Category Archives: Solar PV

Solar output – 2017 Jan – Jun

2017 H1

Now that we’re half way through 2017 it seemed appropriate to have a look at energy usage from the solar panels – especially as those six months reflect the first six months with the battery storage system.

The graph is taken from my ImmerSUN smart controller which automatically diverts surplus solar electricity to the car charger or immersion heater.  The battery storage system has independent controls but its benefits can be seen via the ImmerSUN.

The purple line shows the consumption of electricity (excluding the immersion heater) and is relatively stable month by month.  Consumption is relatively large due to my electric car and cooking with electricity.

The green line shows the generation of electricity from my solar panels.  Not surprisingly output is lower in the winter, but from April we generate more electricity than we use despite our relatively high consumption.  In principle we could be electricity independent during those months but for the time of consumption not matching the time of generation.

The red line complements the green line as it shows the import or purchase of electricity from the grid, and thus reduces as the generation rises.

The blue line shows the diversion  of electricity to heat water via the immersion heater when neither the battery storage system nor the car charger can absorb the available electricity.

Finally on the graph the turquoise line shows export of electricity to the grid when all smart capability within the house to use electricity is exhausted i.e. battery storage system at maximum power or full, electric car battery full or absent, and water in cylinder is hot.

Among the numbers:

‘Savings” at £80 refer only to the value of the water heating achieved from solar electricity versus buying electricity (although our backup is mains gas).

“Self consumption” at 86% refers to the proportion of solar panel output used i.e. not exported to the grid.

“Green contribution” at 59% refers to the proportion of total electricity consumption (excluding water heating) derived from the panels rather than from the grid.


Electricity use 2016

The immerSUN provides a useful app showing electricity use which includes an annual option.  The graph below shows the 2016 annual data:
Although some of the data was only collected from mid-March 2016, the graph still shows useful information. I think that the graph overstates bought / imported electricity in January to March but understates generated electricity proportionately in the same period.

The purple line shows monthly electricity consumption and is broadly consistent month-to-month.

The green line shows the generated electricity from the solar PV system.  Its seasonality is clearly visible.  Solar generation exceeds electricity use in four summer months, and is very close in a fifth.

The red line shows bought electricity.  It’s generally a mirror image of the green line reflecting more purchased electricity in winter and less in summer, but is not zero even in months where generated electricity exceeds used electricity due to time of day issues – cooking and car charging often occur at times when solar output is low such as cooking in the evening and charging at night.

The blue line shows surplus day-time electricity being used to heat water, and thus saving gas.

The turquoise line shows surplus day-time electricity being exported once the water has reached its set point.

It will be interesting to see how this changes in 2017 as a result of a full year of solar car charging in its current mature condition and with the new battery storage that should help get more of the generated electricity used by saving it for evening use.

Solar PV Generation 2016

The end of 2016 marks the end of the first full calendar year of solar PV operation in this house since the system was installed in late September 2015. In 2016 we’ve generated 4,129 kWh – 12.5% more than the 3,668 kWh annual output estimated for the system.

Income for 4,129 kWh generation with 50% deemed export is £641, with further savings made from the self-use of the electricity rather buying electricity or gas.   Detailed monitoring of these savings only started in March 2016, so there’s less than a full year of data, and my ability to use generated electricity improved through the year – particularly with my electric car charger control project.  From March to December 2016 we used 44% of the generated electricity replacing bought electricity, and a further 28% of the generated electricity replaced bought gas for hot water.  Back in September at the anniversary of the solar PV installation I estimated these energy savings as a combined £200 (link).

The chart above shows the minimum, mean and maximum daily generation for each calendar month since installation.

As one might expect, significant seasonal variation is evident month-to-month and well as day-to-day variation within any month. June 2016 looks a bit disappointing compared to May and July. The last few days of September 2015 immediately after the panels were installed were obviously quite good for September, but within the spread for September 2016. October looks quite similar for 2015 and 2016, while November 2016 is rather better than November 2015.  December again looks quite similar for both years.

Solar panel power outputs deteriorate with time.  My panels are supposed to have at least 90% of the original minimum power output after 10 years, and 80% after 25 years.  Generally mine seem to be performing well with annual generation significantly higher than estimated, and no evidence of performance deterioration in the late September to end of December period for which 2 years of data exists.

Solar PV Installation – 1 year on


We’ve now had our solar panels for 1 year, so it would seem time for an update on how things are going. Our 4kW SSE-facing system has an expected annual generation of 3,668 kWh. However after a year in service I’m pleased to see that we’ve generated 4,056 kWh – 10% more than expected.

The financial returns on a solar PV system are a combination of 2 things: (i) payments from your chosen electricity company for energy generated and exported to the grid and (ii) savings from not having to buy so much power as you use that which you’ve generated instead.

When it comes to payments from the electricity company, my electricity company (like many) chooses not to go to the expense of installing an export meter and instead assumes that half of the power which I generate is exported to the grid. The total annual revenue for the first 12 months of operation, including both generation and export (assumed to be 50% of generation) is £629.62.

Then there’s the question of how much electricity I save. I’ve only had monitoring of usage since March (approximately 6 months) but in that time I’ve used 41% of the generated electricity to replace bought electricity. 41% of 4,056 kWh is 1,663 kWh. What is less clear, is what the unit saving for this energy is. Some of this is daytime usage like standby loads, the fridge, cooker and other daytime loads at 11.7 p/kWh; but some would otherwise be night time loads like dishwasher, washing machine, or car charging at 7.57 p/kWh. I don’t measure the split so I’m simply going to assume an average unit rate = (11.7 + 7.57)/2 p/kWh = 9.6 p/kWh. 1,663 kWh @ 9.6 p/kWh = £159.65.

Finally, there’s the question of how much gas I save by making hot water from solar PV electricity rather than gas. Since March I’ve used 813 kWh or 27% of the generated power for water heating, so for a whole year 27% of 4,056 kWh is 1,095 kWh. The immerSUN itself records 995 kWh used for water heating since December. I’m also going to assume that not all the heat from the gas boiler would have ended up in the tank as hot water since some is lost via the boiler flue to the outside world, and some is lost via the pipes to the inside of the house – so let’s say 80% efficient on gas. 1,095 kWh @ 3.01 p/kWh @ 80% efficient = £41.20.

The combined total of my revenue and savings for a whole year would have been £830.46 – a 13.6% return on the investment or payback in 7.3 years.

The tariff scheme will have provided me with about 20 years of income by the time it closes, so the investment in the panels, as well as helping me reduce my carbon footprint, will have generated 12+ years of profit having paid for itself during the 8th year.