Solar Electricity - Frequently Asked Questions

How to size a system?
Excluding electricity that may in some properties be used for heating and cooking the Energy Saving Trust estimates that the average 3 bedroom house consumes 3,300 units of electricity (kWh) a year.

If you are on mains electricity at the moment and have access to the electricity metre in the property your usage can be easily worked out through taking meter readings at the same time of the day in a typical week and working out the minimum, average and peak usage. We have put together a spreadsheet that you will hopefully find useful in working this out - Home Appliance Energy Usage

In a new build, off-grid situation or a new home a useful excercise is to make a list of all the appliances that will use electricity in the property, note their wattage which will be printed on the devices and work out how often these devices are used.

Once you have this information we need to work out your total usage or 'peak load' for a typical day. The common measurement for power usage is Watt Hours or Kilowatt Hours (kWh) i.e. how many watts you use in a hour.

What is kWp?
kWp stands for Kilowatt Peak or maximum load operating under perfect conditions. In our experience kWp is a useful measurement to use when selecting and integrating components such as charge controllers and inverters in system sizing calculations. As a rule of thumb we calculate 75% of a systems rated kWp as the real kW output once installed in a suitable location in the UK.

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 work done, or energy used when a kW of power works for one hour.
What are the components of a photovoltaic (PV) system?

* PV modules or PV panels
* An inverter for converting direct current (dc) into alternating current (ac) and if a Grid-Tied system connecting to the national grid.
* Wiring
* A charge regulator or Charge Controller (for systems using batteries)
* Mounting hardware or a mounting framework.

How does a 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 integrates with existing power systems.
What's the difference between PV and other solar energy technologies?

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.
* Concentrating solar power (CSP) systems, which concentrate the sun's energy using reflective devices such as troughs or mirror panels to produce heat that is then used to generate 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.

What does energy conversion efficiency mean?
Energy conversion efficiency is an expression of the amount of energy produced in proportion to the amount of energy consumed, or available to a device. The sun produces a lot of energy in a wide light spectrum, energy conversion efficiency for Solar PV devices are typically from 7% to 17% efficient.

What About Grants?
At the time of writing grants are available via the governments Low Carbon Buildings scheme. You will need to fulfill some intial criteria before being eligable to apply, these include:

* Having loft insulation that meets or exceeds current building regulations
* Having cavity wall insulation if applicable.
* Having low energy light bulbs fitted into all appropriate light fittings.
* Have basic controls on your existing heating system such as room thermostats and timers.

For Solar PV systems the maximum grant available is up to 50% of the eligable costs up to £2500.

We always suggest contacting us as soon as possible, especially if you do not currently meet the minimum criteria.

We will work with you to ensure that grants are maximised at every step and ensure that work isn't repeated or equipment duplcated saving both cost and time.

In addition many local authorities and other groups provide regional funds for renewable technology installations. We will guide you through these processes and carry out the relevant homework as part of the survey process.

How is a Solar PV system connected to the national Grid?
Connecting a PV system to the distribution network will require permission from the Distribution Network Operator (DNO). The DNOs in the UK have different policies when it comes to connecting PV systems to their networks, and so different rates will be paid for exported electricity. We can help make the necessary arrangements for grid connection, as part of the service we will also explore and share all the options with a view to maximise the financial and or environmental benifits of your system.

What is net metering?
Net metering allows buildings with PV systems to use any excess electricity they produce to offset their electric bill. As the PV system produces electricity, the kilowatts are first used for any local requirements within the building. When the PV system produces more electricity than the building needs, the extra kilowatts are fed into the utility grid.

Does a PV system need batteries?
If the building is connected to the National Grid then you 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 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 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 covered in the survey carried out before installation. Modern arrays can bypass the effected diodes to minimise shade effects; but these effects must still be considered.

How does the angle a PV Panel is installed affect performance?
The maximum total annual solar radiation is usually at an orientation due south and at a tilt from the horizontal equal to the latitude of the site minus approximately 10-15 degrees. For example 30 degrees is an optimal tilt in Southern England, increasing to almost 40 degrees in Northern Scotland.

If the optimum angle is not achievable, over 90% of the maximum annual energy can still be achieved at 10 degree and 50 degree tilts. South-facing vertical facades generate around 70% of the maximum.

What other points should be considered?

* Method of fixing/ integration into the building.
* Planning Permission, conservation area requirements may mean solar tiles, solar slates and other BIPV products will be needed.
* Ensure that the fixings do not cover or shade any part of the PV cells.
* The fixing must allow for thermal expansion without breaking the glass
* Weather sealing
* Ventilation, the back of PV panels can reach 80 degrees if poorly ventilated.
* The mounting option must allow for safe maintenance and possible replacement of individual modules.
* The life of the support structure must be at least that of the PV array. The preferred materials are aluminium, stainless steel or glass-fibre.
* Protection from corrosion
* Wind loading
* Any extra weight
* How and where to run electrical wiring
* Where to place junction boxes.

What are the relevant codes of practice and articles of legislation?
* BS EN 61215
* BS EN 61646
* The Electric Supply Regulations 1988
* The Building Regulations 1991 (and amendments)
* The Construction (Design and Management) Regulations 1994

How long do photovoltaic (PV) systems last?
The basic PV module (interconnected, enclosed panel of PV cells) has no moving parts and can last more than 30 years. It is estimated that performance will decrease by under 1% per year, which would mean that in 50 years they'd still be 60% efficient.

What maintenance and cleaning do PV systems need?
Solar photovoltaic systems are silent in operation, have no moving parts and require no maintenance. At low tilts 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.

Glen Freeman is a renewable energy systems designer and co-owner of In Balance Energy Ltd. In Balance are specialist designers, suppliers and installers of bespoke energy systems for homeowners, businesses and communities in the UK. Their website can be found at http://www.inbalance-energy.co.uk