How much energy do solar panels produce for your home?
Over the last decade, solar technology has grown in popularity as more people begin to see the numerous benefits it can bring, both environmentally and financially.
Homeowners who are looking to switch to solar energy begin the process with lots of questions, including whether installing panels on their home is really a commitment worth making.
• explains how the technology works
• looks at how much electricity solar panels are capable of producing
• tells you what to do if your panels aren’t performing properly
How do solar panels work?
Solar panels use photovoltaic (PV) cells made of silicon to absorb sunlight and create an electric current (a direct current, or DC). Because household appliances can't use this type of current, a small unit known as an inverter - installed within the home, often the loft - converts the DC to the alternating current (AC) that electrical items need to work.
As PV cells turn the sun’s light energy (not its heat) into electricity, normal daylight is more than sufficient for the panels to do their job. They don't need intense, bright sunshine - although they are undeniably more productive on sunny days with no clouds, and during the summer when there are more daylight hours. They also work better in locations that get less cloud coverage.
You’ll still be relying on the national grid for some of your electricity, as the panels are unlikely to generate enough for your needs at all times. And as solar panels can’t produce electricity when it’s dark, you’ll need your normal supply during the night too.
Several factors influence how effective a solar PV system will be.
What makes solar panels fully effective?
The UK’s latitude (its point on Earth in relation to the equator) is 51 degrees north, meaning the sun is always south of your house and never goes directly over it. Consequently, south-facing roofs get the best results, although solar panels will still work on roofs that face east or west.
A roof that’s tilted at an angle of around 30 degrees is said to give the best overall performance.
It must be free from shadows and obstacles (e.g. trees), as anything blocking the sunlight will make the panels less efficient.
Capacity of your system
A solar panel is typically made up of 60 individual PV cells wired together. A solar PV system connects several of these panels, and is known as an array.
Measured in kilowatt hours (kWh), the capacity (sometimes called ‘rated capacity’ or ‘rated output’) is the maximum amount of electricity the system can produce under ideal conditions. This is taken to be 1,000 watts (or 1kW) of sunlight per square metre of panel and known as ‘peak sun’.
Most domestic solar PV systems have a capacity of between 1kW and 4kW. A standard 60-cell solar panel will produce around 265 watts of power. So, four of those panels receiving peak sun for one hour would generate over 1,000 watt hours (or one kilowatt hour, or kWh) of electricity.
If ‘kilowatt hour’ sounds familiar, it’s because your energy providers bill you for your electricity by the kWh, and charge you a rate of so many pence per kWh.
Efficiency of your panels
A panel’s efficiency is in how much sunlight it can turn into electricity. Solar panels are never 100% efficient - in fact, most panels installed on houses have an efficiency of around 20%. It is possible to find more efficient panels (around 40% to 50%), but these tend to be expensive and beyond most people’s budgets.
Panel manufacturers provide their own figures for the efficiency of their products, after testing the panels under what are known as Standard Test Conditions (in other words, the absolute best possible conditions in terms of sunlight, roof angle and so on). However, because no domestic solar panels will ever be operated in ideal conditions, it’s best not to use this as a guide when making your own purchase.
The type of materials used to make a solar panel can also have a bearing on efficiency. Monocrystalline panels are made of higher-grade silicon and are the most efficient (in terms of output and space), while polycrystalline panels are slightly less efficient but cheaper to buy. It’s generally the case that higher efficiency solar panels cost more but use less roof space.
Not all areas of the UK get the same amount of sunlight. The south of England is the sunniest part of the UK, benefiting from high pressure and its effect on clearing cloud from the sky. The amount of sunshine falls incrementally as you move inland and further north, which has a slight effect on how productive solar panels can be.
How much electricity would solar panels produce for me?
Every solar PV system is different, and so it’s hard to be exact about how much electricity yours would generate (given the factors mentioned above). However, there are a few general benchmarks that you can use to estimate your system’s output.
The most popular domestic solar PV system is 4kW. A system of this capacity on an average-sized house in Leeds can produce around 2,850 kilowatt hours (kWh) of electricity in a year. (This does depend on the climate and the amount of cloud cover.)
One method is to work out how many kilowatt hours of energy your panels would produce each day. Use this formula:
Energy produced per day/per month
• Size of one solar panel (in square metres) x 1,000
• That figure x Efficiency of one solar panel (as a decimal)
• That figure x Number of sun hours in your area per day
• Divide that figure by 1,000
So, for example:
• Panel is 1.6 square metres in size:
o 1.6 x 1,000 = 1,600
• Panel is 20% efficient:
o 1,600 x 0.2 = 320
• Your area gets 4.5 sun hours per day:
o 320 x 4.5 = 1,440
• Divide by 1,000:
o 1,440 ÷ 1,000 = 1.44 kilowatt hours per day
Because the number of sun hours varies greatly throughout the year (4.5 hours is an estimate for July), you should expect this figure to be much lower during winter months in particular.
For a monthly total:
• Multiply the daily figure by 30:
o 1.44 x 30 = 43.2 kilowatt hours per month
Energy produced per square metre
A 4kW system will have around 16 panels, each of which is approximately 1.6 square metres in size. If each solar panel is rated to produce roughly 265 watts of power (in peak conditions), the system will have a total output of around 165 watts per square metre.
• 16 panels at 265 watts each to get the system’s capacity
o 16 x 265 = 4,240
• Divide the capacity by the total size of the system (25.6 square metres)
o 4,240 ÷ 25.6 = 165
Again, this is only in ideal weather conditions, and all the other factors that determine a solar PV system’s effectiveness come into play.
Can I store the electricity my panels generate?
When you register your solar PV system for the Government’s Feed-in Tariff scheme, you save money in three ways, one of which is called the ‘export tariff’. This means you receive a payment for electricity you produce but don’t use yourself.
However, because this payment is capped at 50%, it’s still in your interest to use as much as your generated electricity as possible - including storing it through a battery and using it as night.
Companies like Tesla, Samsung and Powervault now manufacture batteries for storing solar energy and have made them available to people in the UK. The technology is still fairly new and so these products can be quite expensive - although, like with solar panels, the cost is gradually coming down.
Any battery you install must be compatible with your solar PV system and have the correct voltage, so you should always seek professional advice. Your solar panel installer will be able to tell you what kind of battery (if any) is best for you.
Is there a way to check that my solar PV system is working efficiently?
When your solar PV system is fitted, it’s connected to a control panel called an in home display. This is usually a wireless device that you can keep in your home and use to monitor how your system is performing and whether it’s generating as much electricity as it should be.
We don't recommend ever tampering with the solar panels in any way, as this can damage the system and invalidate your warranty. If you’re concerned that your panels aren’t performing to their full efficiency, you should contact the installer or the manufacturer, who will likely send out a professional technician to investigate.
Depending on the fault, the technician may use a multimeter. This is a digital device that measures electrical output by voltage, current and resistance and can detect whether an electrical appliance is working as it should be.
How would a technician test my solar panels?
They will test one panel in your system, preferably on a bright, sunny day when the panels are getting direct sunlight (they might have to disconnect a panel to achieve this). On the back of the panel is a converter box, inside which are positive (+) and negative (–) connections. The technician will hook up the multimeter to these connections to measure the voltage (measured in volts) and current (measured in amps) and check that they’re correct.
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