you are retired and want to sit by the generation and export meter and make constant decisions on whether you are maximising the PV electricity produced then it "could" work but in reality - it will end up costing you more purely because you are knowingly increasing your consumption and using the most expensive way to it.

If you really want to maximise it, monitor it for a month or 2 - get an average production between certain hours and then in that time plug in a power bank, or electrical batteries to store the power you are generating and then use that to lower specific items in your home.

[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.

[phn]

Interesting topic this - have been thinking of getting a solar pv setup myself, but still not sure about the economics for an average sized house. It would take a long time to pay back the 10K + you put into getting the panels put up in the first place from the amount you get paid. But if British Gas are offering to put the panels up for free (and take the unsold energy) then I suppose it must be a good economic / business sense in the long run...

[Herbs]

average payback is 8-10 years but payments are guaranteed for 25 years - so easy way to double your money. Not to mention the price of Gas & Electric has gone up by another 7% with British Gas, whilst Scottish % Southern have put Gas up by 9%

Edited by Herbs on Friday 12th November 12:47

[inie meanie]

Hi what you want to do is right on the money. Dont listen to those people who obviously dont understand what you are trying to achive. What you really need is a box called EMMA. It monitors your output from the pv and compares it to the house demand. The difference it will then power an output for the balance. Ideally your hot water tank. have a look at www.coolpowerproducts.com I think it is expensive but it's cheaper than installing a water solar system. I have both water and a large pv system. I too am looking for a low power immersion heater that I can use to add a bit to the water when I am producing more than 3 KW which is virtually every time the sun is out. If I can then sort a cheap way of automating the action of switching on the heater when my system is a 3kw then I will be happy. Have you found a heater yet?

[R500POP]

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