To conclude the year's measurements, Here's the graph for the year's generation from our array of solar panels including completed August. This is for our 2kW 8 panel solar array (Panasonic HIT panels), mounted on our south-facing roof at approximately 35-40° angle from the horizontal.
The panels generated 244.77 kWh for the whole of August, 52.68 kWh higher than the forecast amount of 192.09kWh.
Now we have have an entire year's results in, the change in monthly generation is easily observed. At 27.kWh for the whole month, December is the lowest-generating month, and at 288.60 kWh, July generated the most. That's just over 1000% difference between the maximum and minimum months.
The highest generating day was May 6th, with 13.7kWh. The 2nd, 12th and 13th of December are in joint bottom at 0.1kWh per day. That's a staggering 13,700% difference between the highest and lowest generating days.
Here is the completed annual sun-strength chart:
I've discussed this graph on previous posts so won't go into it again.
One of the major factors when measuring the benefit of having solar panels installed, is the amount of your solar generated electricity you actually use yourself. The more of your own ‘home grown’ electricity you use, the less you have to buy from the grid, and the more money you save. Naturally, solar panels generate electricity during daylight hours, and normally, during daylight hours, people are out at work and so the unoccupied house is not using (very much) electricity.
Measuring how much of your own home grown solar energy you are using yourself is not a straightforward task. On a standard solar installation (like ours) you have two electricity meters. The first is the normal electricity meter that everyone has, which measures the amount of electricity you have bought from the grid. With a solar installation, you get a second meter, which measures how much solar electricity your panels have generated.
The complicating factor is that your solar panels could be generating 2kW whilst you’re out at work, and the electrical appliances in the house are only using, say 0.5kW (fridge, freezer, electrical items on standby etc.). That means that the 1.5kW that you’re not using is being fed into the grid and supplied to somewhere it is needed. There is no meter that shows how much of your solar energy you’ve used yourself, only the total amount generated.
From a purely selfish point of view, to get the most financial benefit out of your panels you want to be using as much of your solar generated electricity as possible. We are fortunate in this regard in that I work from home, and I have an office with computers / printers / air con in use all day. Also, the dishwasher and washing machine are both regularly in use during the day too.
The inverter system is clever, in that it will use up all your solar generated electricity first, before buying more electricity in from the grid to ‘top up’. One way of getting an idea about how much of the solar energy you’re using yourself is to look at the main electricity meter during the day whilst the sun is shining brightly. If the main meter is still counting up (even only very slowly) then it means you are using all the solar energy you’re generating, and you are still topping up with electricity from the grid. If the main meter has stopped counting up, it means you are not (at that precise moment) buying any electricity from the grid. Unless your household is using exactly the same amount of electricity as is being generated by the panels, you are now (probably) exporting to the grid, since you are not using all the solar electricity you’re generating.
I have, at sporadic intervals during the day, during bright sunshine, had a look at our electricity meter, and it’s always been counting. Sometimes it only goes very *very* slowly, but it has never stopped counting on the occasions I’ve looked at it. Now I may be a solar sad act geek, but even I refuse to spend all day sat in front of the meter watching to see if it should ever stop counting.
See below a chart showing the average daily amount of electricity we have bought over the last two years. The early averages are spread over longer periods because I took infrequent meter readings.
Here are the above meter reads chart shown on a graph:
ELECTRICITY BOUGHT FROM THE GRID BEFORE PANELS WERE INSTALLED
1st September 2012 - 22nd August 2013
Total 355 days
8680kWh total bought from grid
Average of 24.45kWh per day.
ELECTRICITY BOUGHT FROM THE GRID AFTER PANELS WERE INSTALLED
22nd August 2013 - 15th August 2014
Total 358 days
5175kWh total bought from grid
Average of 14.45kWh per day.
This means, since installing the solar panels, we have bought, on average, exactly 10kWh less per day (24.45kWh - 14.45kWh) from the grid. I've checked the maths, it really is *exactly* 10kWh - to 2 decimal places! Over the same period, the solar panels have generated, 1929/365 = 5.28kWh per day.
I have made other energy saving measures over the last 12 months:
1) fitted low-energy lights throughout the office/house
2) our fridge/freezer packed in, and it has been replaced with a new A* energy certificated one (it’s turned on 24/7 so despite not using much electricity, it makes a significant saving over the year)
3) I have also been more vigilant (my sons would say dictatorial) in making sure computers/lights/TV are turned off when no-one is in the room.
This means that even if we are using *all* of the solar generated electricity ourselves, the above measures are also saving us 5kWh per day. I don’t know whether these measures could possibly contribute more than 5kWh in savings per day, in which case we would not be using all the solar energy we generate.
There’s no way to determine retrospectively how much of the saving is due to the above measures, and how much of it is due to using our own solar-generated electricity; but I think I’m on fairly safe ground assuming that we are in fact using all of the solar energy we generate. This is the best possible situation to be in, since we are saving the maximum amount of money on not having to buy electricity from the grid. I put this down to the fact that I work from home, and have computers/printers/air con all turned on during the day when the solar energy is being generated.
20 YEAR FORECAST
Based on this year’s results, I have created a forecast to predict the performance of the panels over the 20 year life span of the Feed in Tariff. Note that the panels will still be working (all being well!) after 20 years, so there will be an ongoing saving on electricity usage from that point, but we will no longer receive any money from the Feed in Tariff scheme, so I have used that as a convenient cut off point for the forecast.
Measurement discrepancies
It is the calibrated meter reading for the year ending August 2014 that I have used for the 20 year forecast data, so there will be discrepancies compared to the total of monthly values.
Assumptions on this forecast
Consistency of usage
I’m assuming that I will continue to use as much electricity over the next 19 years as I have been doing for the last year. Also I have assumed that I will continue to use all of the solar electricity that we generate.
Deterioration of generating capability:
As is the case with all domestic panels, the manufacturer’s specifications state that after 20 years, the panels will only be achieving 80% of their brand-new generating capability. For the sake of this forecast, I have assumed a linear yearly deterioration of 1% of the first year’s generation, which translates exactly to 80% after 20 years. I will be able to compare the actual deterioration against this as time goes on.
Feed-in tariff index linked:
The feed in and export tariff is index linked to the Retail Price Index (RPI), applied yearly every April. See here: http://www.fitariffs.co.uk/eligible/levels/indexation/ This year our generation Feed in Tariff rose from 14.9p/kWh to 15.177p/kWh, and the export tariff rose from 4.64p/kWh to 4.731p/kWh. This is an average rise of 1.88% (quite a lot lower than the 2.7% quoted for April 2014 on the website above). To be conservative, for the sake of this forecast I have fixed on the 1.88% rise for the next 20 years.
Increase in cost of electricity bought from the grid:
According to https://www.gov.uk/government/statistical-data-sets/monthly-domestic-energy-price-stastics, electricity costs rose by an average of 7.8% per year from 2006 until 2013, and 6% from 2013-2014. There are many factors that will affect the price of electricity in the UK over the next 20 years. I'm not going to start trying to second guess Hinckley Point C coming online / other nuclear station switch offs etc. I don’t have a crystal ball, but I think I’m being conservatively realistic by allowing for an average price increase of of 5% per year over the next 20 years.
Here is all the data plugged into an excel chart:
(There is no significance to the highlighted box, it's just where my cursor happened to be when I did a screencap)
Here is the financial data from above shown on a graph:
Observations
Over 20 years, our 2kW array will generate in the region of 34-35MWh. Somewhere about seven and a half years is the breakeven point on the original £5200 investment in the panels. After 20 years the panels will achieve a total of approx £21,736 benefit (feed-in tariff + savings on electricity not bought from the grid), which is a return on the investment of 318% over 20 years, or 15.9% per year.
Note that if we discount the savings from using all our own solar electricity, using the feed-in tariff alone it would take about 13 and a half years to recover the original investment, almost twice as long. In this case, the total 20-year benefit is £8567. This is a return of 58% over 20 years, or 3.24% per year. This is a significant consideration if your household does not use much (or any!) electricity during daylight hours.
The chart shows that the savings made per year by not buying electricity from the grid, breaks even with the money received from the feed in tariff after 6 years. Due to the fact that the cost of buying electricity from the grid rises faster than the Retail Price Index (to which the Feed-in Tariff is linked), from year 6 onwards the biggest yearly financial benefit of the solar panels is from the money saved by not buying from the grid.
Conclusions
Practically - The Panels
Our installation was a bit of a disruption - we had scaffold up on the back of our house for about 8 weeks all told, although that was due to holiday date clashes and repairs we had to make to our roof prior to the panels going up - it only took a couple of days to actually put the panels up. As long as your roof is sound and the installation is done by a reputable company then the disruption from panel installation is minimal.
The inverter The inverter is a large box of electrical wizardry that turns solar electricity into a form that can be fed into your consumer unit (fuse box) and then on into the grid (see photo under 'measurement discrepancies', further up). You need to have somewhere to fix it in the house (ours is in the under stairs cubby hole, next to the consumer unit). The installation company said they also mount them in lofts, they can go wherever is convenient for your property - with the only proviso being they have to physically route the wiring from the panels on the roof to the inverter, and then from the inverter to your home's consumer unit. Our wire runs through a sealed hole in the roof immediately under the panels (the guy said they've been installing panels for 10 years and have never had a customer ring up with leaking issues), across the inside of the roof in the loft, down to the eaves, then through the wall and down the outside of the house in plastic cable conduit. Our under-stairs cupboard is against an exterior wall so it could go straight through the wall directly to the inverter.
Once up and running, on a day-to-day basis you don't notice them. You don't need to do anything with them, you don't need to turn them on or off, they just sit on your roof, come rain or shine (although preferably mostly shine) and generate electricity. There's no moving parts to wear out. They make no noise. They don't belch smoke.
There was only one issue we have had with our panels, and that was during installation, one of the snap-fit electrical connectors that join the panels together hadn't been properly snapped on installation. Thermal expansion & contraction caused it to unseat midway through September 2013, meaning we lost a weekend's worth of generation before the company could come out and fix it.
I have been advised that inverters have a life expectancy of 'about 10 years'. No examples of our particular model (Power One Aurora Uno) have yet come to end of life, but based on previous inverter technology, I've been told to budget to replace it around 2023, at a cost of around £600. This should be factored into the financial benefit of the system as a whole: £600 out of £21,736 takes the total benefit down to £21,136, making for an annual return of 15.3% over 20 years on the £5200 investment.
Financially
Buying solar panels is certainly not a get-rich-quick scheme. They are a long-term investment with respectable returns, as the above figures show. If you use most/all of the solar generated electricity yourself, on a £5200 investment you can net yourself a whopping 15% annual return over 20 years, and you don't have to put your money anywhere dodgy to get it!
Ethically
Regardless of any financial benefit, solar panels are totally and utterly an ethical choice (obvs). Our 8 lil' ol' solar panels will save the UK a not-insignificant 35MWh of generation over the next 20 years. A drop in the ocean compared to national usage maybe, but if every roof in the UK had a solar array on it, we wouldn't need power stations, simple as. The numbers are compelling (see my earlier blog post http://gillpanels.blogspot.co.uk/2014/07/a-solar-uk.html ). I haven't done any studies on the environmental cost of manufacturing solar panels, which is something I'd like to look into in future.
The democratisation of energy is a good thing, but we would still need a grid to distribute electricity to where it is needed. And we'd also need to replace the power stations with power storage facilities - and this is the big problem - they'd need to be able to store HUGE amounts of kWh (not yet done the maths...) The majority of solar electricity is generated in the summer, and the excess would need to be stored for use during the winter. In this brave new world, electricity still isn't free. In the absence of a revenue stream from generation, the electricity companies will have to shift to a new model for financing a robust energy storage network and upkeep of the distribution grid.
Roll on the next 19 years...
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