Solar PV

Most of the energy on earth comes from the sun, and one of the simplest ways of using more of it is to put solar PV panels (Photo Voltaic) on your roof and generate your own electricity.

On a good day ours generate 15 to 20 units; in winter when the sun’s not so strong and the days shorter, from 0.8 to 10 units. An average household uses 14 units a day.

 

Solar PV

Product 14 Canadian Sol mono crystalline panels + 8 Enecsys micro inverters
Rating 3.43 kWp
Annual Yield predicted 2,600 units per year
2,705 last year
Cost £8000
Saving £150 + 1.3 tonnes CO2 per year
Income predicted £400 Feed In Tariff + £36 export tariff per year
£480 last year
Payback time 13 years
Supplier Pure Renewables

 

Payback is still often less than 10 years. But didn’t they halve the Feed In Tariff?

You get paid for renewable electricity you generate, through the Feed In Tariff (FiT).

The rate [in 2014] was about 15p per unit of electricity, which for us should bring in around £400 a year. You get the FiT for every unit generated, whether you use it yourself or export it to the grid, and you’ll receive the payments for 20 years. We should also save a couple hundred quid on our electricity bill because we won’t need to buy as much, and get a further £36 for selling surplus electricity that we don’t use back to the grid. The exports are estimated at half of what we generate.

It is indeed true that the FiT rate was halved a few years ago. What most people don’t seem as aware of is that the price of solar PV has also plummeted – up to 80% in the previous 4 years. So the payback time, the amount of time it takes for the savings to cover the cost of the initial investment, is about the same.

So it’s still a great time to get solar PV. In many cases the payback time will be less than 10 years. After that the income is a return on your investment (profit).

However, in order to encourage people to get PV sooner rather than later, the FiT rate is reducing over time – so the sooner you get it, the more you’ll get paid.

Solar Energy Calculator

Our System

Inglehome compass

Our roof was most definitely less than ideal. No large, south facing roof in direct sunlight, oh nooo, that would be too easy.

The main house has a hip roof – it’s in 3 sloped sections, angular shapes awkward for fitting on rectangular panels. Our sections face east, south and west; facing south captures the most sun. Although the triangular end section was facing south, it had 2 large chimney stacks casting shadows over it. We also noticed that in winter, when the sun is low in the sky, there is some over shadowing from the taller house next door to the south.

What we didn’t

We considered mounting 7 panels on the south facing end terrace wall, but being at a lower height the overshadowing from the neighbouring property would be even greater than on the main roof; even more so for the garage roof. Theft may even be an issue on a roof that is that accessible.

The rear extension roof is south facing and oblong; but at only one storey high suffers partial shading from the main house. We decided to reserve this roof space for solar thermal.

What we did

In the end we decided to completely remove the larger, redundant chimney stack and put panels on all 3 faces of the main roof – facing East, South & West. We retained one chimney with a slightly reduced height for use with a wood burning stove. Fitting solar panels requires full scaffolding, so we arranged for that to be put up a week earlier so we could work on the chimneys.

We managed to find an arrangement that would fit 5 panels on each roof section. However the east section has a skylight providing natural light to the landing which we wanted to retain, so we only put 4 on that face. Purely from an energy saving point of view we could save more by covering the skylight with another panel, which would generate more than a light bulb on the landing would consume… but we like having natural light in an otherwise dark part of the house.

4 on the east face + 5 south + 5 west = 14 panels  x  245 watts each = 3.43kWp (kilowatt peak, the maximum theoretical power output)

Micro Inverters

The solar panels produce DC electricity, but the sockets in our homes supply AC, so an inverter is needed to convert the DC into AC that we can use, and feed into the grid.

Traditionally all the panels are linked together in a single array, that feeds into one inverter.

But this causes problems if part of the array is in the shade, even if the rest is still in the sun.  Even partial shading leads to a drop in power from the whole array.

That would be a big problem on our installation because we have panels on 3 different roof faces (receiving different levels of light), the remaining chimney sweeps a slender shadow, and in winter there’s partial overshadowing from the taller property to the south.

The solution was micro inverters. Each panel, or pair of panels, feeds into its own inverter. So if output from some panels drops, the other arrays are unaffected. It also makes it possible to monitor the output in detail.

The panels are expected to last 20-25 years, but the inverter is the one bit of kit you might expect to have to replace after about 10 years. But these micro inverters come with a 20 year warranty. Bonus!

 

Scaffolding was erected a week early to work on the chimneys.
Scaffolding was erected a week early to work on the chimneys.
Roofers tile the hole where the large, front chimney used to be. Stove installers install a flue liner and new pot on the rear chimney. Then the PV: Bolts through tiles into rafters. Rails on bolts. Micro inverters fixed to rails.
Roofers tile the hole where the large, front chimney used to be.
Stove installers install a flue liner and new pot on the rear chimney. Then the PV:
Bolts through tiles into rafters.
Rails on bolts.
Micro inverters fixed to rails.
Panels on rails. Plug it all in.
Panels on rails.
Plug it all in.

 

Online Monitoring

The micro inverters transmit data to our wifi and (like everything else these days, including me I suppose) on to the internet. So we can see how much has been produced throughout the day by each individual panel or pair of panels.

We can see that on this winter's day it became sunnier in the afternoon. In the middle of the day the south face (orange) is producing more, but as the sun moves around, the west face (pink) produces more.
We can see that on this winter’s day it became sunnier in the afternoon. In the middle of the day the south face (orange) is producing more, but as the sun moves around, the west face (pink) produces more.

 

You can see how they performed on the Results graphs.