We installed photovoltaic solar panels on 25 October 2010 and these notes are based on our experience so far.
We use electricity for lighting and cooking, but not currently for heating except to drive the central heating and water pumps. Our usage is 15 plus or minus 1 units per day and is remarkably consistent. The cost was £0.1092 per unit (1kWh) or £1.64 per day (all monetary values exclude standing charges and the various discounts, but include VAT at 5%).
We live at about 51 degrees latitude. There are 18 solar panels, each with a nominal power of 220 watts giving 3.96 kWp (this is measured with an illumination of 1kW per square metre at 25 centigrade, rather optimistic for us). The panels are split equally across a SE and a SW facing roof, each with its own inverter with a capacity of 2kW. The inverter converts from the panels DC to the mains AC at the local voltage and frequency. The panels attached to one inverter are collectively called a string. Our panels are installed at an angle of approximately 40 degrees to the horizontal and hence are most efficient when the sun has an altitude of 50 degrees which can only occur around noon between 20 April and 23 August. There is some shade from trees especially in the late afternoon between October and February inclusive (the SW panel produces less electricity than the SE in winter), but are shade free in summer.
Each inverter requires at least 20 watts to function, and while 95% efficient at full load (2kW), this drops to 75% at 5% load (100 watts) and falls rapidly below that so the inverter goes into standby at 20 watts (and then consumes 1 watt from the mains).
The inverter switches on 30 minutes after sunrise on a clear day and 1 hour after sunrise on a cloudy day with light grey clouds. It switches off 30 minutes before sunset on a clear day and 1 hour before on a cloudy day with light grey clouds. With dark grey clouds switch on may be delayed until 3 hours after sunrise. On a few days it doesn't come on at all, especially if covered with snow. Using a digital camera set at ISO 100 the switch on light level corresponds to a sky exposure of 1/160 at f8 or landscape exposure of 1/30 at f3.5. However the October power levels are extremely low for a about 2 hours after dawn and before sunset. The following links give an idea of the power generated during a perfect sunny day in winter, spring/autumn and summer.
A rule of thumb is that if you imagine looking out from the centre of the string of panels perpendicular to their plane and cannot at a minimum see the outline of the sun because of cloud, trees or other obstructions, or because the sun is not in that general direction, the inverter output is unlikely to be exceeding 10% of the panels nominal output (generating 200 watts per string in our case), and may well be much less. Thus a south facing panel in summer would start generating much later than our SE panel and likewise stop much earlier than our SW panel.
It also appears that while the statement that "direct sunlight is not required because any light will generate electricity" is true, it is also very misleading. With 2000kWp of panels facing a clear blue sky with no direct sunlight on the string the output was about 25 watts (i.e. about 5% of that from a string facing the sun at the same time). On a cloudy day (light grey clouds) it does not matter whether the string is facing the sun or not - the output is about 10% of that which would be obtained if the sun was shining on the string. The conclusion is that if the sun is not shining on the string a cloudy sky is better than a clear sky. This is because the panels are not sensitive to blue light. Dark grey clouds generate about the same as blue sky.
Any shadow on the panels has a significant effect - a 10% shadowed area reduces power by about 20%, and the difference between 100% shadow and 50% shadow is very small. Hence the requirement for unobstructed sunlight. Even a small shadow on one panel significantly reduces the power output of the inverter. This is because the inverter configures the DC circuit to maximise the power generated by the string of panels attached to it. It assumes that the illumination is constant over all the panels. If this is not the case the configuration will be suboptimal for all the panels resulting in a significant fall in efficiency. This is the reason we have two inverters.
It should also be noted that this system does not give immunity from power failures. The inverter requires mains power to function. It synchronises its AC outout with that of the mains, and if there is no mains votage it is synchronising with nothing. In fact for safety reasons if the mains goes outside the national specification for voltage or frequency the inverter will switch off. This is a legal requirement otherwise someone attempting to repair a faulty transmission line is likely to be electrocuted. (A different type of inverter for use with batteries is required for standalone systems).
It would be interesting to compare our SE-SW setup with a similar one facing due south. The maximum power should be 1.4 times what we are getting, but the maximum generation period would be about 10 hours a day against our 16 hours per day. We would generate less power in the winter, but should be better off once the day length exceeded 12 hours (18 March to 25 September) provided we generated an extra 1.14 units in those extra hours. Assuming a rate of 0.5 units per hour that would equate to a day length of greater than 14 hours or 18 April to 25 August. That implies that a 4 kWp south facing panel would be better than our split system, but not sufficienctly so to make replacement of the bungalow economic!
I am not convinced that solar power is particularly green (it takes over four years to produce more energy than that used to manufacture it), and the UK certainly could not run off solar even if all roofs were covered with solar panels. In fact it would require 12.5% of the UK's land surface to be covered in solar panels and even then there would be no electricity at night and very lttle in winter. However financially it was currently the best low risk investment that can be made provided the capital was available and the roof was suitable. With the reduced rates for the feed in tarriff introduced in December 2011 I do not believe it to be financially worthwhile any more. It should also be noted that buildings must have an EPC rating of level C or better to get 21p per unit, otherwidse they will only get 9p per unit.
9 November 2010
The following gives an idea of the total power generated in units per day per 1kWp of nominal power for different weather conditions averaged over the 2 hours either side of noon in mid autumn (i.e. our figures are 4 times these):4 December 2010
The results for November were:
Power generated 63.9 units giving £26.39 income (82.9 units giving £34.24 to date)
Power exported 32.0 units giving £0.96 income (41.5 units giving £1.24 to date)
Power saved 56.04 units or £6.12 (72.99 units or £7.97 to date)
Taking loss of interest into account we have lost £32.50 in the month and £37.87 to date. There were only three days in the month in which we made a gain.
On the best day in November we generated 5.9 units and on the worst 0.08 units. Most of the power was generated in the two hours either side of midday so the amount of sun in that four hour period is critical. One tree was casting a significant amount of shade due to the low sun altitude (17 degrees at the end of the month) so it was felled on the 25th.
The two strings have generated almost identical amounts - 31 units SE and 32 units SW in November and 41 units each to date.
The data for October and November can be found here in an Excel spreadsheet.
It should be possible to make a profit on a day in December but that requires completely clear sky for the critical hour either side of noon, and this we didn't get. The best was fair weather cloud when we generated 4.2 units and made a loss of £0.19. On two more days we generated 3.5 units, but there were 9 days when snow cover prevented any electricity being generated (sun shining through half an inch of snow generates a few watts). The low sun altitude means that any cloud cover has a more pronouced effect, and a generating time of 10am to 2pm was not uncommon. Overall we generated 28 units in December giving an income of £12, and saving an additional £0.22 on our consumption. Taking loss of interest into account this gives a loss of £56 in December and £94 to date. The data for December can be found here in an Excel spreadsheet.
1 February 2011We did manage to generate 7 units on a particularly sunny day at the end of the month with a gain of £0.78, but this was exceptional. Overall we generated 53 units giving an income of £22, but a loss of £45 in January and £139 to date.
The following gives an idea of the total power generated in units per day per 1kWp of nominal power for different weather conditions averaged over the 1 hour either side of noon in December/January (i.e. our figures are 4 times these):
The second stage of the project was completed on 11 January. This was the installation of an air source heat pump to provide hot water for radiators and domestic use, and the removal of the oil boiler. The data for January for both the solar panels and heat pump can be found here in an Excel spreadsheet.
1 March 2011We did manage to generate 8 units on three days with a gain of £1.20, but the month was dominated with cloudy weather. Overall we generated 75 units giving an income of £32, but a loss of £30 in February and £169 to date.
The following gives an idea of the total power generated in units per day per 1kWp of nominal power for different weather conditions averaged over the 2 hours either side of noon in November and February (i.e. our figures are 4 times these):
1 April 2011
A doubling of output under the same sky conditions and a few sunny days meant we produced more units in March than in November to February inclusive. There is a six hour period in which most of the power is produced. There was in increase in the price of electricity at the start of the month from 10.92 to 12.31p a unit.
The following gives an idea of the total power generated in units per day per 1kWp of nominal power for different weather conditions averaged over the 3 hours either side of noon in October and March (i.e. our figures are 4 times these):
The data for March for both the solar panels and heat pump can be found here in an Excel spreadsheet. 5 generated units per day covers the cost of our consumption excluding hot water and central heating. Another 5 units covers the hot water which occured on half the days in the month. On one day we also covered the cost of central heating. Total income was £126 in the month and £229 to date.
1 May 2011
The first half of April was similar to March generating between 1.1 and 12 units per day, but for much of the month it was unusually sunny generating up to 21.8 units per day. The units generated in the month were very nearly equal to that for October-March inclusive. There is an eight hour period in which most of the power is produced. There was in increase in the price of electricity at the start of the month from 12.61 to 12.63p a unit, but the generation tariff rose from 41.3p to 43.3p per unit and the feed-in tarriff from 3.0 to 3.1p.
The following gives an idea of the total power generated in units per day per 1kWp of nominal power for different weather conditions averaged over the 3 hours either side of noon in March (i.e. our figures are 4 times these):
The data for April for both the solar panels and heat pump can be found here in an Excel spreadsheet. From the 19th the heat pump was run completely from the solar panels. Total income was £221 in the month and £451 to date, plus we saved £7 on importing electricity.
3 June 2011
In May we generated 509 units giving an income of £228, and saving £15 on imports plus possibly £11 on electricity for the heat pump.
4 July 2011
In June we generated 435 units giving an income of £188, and saving £11 on imports plus possibly £11 on electricity for the heat pump. Over the six months from the start of the year the income from the panels has just about balanced our total electricity usage (a shortfall of £16) so we have had free energy during that period. It has proved difficult to get more than one half of our consumption from the panels, even with using the washing machine, dish washer, heat pump etc. only during the day. Since the 19th April the heat pump has only been on for an hour around lunch time. In the winter as little as 5% comes from the solar panels even on a bright sunny day. Unfortunately solar electricity is only generated when there is little use for it - very little is generated in winter and none at night when domestic demand is at its highest. It would not be worth installing the panels if it were not for the very high generation tarriff. Without this we have only saved a total of £42 on electricity we have generated for our own use in the nine months and £30 from the electricity we have exported. The generation tarriff of £0.433 is effectively a tax on those who cannot install solar panels (for either physical or cost reasons) paid to those who can afford the installation cost. The data for June for both the solar panels and heat pump can be found here in an Excel spreadsheet.
6 August 2011
July's generation total was 446 units, that is an income of £200 with £17 saved on imports plus another £11 on electricity for the heat pump. I have stopped monitoring the maximum power produced because this seemed to have the highest value on cloudy days with a few bright periods indicating there is something adrift in the inverter's software. Instead I am calculating the average power during the generating day which is about 300 watts per 1kWp over 12 hours in July.
The following gives an idea of the total power generated in units per day per 1kWp of nominal power for different weather conditions averaged over the 6 hours either side of noon in June and July (i.e. our figures are 4 times these):
This table gives an indication of the power distribution over a completely clear day in mid summer.
2 September 2011
In August we generated 388 units giving a total income of £174, well down on previous months due to below average sunshine. Additionally we saved £11 on imports and another £11 on the heatpumpby only running it around midday. By the end of the month the days were noticably shorter and the bulk of the power was produced in the 5 hours either side of noon with an average of just over 300 watts per 1kWp. More data for both the solar panels and heat pump can be found here in an Excel spreadsheet.
2 October 2011
The rapid fall in power produced during the month is illustrated by the fall from 14.1 to 13.4 units in identical conditions over the last three days of the month. We generated 305 units giving an income of £136 and saved £6 on imports. An idea of the generation of power around both the spring and autumn equinox is given here.
21 November 2011 At the end of the first year (on 24 October) of the solar panels we had generated 3311 units - the amount predicted prior to the installation was 3100 units which was surprisingly close.
During October we generated 217 units, and to date a total of 3332 units giving an income from the feed-in tarriff of £1432, and another £51 from the units exported. Additionally we saved a total to date of £78 from the power we generated and used ourselves excluding that used by the heatpump which would add £75 to the savings. This shows that the feed-in tarriff dominates, and under the new system will be halved for new installations, which I believe makes new installations uneconomic when loss of interest on the money invested and inflation are taken into account. It has proved very difficult to use our own electricity because we use so little when the sun is shining, even though the washing machine and dishwasher were only used around midday in the summer, and the same for the heat pump. The year's results can be found here.
The generation rate falls very quickly in October - a sunny day mid month only produced about 9 units, down from 13 at the start of the month.
7 January 2012
The cost of electricity went up to 13.17p per unit at the start of the month. By mid November a sunny day produced only 7.6 units, and mid December 5.1. However this December we generated 69 units compared to 28 last year showing how sensitive it is to the weather. At the end of the year the net cost of electricity (cost of imports - income from generating tarriff and exports) was £46 for the whole year (note that includes cooking, domestic hot water and all heating). The total income to date is £1552, plus savings on imports of £246 (although of this value £166 has been assigned to the heat pump leaving £80 saved on ordinary domestic use). A breakdown by month is given here. The overall cost to date (all costs less income) is £16,221. The year's results by day can be found here.An idea of the generation of power around the end of the year is given here.
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