A question we are often asked at Exeo Energy is ‘What are the most efficient solar pv panels?’ This is an interesting and constantly evolving topic. However there are a few key areas worth discussing when we are talking about solar panels and efficiency.
The most efficient solar pv panels can be found in research labs – and are not commercially available. So an article in a New Scientist magazine stating some world record in solar panel efficiency is probably talking about a single photovoltaic cell, using a new technology that could be years away from mass market, if it ever reaches mass market at all. At Exeo Energy we are interested in new developments in the solar industry but we are primarily concerned with commercially available solar panels that we can integrate into our systems.
Solar panel efficiency is not really about solar panel power output – it is more about area and cost. So a 4 kWp solar panel system using high efficiency panels is not going to generate more energy than a 4 kWp solar panel system using low efficiency panels – it is just going to take up less room on the roof and cost a bit more. Great if you have a limited roof are but if you have lots of roof area, not a big concern.
Solar panels are only one part of the solar pv system. If you are looking at the entire systems efficiency you also need to look at the efficiency of the solar panel inverter and any energy losses in the cable runs.
Interestingly (and counterintuitively) mono and poly crystalline panels get less efficient as the panels temperature increases. So on a hot summers day the efficiency of the panel will decrease, but the total solar energy hitting the panel is much higher so you still generate much more power when its sunny.
Before we answer this question in any depth we need to establish clearly what efficiency actually means in the context of solar panels. Generally efficiency can be described as the ratio between energy into a process and the useful energy out of a process.
Lets take a light bulb as an example. Light bulbs, especially older ones give out some heat energy (i.e. in the context of lighting, nonuseful energy).
So lets say the light bulb consumes 100 W but only gives out 60 W of useful light energy. There is another 40 W of nonuseful energy going to waste.
Efficiency = useful energy delivered from system / energy supplied into to system
= 60 / 100
= 0.6 also expressed as 60%
In the context of a solar panel we would be looking at:
Energy supplied to the system – this is the solar energy hitting the solar panels – this is measured in W/m2
Useful energy delivered from the system – this is the electrical energy that is generated from the solar panels this also need to be measured in W/m2
Lets take a nice sunny day in the summer in the UK, you could expect say 750 W/m2 of solar energy reaching the surface of the earth.
Now a 250 Wp panel normally covers 1.6m2. so a single m2 would be taken up by 156.3 Wp.
Now if we measure the power out of that panel to be 220 Watts of electrical energy this needs to be divided by 1.6 to take into account the fact we are only looking at a m2.
220 W / 1.6 m2 = 137.5 W/m2  this is the useful energy delivered from the system
Therefore we have an efficiency of:
137.5 / 750 = 0.183 or 18.3%
We could do the calculation the other way around too – if we know the efficiency of a panel we could work out the output under certain light conditions.
sunny, clear sky summer: 600  1000 W/m² winter: 300  500 W/m² sunny, skattered clouds or partly cloudy summer: 300  600 W/m² winter: 150  300 W/m² cloudy, fog summer: 100  300 W/m² winter: 50  150 W/m²


Now a key thing to remember with efficiency is that if you have two panels that are say 250 Wp but different efficiencies, one will not generate more than the other, they will both generate the same, but because of the better efficiency one will need less area to generate the same electrical power output.
This is the same on system level, if you have two 4 kWp solar panel system each with a different efficiency panel they will both output the same electrical power but the more efficient panels will take up less room that the less efficient panels.
Solar panel do tend to cost more the more efficient they are. If you are trying to fit the maximum amount of kWp of solar panels on a limited area in order to get the maximum yield from the available are then using high efficiency solar panels makes sense because the higher yield often justifies the higher price of the efficient solar panels. However if you have unlimited room and you only want a small system then there is no need to install highly efficient panels  you can use lower efficiency panels that will take up more room but still generate the same amount of power and crucially cost less to install per kWp.
A couple of examples:
Example 1:
A small domestic property with a roof area only suitable for 6 solar panels.
Standard efficiency solar panel option: If we used a standard efficiency panel for example a 250 Wp solar panel with an area of 1.6 m2 we would end up with a 1.5 kWp system.
Estimated annual yield: xxxx kWh
Estimated cost: £
High efficiency solar panel option: If we used a high efficiency panel, for example a 315 Wp panel, taking up exactly the same area of 1.6m2 we would end up with 1.89 kWp solar panel system
Estimated annual yield: xxxx kWh
Estimated cost: £
Example 2:
A domestic property with a very large roof area that could fit 40 panels.
We might decide with the client that they wanted a 4 kWp system as this would provide a reasonable amount of energy and be within their budget
Standard efficiency solar panel option: Again if we used a standard efficiency panel for example a 250 Wp solar panel with an area of 1.6 m2 we would need x16 units to make up a 4 kWp system.
Estimated annual yield: xxxx kWh
Estimated cost: £
High efficiency solar panel option: If we used a high efficiency panel, for example a 315 Wp panel, taking up exactly the same area of 1.6m2 we would end up with 1.89 kWp solar panel system
Estimated annual yield: xxxx kWh
Estimated cost: £
It is worth noting a few things when talking about efficiency. The first is that there is a difference between cell efficiency and solar panel efficiency