So now I have a solar PV system how do I make the most of it???

Cardew wrote: »

The economics of this surely cannot make sense and of course it rules out those who have a combi boiler.

£999 invested in a long term savings account will produce presently £46 (£37 after tax)

If you have gas, the 1kW immersion heater has to run for approx 1,000 hours a year just to break even with the lost interest.

Then if you manage to get the immersion to run for another 1,000 hours a year(2,000hrs in total) and save a further £37 the system will have paid for itself in 27 years!

Of course systems like this never need repair!!!

Discuss!!

At £999, I agree it's not a good investment. However for cheaper systems (eg £240 plus fitting of around £160 as quoted in one of my links above) the economics look considerably better.

Our summer usage of gas (which is for twice-daily water heating only) averages around 430kWh/month. The timer program stays pretty constant through the year, so around 5160kWh is used annually on water heating. For each kWh that we can replace gas consumption with electricity usage (generated by PV), the saving would be 3.51p (inc VAT and 6% discount) for us on our tariff.

It would therefore take 11400 kWh of gas savings to break even, unless you can get it fitted cheaper.

It's quite speculative to estimate how long it would take for the panels to dump excess energy into our water tank at a rate of 1kW, as it's very dependent on the season (and the current-switching threshold you choose). However on any given sunny day irrespective of the season, we'd see some benefit (even if it means in the evening the boiler fires up for less time in order to reach the thermostat setting). In summertime when the sun rises very early here in cider country, the morning boiler program should be shortened or avoided too.

Lets guestimate that over the course of a year, our water heating bill was cut by 1/3 through this device (1720kWh, or £60 at current rates) - which is a quite conservative estimate I think. On that basis it would take around 7 years to break even, less if the upfront cost was lower or the estimated savings were too conservative. Increasing gas prices would also bring forward the break even point.

If the upfront cost were saved in a long-term savings account instead, then compound growth over 7 years would be about 29%, so it would take another couple of years to be truly break-even. Of course if the actual savings are higher, the break-even point is shortened. After the break-even point, the gas savings considerably exceed what your savings pot could earn.

Such a system would require far less maintenance than a solar thermal system, perhaps a new immersion element (at £50) but at what frequency I don't know?

As Smart Meters are rolled out, and the deemed 50% export assumption is replaced by actual measured export (so I hear), then the above calculations would need updating. This would be necessary to account for the marginal saving between export rate (3.1p currently) versus Gas cost (3.51p currently), as opposed to the current situation where the whole Gas cost is saved for each kWh used in the house (because export is deemed so you get paid the same FIT income regardless). This would dramatically lengthen the payback period, but I do wonder when we'll actually see widespread roll-out of smartmeters.

I also wonder if the above calculations could be refined to consider the efficiency of boilers against an immersion element, given that there is no need to pump the water between the boiler and tank and no heat losses from the pipes inbetween. In theory then an immersion element may be more effective, kWh for kWh, than a gas boiler.

r.cordrey wrote: »

Your logic and mathematics are sublime - if we are comparing like for like.

I would not pay £999 for the installation: the current-sensitive power relay costs about £150 and the wiring is not onerous (supply companies want to make a profit).

Unless you have a very large solar array the utility companies will not fit an export meter, preferring to assume that 50% of the output will be exported (they know that the average household will in fact be exporting 70%+). You will be paid the output in kWh/2*3.1p regardless of how much you actually use.

If you can utilise as near as possible all the power the panels are generating, the power drawn is free. Employing an immersion to heat water is a simplistic way of storing the PV output during the day when the sun is most productive, which is also the time that many are at work.

Now do the sums for heating water by gas in the evening, if you have failed to utilise the free solar energy.

I don't know what you mean by the opening paragraph??

You originally stated:

it makes power available to a separate immersion heater (supply and fitting is included in the £999) which can be switched off if you want to use the power elsewhere.

It is a simplistic solution, which (for me) is a benefit!

Does that not imply the device is worth £999?

Everyone on this thread must surely be aware that you can utilize all the PV generated electricity. The issue simply is to weigh the cost of the 'device' against the 'value' of the energy consumed by the immersion heater.

Cardew wrote: »

Nice post - and we are in violent agrement that at £999 it makes no sense!

The only part of your post I would question is the saving of 1,720kWh pa. You indicate it is a conservative estimate - I would consider it an over-estimate.

It means that you have to have the device powering the 1kW immersion heater for an average of 4.7 hours every day 365 day a year.

It is also not just a question of the ability to generate that amount of surplus electricty, but the ability of the Hot water tank to absorb large amounts of electricity(particularly in summer) without the immersion heater thermostat operating.

I must admit not a great deal of thought went into the 1/3 savings, but it seemed a reasonable starting point.

My thinking was that during the longer day, sunnier parts of the year, our gas usage would be nominal as the PV would be sufficient to warm the tank to it's relatively low target temperature. I appreciate though that once up to temperature, we won't get further savings no matter how sunny the day is. Hopefully on such days the gas usage would be minimal or zero. For shorter, less sunny days the savings would be less, but only on overcast days would the savings be £0. I've asked one of the suppliers if they have evidence of savings generated, this may help us determine the payback time more accurately.

In Febuary on a good day we are generating >2kW between 9am and 2pm, so more than the 4.7hrs you mentioned. Between now and June 21st that will only get better, and we are only 2months after the shortest day. This would suggest that for 8 months of the year, we have a potential window of hot water heating of in excess of 5hrs, or more if the current-switch is set to less then 2kW.

I think one of the main factors to consider is the retention of the stored heat in the tank. Our house is a new build (2010) house so the tank itself (200 litre capacity) is well insulated according to recent building regs. Setting the immersion thermostat to be slightly higher than the boiler thermostat would mitigate some of the losses between sunny mornings/afternoons and the evenings (when we run bath for our toddler) and hopefully avoid the boiler firing up unless needed.

Doing some quick calculations based on the specific heat capacity of water, it would take 16800kJ to raise 200litres of water from 20C to 40C. A 1kW heater would deliver 3600kJ/hour (ignoring inefficiencies), so would take 4.7hrs (4.7kWh) to reach temperature. This assumes that tank was near-cold when heating starts, as opposed to a higher starting temperature. I do like the symmetry that this figure tallies with your 4.7hrs figure above Cardew, which is a complete coincidence!

Using my 430kW/month figure from my previous post, it suggests that in total each day we use about 14kWh/day for the two water heating programs. This is somewhat higher than the calculation above (ie 9.4kWh), but the cold water temperature and thermostat (45-50C) difference is probably more than 20C. There's also the heat losses in the boiler as well to take into account.

Obviously the morning heating requirement (for 2 showers) is less likely to be satisfied by PV than the evening heating requirement for a bath for our toddler. I don't know how efficient tanks are these days, but it might not be unreasonable for most of the energy to be retained overnight, thus reducing the extra heat required in the morning to bring it back to temperature.

My aim is to minimise gas usage, not necessarily make it zero and thus far I've still not ruled out the 'immersion dumping' method. I'm not 100% convinced the numbers stack up yet, but I'm getting there. Would appreciate any other opinions from Cardew or anyone esle.

Edit: Remembered I could do some number crunching from the stats that my SoloPV generates. It logs average power output over each 15min period 24/7, which I used to work out for how long the output exceeds a particular threshold. The following data is for the 19days to date in February, and shows how many hours the system has generated in excess of 2kW, 1.5kW and 1kW:

Avg power exceeding:           2kW    1.5kW    1kW
15Min periods                  98        139   184
Total Hours                    24.5    34.75    46
Hours/day                      1.29     1.83  2.42   

Of course these stats are the average of good and bad generation days. Armed with this information and with a broader date range, I'll be able to work out what the optimum current-switching threshold in order to achieve X hours of heating into the tank, on average. The lower the threshold the better in terms of gas savings, but the higher the risk of importing electricity at the same time if other appliances are operating.

sly_dog_jonah wrote: »

Setting the immersion thermostat to be slightly higher than the boiler thermostat would mitigate some of the losses between sunny mornings/afternoons and the evenings (when we run bath for our toddler) and hopefully avoid the boiler firing up unless needed.

It's not the simple option for you that it would be for me, but you could increase your tank temperature massively and hence store much more heat.

In my case, I have a Heatbank rather than a conventional storage cylinder so boiling point is my only limit and control gear takes care of the rest.

For a conventional cylinder, you can't risk delivering DHW at much more than 40 deg C - particularly if there's a toddler around ! You could however run the tank much hotter if you fitted a blending valve after it so that dangerously hot stored water was mixed with cold before getting anywhere near taps.

sly_dog_jonah wrote: »

Thanks for the suggestion. By blending valve do you mean a 'thermostatically controlled mixing valve'? Like mentioned here:

Rather bizarrely, the url that appears in the posting was a Wiki reference but when quoted it changed to a page from a Canadian firm called safari plumbing !

Both do indeed describe the sort of thing I had in mind though neither are much help in sourcing one in the UK.

Try eBay Item number: 110781332540 for a UK manufactured item at around £50. Not entirely sure whether that's all you'd need or if you might need an additional non-return valve or two to install it.


The Canadian article seemed to make a lot of sense in that there's only a narrow window between water that's cold enough to breed legionella and a DHW supply that may scald. No doubt our own building regs will follow suit eventually.