Glossary
Solar PV
Photovoltaic (PV): the effect that certain materials generate electricity upon exposure to light. Photovoltaic technology is used in solar panels on your roof that generate electricity for your house.
Solar Panel (Module): A photovoltaic panel made up of a number of 'cells', each of which produces electricity when exposed to light. Important panel characteristics are the size in watt-peak (wP), the efficiency (%), the lifetime (years), degradation factor(% after 30 years) and panel aesthetics.
Inverter: box used to transform the direct current (DC) electricity generated by the solar panel into alternating current (AC) electricity that can be used in the house. The most important characteristics of the inverter are the efficiency (% transformed into AC – usually around 90-95%) and the lifetime.
Power optimizers: With traditional (string) inverted Solar PV systems, the weakest panel reduces the performance of all the panels in the string as all the panels are connected in series. By using power optimizers this condition is eliminated as each individual panel is optimized to produce maximum energy and a single faulty panel does not affect the performance of the remaining panels. more info
Solar System: a number of solar panels, the inverter and the installation materials combine into a photovoltaic system. The size of a system is denominated in kWp (kilo-watt-peak), which is a measure of performance under standard testing conditions. Actual system outputs in kWh (kilo-watt-hours) depend on your location and roof characteristics (discussed in detail in solar education) and the solar system derate factor.
MCS accredited installer: the solar installer who sells and installs the solar system. MCS is a government regulated accreditation of installers. Currently there are over 4000 accredited installers in the UK. CompareMySolar helps you to compare and make a selection out of these installers and invite up to three for a free and non-obligatory site visit.
Your roof and solar system outputs
kWp (kilo-watt peak): The number of kilo-Watts (1.000 watts) a photovoltaic system will produce in peak conditions, which is the basic measure of performance under standard testing conditions. Needs to be combined with solar potential, insulation and the derate factor to get the actual system outputs in kWh on a specific roof.
Solar Panel efficiency: Efficiency is the amount of energy that a solar panel transforms per square meter. Standard testing conditions state that 1.000 kWh falls on one square meter in a year. Solar panel output given their efficiency is expressed in Watt-peak (Wp). This means a low efficiency (6%) panel would produce 60 Wp per square meter, a medium efficiency (12% panel) about 120 Wp per square meter and a high efficiency (18%) panel about 180 Wp per square meter.
kWh (kilo watt hours): Measure of electricity as generated by your solar system. Defined as power (watt) used for a period of time (hours). All your domestic appliances use a certain power (e.g. light bulb is 15watt, coffee machine is 800 watt) for a certain time (e.g. light bulb 4 hours per day, coffee machine 12 minutes or 0.2 hours per day). Thus they all use electricity (light bulb 60 watt-hour equals 0.06 kWh, coffee machine 160 watt-hour equals 0.16 kWh). Solar system outputs are measured on a yearly basis, and a 1 kWp system can generate around 800 kWh per year (light bulb uses 365*0.06 =22 kWh per year, coffee machine uses 0.16*365 = 58 kWh per year)
Solar irradiation: Solar system energy outputs depend on the location of your roof, as the level of solar irradiation varies across the UK. For a 1 kWp system this can mean a yearly output of between 675 kWh in the North and 975 kWh in the South. Roughly, Devon and Cornwall are the most attractive with over 900 kWh for 1 kWp, while in most of England and Wales a 1 kWp system generates between 800-900 kWh. Scotland and Northern Ireland are less attractive with between 700 and 800 kWh generated for a 1 kWp system.
Solar potential: the combined effect of roof factors of orientation (south is optimal), roof angle (between 35 and 40 degrees is optimal) and shading (none is optimal). Expressed as the percentage of electricity output that your roof can generate, compared to the optimal roof on that location. Perfect roofs have 99-100% solar potential, while 95% or more is excellent and 90% or more is still very good. Below 90% we would advise against going solar from a financial attractiveness standpoint, unless there is a strong wish to generate green electricity.
Derate Factor: the electricity losses throughout the system. Solar panels, the inverter and the cables each have their own inefficiencies which can be seen as electricity losses. Overall, it is safe to assume that only 80% of the actual electricity generated will reach the meter (to be either consumed in the house or exported to the utility).
System Output: calculated as solar irradiation * solar potential * system size * derate factor. Measures output from solar system in a year in kWh. For example a PV module could have a rating of 200 Wp. 12 modules are connected together on a roof to form a 2.4 kWp array (200 * 12 = 2.400 Wp or 2.4kWp). Using an irradiation of 1.000, a solar potential of 90% and a derate factor of 80% we will have system outputs of 2.4 * 1.000 * 90% * 80% equals 1.728 kWh per year.
Financial terms
Feed-in-tariff: for each kWh generated you will receive a government incentive. The amount depends on your solar system size and whether it is a new built, retrofit or ground mounted system. In systems below 4 kWp it currently is 15 pence per kWh generated. This incentive is guaranteed for 20 years, tax free, and will rise in line with inflation. The Feed-in tariff only applies to England, Scotland and Wales.
Export tariff: when you don’t use the electricity in your own house, you can sell it back to your electricity supplier. Current rates are 4.5 pence per kWh exported, and all electricity companies are legally obliged to pay these tariffs. This tariff is tax free and will also be adjusted to inflation.
Year 1 return: the money you earn for the electricity generated. Calculated as the sum of money from the feed-in-tariff, electricity savings and export tariff. For example, for a 4.0 kWp system that costs around 6.000 pounds and generates 4000 kWh per year you will receive 4000 * 15 pence = 600 pounds due to the Feed-in-tariff. Assuming you’d use 50% of the electricity yourself, you will receive a further 2000 * 15 pence = 300 pounds of electricity savings plus 2000 * 4.5 pence = 90 pounds of export tariff. This results in a year one return of 990 pounds, which is more than 15% return on your investment.
Payback time: a similar calculation as described in Year 1 return for all future years until the solar system has generated enough electricity to pay back the original investment. Factors like panel degradation, inflation and future electricity prices are assumptions, hence there is uncertainty in this estimate. On average, a payback time of around 7-8 years is realistic, after which it is expected that you can enjoy another 12 years of financial benefits and free electricity!
Return over 20 years: for the total return we use a conservative time period of 20 years, as it has been shown panels are capable of still performing well after 30 years of life, albeit with a slight drop in performance. Total return can be interpreted as a multiple of the initial investment, and returns of 3-4 times the initial investment are possible.
