Solar Panel Facts

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Specifying and designing photo-voltaic solar panel installations will always involve some level of Electrical Engineering. Unfortunately for many people this means exposure to a mind boggling number of terms and expressions some of which are vaguely associated with gladly forgotten physics lessons. Fortunately it’s generally sufficient to know a few basic concepts and terms in order to successfully choose and understand what will work for you.

The key to getting your head around the concepts is to do the same thing that many Electrical Engineers who have studied electromagnetic principles for most of their lives do in one way or another:

  • Admit that understanding what electricity and magnetism actually are is rather hard, and maybe impossible.
  • Come up with models (in an engineer’s case these models are often mathematical equations) that describe how electricity behaves.
  • When needing to understand how electricity works, don’t worry about what electricity is, just use the model.

So in this vein, in the explanations that follow, we’ll use the model that electricity is a fluid, in fact we’ll pretend it’s water. Whilst we all know that electricity isn’t water and that if we cut a live wire we don’t usually get a puddle of electricity on the floor, the concept is still very useful. In fact many of the first mathematicians and engineers to investigate electromagnetic phenomena thought of it in exactly this way, as many of the principles are very similar. For those who find the voyage of electromagnetic discovery interesting please browse the pages we’ve collected here. We’d love to hear from you if you’re an enthusiast and would like to enhance the material we have on the site.


The standard unit of Power is the Watt, symbol ‘W’ 1000 Watts = a kilowatt, symbol ‘kW’ In this context the number of kW assigned to a process or piece of equipment represents the amount of energy that that process or equipment consumes or produces per second.

This can also be explained using the ‘electricity is water analogy’. If someone is using a hosepipe to water their garden then every second a certain amount of water will be pushed out of the hose onto the garden. You might measure this in litres per second or gallons per minute or any ‘capacity per time’ kind of measure. Similarly a spring in someone’s garden might produce so many gallons of water per hour that flows into a small stream (nice garden!). In either case this ‘rate of water flow’ or ‘amount of water per second’ tells us something important about a source of water or something consuming water. A quantity of water flow under pressure can be considered the equivalent of ‘power’. In a similar way, electrical power may be thought of as the ‘volume’ of electrical energy used or produced by something in a second. Electrical power is in effect the electrical energy per second that might be thought of as ‘flowing’ into or from an electrical device. So a piece of equipment that has a power rating of 3 kiloWatt (3000 Watt), such as a kettle, requires a relatively high flow of electrical energy per second and so consumes a lot of electrical energy whilst it is switched on. On the other hand a device that consumes 10 Watts of power (for example a low power light bulb) requires a comparatively small flow of electrical energy per second.

Importantly, note that the pressure and rate of water consumption or production (the power) doesn’t tell us all we might need to know about the water being moved. For a start the actual amount of water used (measured in litres, or gallons, or cubic metres, etc) depends how long the flow continues for. So for example if someone filled a bucket for 10 seconds with a hose supplying half a litre per second they would have used 5 litres of water. If someone else had a tap dripping at a rate of a litre per hour (a much slower rate of consumption), for 12 hours they would use 12 litres of water. In other words the amount of water actually transferred is given by the rate of water flow multiplied by the time it is flowing at that rate. Or in other words, at a certain pressure or height:

Volume of Water = Rate Of Water Flow X Time Water Is Flowing

In exactly the same way, the amount of electrical energy used or produced by something is equal to the rate at which the energy is consumed or produced (i.e. the power) multiplied by the time that the energy is ‘flowing’. Or in other words:

Amount Of Electrical Energy = Electrical Power X Time

Which brings us to the exciting topic of Energy…


Energy is the capacity to do work. The standard unit of Energy is the Joule (J). However there are many other units that are used, e.g. calorie, kilocalorie, Btu, kilowatt hour. For the purposes of PV solar systems it’s probably fair to say the only unit of energy needed is the kilowatt hour, so there’s no absolute need to know or remember the others. For the more ‘energetic’ there is more information on energy, work, and power at the bottom of the page!

kWh – kilowatt hour. Unit of energy. The amount of energy used or produced if a process that is working at a rate of 1kW runs for one hour.

When using a hosepipe, the amount of water the water company will charge you for is the rate the hosepipe is gushing the water out multiplied by the time you have the hosepipe on. So in a similar way we can repeat to ourselves that:

Amount of Electrical Energy = Electrical Power x Time

So if an electric immersion heater of power 3kW is on continuously for 2 hours it will have used 2 X 3 = 6kWh of energy. If a small generator is producing a power of 2kW continuously for 20 minutes it will have produced 2 X 1/3 = 0.67kWh of electrical energy.

Some water companies charge per litre or perhaps per cubic metre of water used; in a similar way electricity companies charge per kWh of electrical energy used. Sometimes electricity companies refer to a kWh as a ‘unit’, and prices are quoted as so much a ‘unit’. This may be rather confusing, but is just part of the language that has grown up around electrical energy supply.

Grid Connected or Grid-Tie

Imagine a situation where water is more expensive than it is now, let’s say it costs around £2 per litre. Actually this may be about the price we currently pay for bottled water, but in this theoretical example we’re imagining that all the water we use in the house for showering, washing, cooking, etc has to be paid for at a rate of £2 per litre. Under these circumstances it’s perhaps some people would collect water from their roof and possibly purify it for their own use. Of course it would depend how expensive the equipment would be to buy and install, how reliable the equipment was, whether the water tasted OK, how much it rains, etc. It’s probably fair to say that if a reasonable capital outlay supplied a decent quantity of good water that was basically free, then folks may well consider it. So you know where this example is going! Every day, solar energy pounds down onto our roofs (and in fact on all our land) as solar electromagnetic radiation (light is electromagnetic radiation). For a reasonable capital outlay you can install energy collectors (solar panels) on the roof that collect the sun’s light energy and convert it into useful electrical energy.

Unfortunately, there’s something of a cloud hanging over this idea as it stands. One obvious problem with our water collection scenario is that it might not rain for several days or perhaps weeks (yes, even in the UK!). Of course at other times there will be absolute downpours, and you can bet your bottom dollar that will be when nobody in the house is particularly using water. What would our roof water collectors do? Well one choice is to build a storage reservoir big enough to hold enough water to last for rain free periods. This will work, but obviously has the downside that the reservoir would require additional capital, space and maintenance. However, if someone is in a remote area with no other source of water then this is probably the only sensible option available. Another solution might be described as ‘grid connected’ or ‘grid-tie’. In the grid connected arrangement, the householder would remain connected to their mains water supplier, but would only use water from the water supplier’s grid at the times when insufficient water was being collected from the householder’s roof. In our imaginary scenario with water being very expensive, it would also be quite expensive for water suppliers to pump water long distances through their pipe networks due to losses from water leaks etc. In this case the water company would possibly agree to also purchase excess water of a given quality from the householder at times when more water is falling on the roof than the house can use. The money paid to the householder by the water company for this excess water would offset the charges for any water that the householder bought from the water company during dry times. To make use of this arrangement the householder would need to install a pump/purification unit to ensure that during times when excess water is available to sell back to the water company, the pump can push the water back along the property’s supply pipe back to the main water grid. Here it will be available to be used by others in the vicinity through their normal mains water connections.

Whilst grid connected water might well be a little farfetched, and raises all sorts of practical questions (hygiene not least), it usefully demonstrates how the principle of electrical grid connection works. Suffice to say the method works extremely well for user generated renewable electrical energy from sources such as photo-voltaic solar panels. The equivalent to the pump/purifier unit in the water example is known as an inverter in the electrical world. Electricity companies in the UK, as well as many other countries, are required to buy electricity generated from a range of renewable sources. Although it’s possible to use rechargeable batteries as reservoirs of electrical energy, grid connection generally makes energy storage unnecessary, greatly simplifying and reducing the cost of solar PV installations.

Maximum Power or Power At STC

Maximum Power is measured in kWpk – kilowatt peak (sometimes kWp). In the context of energy generation – the useful power that a piece of equipment will generate under standard, close to ideal conditions. The capacity of most renewable technologies to generate electricity is specified in kWpk. Of course in reality over a year a given installation will produce energy at an average rate that is much lower than this as conditions are not always ideal.

There is an obvious need to compare solar panels and solar panel installations, so that consumers, suppliers and installers can determine how systems, manufacturers, etc are performing. A would be purchaser might ask a supplier, “How many kilowatts of power do your solar panels produce?” Similarly, once an installation has been completed, it would be a useful to be able to specify the power level that you would expect from such a system. Of course the answer to the question depends on a number of parameters: how sunny is it?, what temperature is it?, how much area of solar panels do you have in your system? kWpk is a unit the solar PV industry uses to provide standardisation. The industry has settled on a set of standard test conditions (STC) that pretty well everybody uses. The test conditions basically represent good generating conditions that you might reasonably come across in the field. For example the STC specifies usable sunlight landing on the solar panels at a power level of 1000 Watts per m2 (equivalent to a clear sunny day), and the temperature of the panels being 25 degrees Celsius (note that PV solar panels actually get slightly less efficient as they get hotter). Whatever electrical power a solar panel or a given installation produces under these STC conditions is said to be the maximum power of the system measured in kWpk. Thus someone might refer to a given installation of solar panels as a 4 kWpk system. By this they would mean that whilst exposed to very good conditions you would expect that system to produce 4kW of electrical power.

In practice it is possible for a system to produce slightly more than its official ‘Maximum Power’. For example a 2 kWpk system might, at times that are very sunny but cool, produce a power of 2.2kW whilst those conditions last. This is because the agreed STC, whilst designed to represent excellent practical solar PV generating conditions, do not necessarily represent the very best conditions theoretically possible.

Expected Energy Generation per Annum

The amount of energy you would expect a given installation to produce in a single year. For example in the UK, a solar PV installation of 1kWpk maximum power, on a pitched, south facing roof would have an expected energy generation per annum of approximately 850kWh.

So how much energy would we expect to get from a given installation in a year? To put a figure on it, you have to take into consideration all the things like cloud and rain and fog and mist and pollution and the fact that the earth inconsiderately spins on its axis every 24 hours such that half the time there’s no sunlight even without climatic factors. You also have to take into consideration that depending at what latitude you live, the sun’s rays will arrive at a different angle according to the time of year, resulting in the energy sometimes tending to glance off the roof rather than hitting it directly. All this seems a bit depressing in terms of harnessing the sun’s power.

Fortunately, the sun pours out so much energy that even with all these factors taken into consideration, even in a high latitude, wet place like the UK, then averaged over a year there’s still plenty of useful solar energy hitting our roofs. How much? We have excellent records going back decades, so we don’t have to guess. On average, every square metre of pitched, non-shaded, south facing roof in the UK will receive between 800kWh and 1100kWh of useful solar radiation over the course of the year. Of course, with the theoretical water collection example, some of the water that lands on the roof will be lost immediately (it will bounce off or evaporate), or lost during the purification process. In a similar way, solar panels convert only a comparatively small proportion of the energy that hits them into useful electricity. A good rule of thumb to use in the UK is that for every square metre of non-shaded solar panel, facing between south west and south east, approximately 100kWh of electricity will be generated each year.


Insolation is the amount of solar electromagnetic energy (light radiation) per second received by a certain area of the earth’s surface. As energy per second is power, insolation would therefore normally be expressed in terms of power per area, e.g. kW/m2. Under normal circumstances most insolation heats up the area it impacts, and just as well, we’d be very cold without it! PV solar panels convert a comparatively small proportion of the insolation energy that they receive into electricity.

In terms of water, insolation might be thought of as the intensity of rainfall. A heavy shower where a large amount of water per second falls on each square metre of an area, would be the equivalent of high insolation in terms of solar energy.

A number of things affect the level of insolation that might be received by a certain area. These include, the degree of cloud cover, the amount and density of atmosphere the sunlight has had to travel through, the time of day, the time of year, the latitude (north or south) of the area, the angle of tilt of the area that is exposed to the sunlight.

The graph below uses insolation data measured over several years by NASA and applies to an area known as the East Midlands in the UK. Few people who live in the East Midlands would say that they live in a particularly sunny part of the world, but as can be seen the insolation figures represent a considerable amount of energy. To put the figures in context: during the course of one year a standard football pitch in the East Midlands will receive about 4,600,000 kWh of energy from the sunlight that falls on it. Remember that a 20 Watt light bulb running for 50 hours uses 1kWh of energy.

Energy, Work, Power

Energy is basically the ability to do work. The standard unit of energy is the Joule. We can use the initially un-illuminating, but surprisingly useful, definition of a Joule of energy as…

“One Joule of energy is the amount of energy required to do one Joule of work”.

Yes in a peak of laziness it was noticed that you could use the same unit for Work and Energy. It’s just like we take for granted we can use the same unit to describe a volume of water as we do to describe the capacity of a bucket we might put water in. An empty bucket by definition has no water in it, but we might still describe it as a 2 gallon bucket. It reflects an automatic mental trick that we do, where we recognise that to fill the water bucket we would need 2 gallons of water. Similarly we would need 1 Joule of Energy to do 1 Joule of Work. So we can also say…

“One Joule of work is the maximum amount of work that can be done with one Joule of energy”

Whilst this is true, it hasn’t really moved us very far forward! Actually, it’s in moving things forward (or in fact in any direction) that we can begin to get a definition for work and hence energy.

“One Joule is the amount of work done when a force of 1 Newton is active on an object for a distance of 1 metre.”

Which helps if you’re the kind of person who’s constantly thinking of forces in terms of ‘Newtons’ – there are people who do, but they are probably in a minority. An easy way to get the feel of a force of one Newton is to imagine yourself on earth (so think ‘kitchen’ rather than ‘moon’ or ‘orbiting space station’) looking at an object having a mass of roughly 100 grams (just under four ounces). The weight that we feel when we pick the object up is the gravitational force of the earth and the object attracting each other (we think of this as the object trying to fall to earth). To hold the object stationary we have to resist that gravitational attraction between the earth and the object. The force we need to exert to hold the 100 gram object in place and stop it falling is about 1 newton (it’s actually more accurately about 0.98 Newtons). So that’s what a force of 1 Newton feels like – not very big really. So now if you imagine something pushing, or pulling, or lifting, or squeezing, etc, with that kind of force for 1 metre of distance, then you have imagined one joule of work, and you have imagined one joule of energy being used to do that work.

As an aside, you can see that you are unlikely to get tired out by doing one joule of physical work. Because a joule is quite a puny amount of energy, we end up using kilojoule quite a lot. A kilojoule is 1000 Joules and is abbreviated as kJ.

Now imagine that force of 1 Newton pushing, pulling, or whatever it is doing for 1 metre, but now think of it doing this repeatedly every second. Maybe the force keeps moving over the same metre length, or maybe it’s like a model car that keeps pushing itself with a force of 1 Newton along a metre of track every second. Then it can be seen that one Joule of work is done every second. Or in other words, one joule of energy is being expended per second. A rate of working (or energy usage/production per second) is such a useful concept that we have a name for it, power. And we also have created a specific way of saying ‘one joule per second’, this is called a Watt. A process that consumes or generates energy, or does work, at a rate of one joule per second is operating at a power of 1 Watt.

Because a Joule is a fairly small amount of energy, a rate of energy usage per second of 1 Watt is not a massive amount of power. Therefore just like kilojoules often are used in practice, so are kilowatts (kW). As you would expect, a kilowatt is 1000 Watts, which is the same as converting energy at a rate of 1 kilojoule per second.

Of course there are lots of ways of using energy which are more interesting than moving 100 gram masses 1 metre. In fact whilst a fair proportion of our energy usage goes into moving things (i.e. transport), much energy usage doesn’t involve moving large masses at all. For example electrical energy is used to create heat and light. However it’s still possible to equate the use of energy in a light bulb to the definition in terms of force and distance.

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