What is kWp?

kWp stands for Kilowatt Peak or put another way the maximum performance obtainable from a solar panel / module when operating under perfect conditions. kWp is a useful measurement to use when selecting and integrating components such as solar PV panels, cables and inverters in system sizing calculations.

Ensuring that cables, inverters and isolators are of a sufficient size to handle a solar PV arrays kWp output will ensure that these components will not overheat, get damaged or be a cause of power losses.

The actual output of a PV array is dependent on the light available to it. In the UK under normal operating conditions a PV array will rarely if ever reach it’s kWp potential; for example a 2kWp system will not run at 2kWp all of the time, instead PV array outputs fluctuate second by second based on the changing light conditions.

What is the difference between a kilowatt (kW) and a kilowatt hour (kWh)?

A kW is a thousand watts and a unit of power. A kWh is the amount of energy used (or in our case generated) when a kW of power works for one hour.

We use Watts (W), kilowatts (kW) and kilowatt peak (kWp) to measure power when sizing a system to ensure that all of the different components will work efficiently and safely together when connected.

We use kilowatt hour (kWh) and kilowatt hours (kWhrs) to measure how much usable power will be generated and or available for export/sale. Conventional electricity bills are measured and payable based on kWhrs, Feed in Tariff payments are also based on kWhrs.

Accurate kWh estimates can be prepared in advance of an installation based on historical solar radiation data combined with the characteristics of the installation and the location. In the UK we typically see that for every 1kWp of installed PV you get a return of between 800 and 1000 kwhrs of electricity per year. The better the system design in relation to it’s location the greater the kWhrs return will be.

If you would like more information about the various components found in both on-grid and off-grid solar PV systems; we have written an article where we describe the different components and the functions they carry out here: Solar PV System Components

How does a solar PV system work?

Daylight hits the photovoltaic cells and is converted to electricity. The inverter converts the electricity from direct current (dc) to alternating current (ac) which can either integrate with existing power systems or charge batteries.

Further information about the types of solar PV system and an overview of how they work can be found here: Types of Solar Photovoltaic (PV) System.

What’s the difference between solar photovoltaics (PV) and other solar energy technologies?
p>There are four main types of solar energy technologies:

Photovoltaic (PV) systems: which convert sunlight directly to electricity by means of PV cells made of semiconductor materials such as silicon. Concentrating solar power (CSP) systems: which concentrate the sun’s energy using reflective devices such as troughs or mirrored panels to produce heat that is then used to generate steam and then electricity.

Solar water heating systems: which contain a solar collector that faces the sun and either heats water directly or heats a “working fluid” that, in turn, is used to heat water.

Transpired solar collectors, or “solar walls,”: which use solar energy to preheat ventilation air for a building.

How is a Solar PV system connected to the national Grid?

Connecting a PV system to the distribution network is arranged by the installer working with the Distribution Network Operator (DNO). The DNOs are the power companies that run the power distribution networks (EDF, Scottish & Southern, UK Power Networks etc). There are two engineering recomendations (G83/1 & G59/1) that are followed by both the installer and the DNO to ensure that the solar PV system will integrate safely with the national grid.

Solar PV systems under 16A per phase (i.e under 3.68kWp) come under engineering recomendation G.83/1. Solar PV systems installed under G.83/1 can be installed without prior notice to the DNO, with the DNO being informed afterwards (within 30 days).

Solar PV Systems over 16A per phase will need to consult the DNO before the PV system is connected to ensure that the distribution network can handle the extra power being provided. Depending on the system size and the DNO’s preferences, larger systems may be installed either under G.83/1 or G.59/1.

Does a solar PV system need batteries?

If the building is connected to the National Grid then your system will not need batteries as any excess energy can be sold to and fed into the grid. During the winter this process can be reversed and the grid can feed your buildings’ electricity system, you will essentially use the grid as your energy store.

If mains electricity is not available or you want to store electricity locally you can use batteries. Batteries can also be integrated into hybrid grid tie / battery systems as a backup, protecting against mains power cuts.

What are the effects of shade on solar PV Panels?

Shading has a disproportionate affect on a systems performance. This is because the cell with the lowest illumination determines the operating current of the series string in which it is connected. This is one of the areas that must be covered in a survey carried out before the installation.

Modern arrays can bypass the effected diodes to minimise shade effects; but these effects must still be considered as system yields (kWhrs) as well as other factors will be affected.

What maintenance and cleaning do solar PV systems need?
Solar photovoltaic (PV) systems are silent in operation, have no moving parts and require minimal maintenance. At low tilts (less than 16°) horizontal glazing bars can sometimes trap debris which could lead to shading of part of the array. The design of the system should aim to minimise uneven soiling. Panels are pretty robust so if they get dirty rub off the dirt with a damp cloth or blast them with a hose (no detergent).Bird nuisance in particular roosting under panels has become a problem in certain areas and as a result we have developed a system to prevent this happening, please refer to our maintenance page.
How long do solar photovoltaic (PV) systems last?

The basic solar PV panel (interconnected, enclosed panel of PV cells) has no moving parts and can last more than 50 years. It is estimated that performance will decrease by less than 1% per year, which would mean that in 50 years they’d still be 60% efficient. Most solar PV panels are provided with 25 year power guarantees direct from the manufacturer as standard.

As well as solar PV panels a DC to AC inverter is needed to supply power to a property. The current lifetime estimates for solar PV DC/AC inverters is from 10-12 years. Although some manufacturers offer warranty extensions up to 25 years, solar pv system owners to be on the safe side should factor in a cost to replace the inverter every 10 years. Inverters cost from approx £800 for a sub 4kWp system.

Will I need Planning Permission for a solar PV system?

Installations above a certain size and in certain locations such as in national parks and world heritage sites will need planning permission. Planning permission is not usually required for residential properties unless the installation is over 4m high (unless on a roof, then no bigger than the roof and no higher than 200mm above the roof line). There is more about planning permission for solar PV systems here: Planning Permission for Solar Photovoltaic Systems.

What is The Code for Sustainable Homes (CSH)?

The Code for Sustainable Homes (CSH) aims to achieve a step-change in environmental performance of new UK homes setting out a timetable to reduce carbon emission in new build housing to Zero by 2016 via new Building Regulations.

CSH is targeted at architects, home designers and builders of new homes and covers water use, waste generation, and the use of low-polluting materials and processes as well as energy.

What grants and incentives are available?

From April 1st 2010 the UK Solar PV Feed in Tariff (FIT) sometimes refered to as the ‘Clean Energy Cashback Scheme’ was introduced. This is an incentive designed to encourage the installation of solar PV systems by providing a guaranteed repayment on the investment.

Payments are based on the size of system that is installed (the kWp) and the actual amount of power the system generates (kWhrs measured by an OFGEM approved generation meter). The Feed in Tariff (FIT) incentive is available for all types or customer including, homeowners, businesses, industry, schools and charities. We provide more information about the Solar PV Feed in Tariff (FIT) here: Solar PV: UK Feed in Tariff (FIT).