Posts tagged with: p.v


How do I know if my PV system is working properly? This is a very good question and something that many customers of solar energy systems wonder. The answer is to spend some time and perhaps some money in monitoring the performance of your system over an extended period of time.

Perhaps the first point to mention is that a good installer should do this for you. They should care deeply whether or not their installations perform as well as they will have predicted. Before choosing your installer try to determine what they offer in terms of after sales support and care.

If you like to take matters into your own hands there are a number of ways to monitor your system. If you are not inclined to pay extra for monitoring hardware, the simplest solution is to use information from the displays of the generation meter or the inverter (or both). Both these instruments will be present in all PV systems installed in the UK by a microgeneration certification scheme installer and will be able to tell you the number units of electricity generated since the system was put in. Get into the habit of checking this number at the end of each day, along with a record of what the weather was like – clear sky, partial cloud, heavily overcast etc…

Your installer will have provided you with an estimate of the annual energy production in kWh – in southern England this should be in the region of 850 kWh per kWp). This means that over the course of the year you should expect an average of two and a half kWh of energy each day for every kWp you have installed. This is heavily dependent on the weather of course. On a clear day you could expect 8 kWh, and on a very dark day as little as 0.2 kWh. After several days of checking you should be able to have an idea of whether your system is significantly under-performing.

Over time, your measurements will become more reliable. Each month of the year has an expected solar energy output. Ask your installer for a chart showing the average monthly variation of solar energy for your location. After one month you can compare the energy you got with what you would expect. Be warned however, monthly solar energy output can vary widely, some months can be particularly good or bad for solar compared to normal, so comparing with the average is not necessarily accurate. Still, you should be able to tell if you are getting 30% less energy than you should be.

Getting a more accurate picture of your system’s performance is challenging. For instance, if your system is underperforming by 10%, how do you prove this?

A good method is to compare it to a nearby reference system which is known to perform well, and compare daily production to that. This can be challenging to find however, especially in the UK where there are still very few PV installations.

Another technique is to try to determine if there is something wrong with your system. Your inverter should also tell you the DC voltage and current coming from your solar panels. Under a clear sky, you can check if these values are in line with what they should be from the datasheet. Whether the voltage or current is lower than expected can provide information on what might be wrong.

If your system is underperforming, it could well be because of shading. If shadows are passing across the solar panels during the course of the day that weren’t accounted for in the system design, then this can really contribute to underperformance. Try to get in a position where you can see the solar panels at different times of the day. Any shadows on the module during the middle of the day (when they should be producing the most energy) can be serious. Many systems will be shadowed in the morning or evening, but this is generally less severe. Make sure to also check for dirt or muck on the panels, even small markings can cause big performance losses for solar systems.

It is also worth looking out for long term degradation. Whilst rare, it has been known for systems to get significantly worse over time.

If you would like buy a monitoring solution yourself then the simplest product is a power meter. Examples are the ‘OWL’ meter or the ‘Wattson’ made by DIY Kyoto. These are both simple meters that can easily be installed by clipping a sensor onto the AC output cable of your inverter. What’s useful is that the data can be transmitted wirelessly and viewed in real time and even stored to show you how energy production varies over the course of the day.

Depending on your installer, many will offer to monitor your system for you. This can be advantageous since they will have access to data from a large number of systems on which to benchmark performance. Make sure to ask them exactly how they plan to do this however.

On the upside, most systems should perform fine and are unlikely to go wrong but when you’re investing such a large amount of money in a PV system, its nice to know how to check its working.


Here at Solarfeedintariff we like everybody to share their feelings on solar energy. One of our readers has been kind enough to give his thoughts on his new solar system and the report is below. Please feel free to submit any articles you feel would help educate the world on solar energy.

We have an unshaded South West facing roof at about 45o angle.  We had scaffolding for some quite substantial roof work, so decided to use the opportunity to have PV panels installed.  Because of a dormer, we had little space and could accommodate only four panels, mounted horizontally, giving us a maximum of just less than 1kw. The supplier was a JHS, a small company in Banbury.

The inverter gives a reading of the current power and total units each day – this gives lots of opportunities for taking readings and doing all sorts of nerdy analysis. Over the first four weeks (July) we are averaging 3.5 units per day.  On about the 15 August the sun will actually hit the panels square on at one point in the day – will this be the best day overall?   Facing SW, the panels do not see the sun at all until after 11 am (BST), until that time we generate more power from bright clouds than blue sky.  Hence it is ideally white cloud until mid morning and then sun – although we do get several hundred watts from bright white clouds.

Getting ourselves registered through our utility company (SSE) took a bit of effort.  The web-site was uninformative, emails were not replied to and I did not have the patience to wait for them to answer telephones.  Writing a letter worked, we were put in touch with the ‘microgeneration’ department and now have a feed-in contract.  There does not yet seem to be a formal scheme for submitting readings of our solar generation, we are asked just to write or email the reading every three months.

There are four ways you save money, three of them legal.  (i) The feed-in tariff, 41.3p per unit generated is very generous.  (ii) For the power you actually use while being generated, you obviously save on your electricity bill – around 11p per unit.  (iii) They also assume (they cannot measure, without extra equipment) that 50% of your power goes back into the grid, effectively this gives you get an extra 1.5p per unit generated.  (iv) If you have an old fashioned meter, the little wheel goes backwards  when you are not using all the power, and this drives the meter backwards – the effect of this is that you are saving 11p per unit on all the power you generate, not just that you use.  I suppose the utility company knows this – the meter does belong to them!

Our installation cost around £6000.  Will we get out money back?  Well at my age (67) maybe not, but we should see a substantial saving each year – and  it’s all been very interesting.

C J Pavelin July 2010

Solar Farm Land Required

Are you a land owner in the South of England?

We are looking for land that has the following requirements in order to build Solar Farms in the U.K and to start moving towards a greener Britain.

Basic Site Specification for Solar PV, UK

Size 5 – 40 hectares

Tenure: Freehold or long-term (25years minimum) leasehold

Orientation: Level ground or south facing angled, south of Birmingham

Electrical connection: Close proximity to any line to export the electricity generated is good. A 33kV line, or above would be preferable for larger sites

Shading: Avoid sites surrounded by shading from trees, buildings, terrain

Neighboring properties: No specific requirements, but large energy consumers who would directly purchase electricity generated may help economics.

Planning: No specific requirements, but avoid areas where adverse visual impact will lead to objections

Access: Vehicular access required for construction and O&M

Suggested sites: Brownfield development sites, former airfields, public sector landholdings, private landowners, former mining areas, industrial land, etc

If you believe you have a site we may be interested in please contact