![\"SHW](\"https:\/\/www.theoillamp.co.uk\/wp-content\/uploads\/2014\/07\/SHW-payback.jpg\")
<\/a>The first thing to say is my system was subsidy free and has to compete with mains natural gas.\u00a0 Six months after the installation the grant scheme was introduced,\u00a0 but grants could not be retrospective.\u00a0 So the system has always had two financial handicaps.<\/p>\nI have used two methods of calculating the solar hot water payback.\u00a0 Both use the average Q2\/Q3 gas usage before the SHW system was installed as a basis to model the savings using the difference between what it is and what it would have been. \u00a0 The first method is just that.\u00a0 Its a bit crude but this one is showing that solar hot water payback has occurred in Q2 this year.\u00a0 The second is my preferred method (red lines on graph above).\u00a0 Solar hot water output is not metered, it is possible to buy meters but I don’t have one.\u00a0 In 2004 however I had a PV system installed on the same roof.\u00a0 This is metered and using the relative efficiencies of both systems allows me a reasonable guess at how much energy I’m capturing using the SHW system.\u00a0 I can then work out assuming a 25 year lifespan of the system how much I have bought the energy up front (cost of system\/average output kWh\/year*25).\u00a0 This is currently around 8.2p\/kWh.\u00a0 I then use direct proportion to multiply the savings in the first method (saving x cost of gas\/8.2).<\/p>\n
In 2008 churches in Edinburgh organised a climate change conference.\u00a0 I was talking on SHW and PV and thought I ought to look at the costs.\u00a0 I had used another method of calculating the\u00a0solar hot water payback and had assumed since gas prices had started rising payback was humming along nicely.\u00a0 Looking again at the solar hot water payback method I had used up until then I decided I didn’t like it and came up with the alternatives described above. \u00a0 When I bought the system I was paying around 1.3p\/unit and as can be seen from the graph payback was negligible.\u00a0 (Although carbon savings were good, which was the aim of buying the system.) However, by the time the conference came around I could see it was all change in 2008.\u00a0 Global oil prices reached an all time high as did gas prices in the UK (too that time) and the wholesale electricity prices.\u00a0\u00a0 The savings jumped massively that year.\u00a0 This trend has continued and accelerated since.\u00a0 I am now paying 5.57p\/kWh, down slightly from an all time high this spring.<\/p>\n
Other points to note.<\/p>\n
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- The solar hot water payback calculations are a bit crude but I have yet to find a better way.<\/li>\n
- Running costs are zero.\u00a0 The system is freeze tolerant, uses the existing pipework and is solar powered.\u00a0 Its never gone wrong.<\/li>\n
- 2014 includes Q2 only.\u00a0 Q3 savings are about half this so on my preferred measure we will be at about 40% still to go.\u00a0 But even on this trend the system will pay for itself in 2-3 years if gas prices keep rising at the longterm historical trend.<\/li>\n
- I include a small boiler maintenance saving of \u00a320\/year, a hidden saving of SHW.\u00a0 My gas hot water system unlike some does not use a pump.\u00a0 So there are no electricity savings from this.\u00a0 There might be on some hot water systems.<\/li>\n
- The payback is slightly higher though in reality since we used to have a washing machine with a hot water feed.\u00a0 Obviously when using hot water heated by the sun we were saving on electricity costs.\u00a0 But the savings are impossible to quantify. \u00a0 Our current machine like almost all now is cold water only.<\/li>\n
- The system used not to work in Q1\/4 at all. \u00a0 In fact on the 30th September for two years in a row we had a glorious warm sunny day.\u00a0 No solar water.\u00a0 Then I had a burst lead pipe elsewhere in the house.\u00a0 I asked the plumber to remove all lead pipes.\u00a0 Some were those used by the SHW.\u00a0 He insulated these after replacing them.\u00a0 This cost is negligible and is not included since its unrelated to the SHW system I would have had the pipes removed anyway and the insulation costs were a few minutes work.\u00a0 This made the system usable in Q1\/4.\u00a0 I use a slightly different payback system of avoided gas\u00a0use (gas use measured over 30 minutes and converted to kWh)\u00a0 to calculate the savings in these quarters since the heating system is more likely to be on which means the above calculation method would not work.\u00a0 To count I must use the water before the heating system goes on.\u00a0 Q1\/4 savings are pretty small, a few years ago they were \u00a319, less last year.<\/li>\n
- The plumber left some of the existing pipe run uninsulated.\u00a0 We had some electrical work done and whilst the floorboards were up I finished that section off myself.\u00a0 The costs are so small I have not included them.<\/li>\n
- In 2008 I came up with the idea of switching the boiler off completely.\u00a0 Often we were not using it and we have a pilate light that was ghosting.\u00a0 This makes the 2008\/9 jump a bit bigger than just gas prices influenced, but since then the increasing trend is gas price influenced.\u00a0 I use the immersion heater to fill in.\u00a0 We have a small double tank and the immersion is fast 15 minutes is enough for a shower\/washing up.<\/li>\n
- I discovered I had to turn the upstairs radiators off otherwise they get warm from the SHW panel and the water is a lot less warm.\u00a0 A pain in Q1\/4 when you are using both.<\/li>\n
- A major pain was when I used to pay variable rates for gas.\u00a0 You paid a lot (x<\/strong>) for the first units then less (y<\/strong>) for the next and sometimes z<\/strong> for the last bit per quarter where z<\/strong> would be even less.\u00a0 Since we always used the boiler a bit in each quarter we would always be in one of the cheaper bands lowering our savings.\u00a0 It also made the calculations complicated and a nightmare if the price changed mid quarter.\u00a0 On one such occasion I spent an entire evening on the spreadsheet when the price had changed mid quarter.\u00a0 Its one reason I am with my current supplier, who have one rate only and no standing charge.<\/li>\n<\/ul>\n
Solar hot water payback is possible and the gas savings are impressive.\u00a0 SHW is as we said in our book<\/a> a basic technology that most of us should fit.\u00a0 My woodburner will also pay for itself in Q4 this year.\u00a0 Payback on this will have been much faster than for the SHW.\u00a0 I will blog on this in the new year.<\/p>\n
Neil<\/p>\n","protected":false},"excerpt":{"rendered":"
Solar hot water payback is possible.\u00a0 In the summer of 2008 I was helping man a transition stall when a very posh woman told me that my solar PV system would never pay for itself.\u00a0 She said should know since … Continue reading