Surveying Your House For Optimum Usage

When surveying your site for optimum usage there will be three main things that you'll be looking for:

1. Your Household's Power Requirements: Typical power consumption on a daily basis, taking into consideration all electrical devices and lighting requirements, so you know what your system must achieve

2. Your Location's Solar Irradiance: Solar Irradiance or Insolation, as it's also called, is made up of the number of hours of sunlight and the strength of that sunlight at your particular location, taking into consideration the changes that occur through the course of the year.

3. Your Site's Peculiarities:This could be anything about the property or surrounding area that could influence the output and income capabilities of your solar PV system and will be assessed through a thorough survey of your site. At the end of this chapter you should know what type of system you will require to meet your household requirements, what you can expect from a solar PV system and what you need to do in order to maximise the income you can generate from the solar energy you will be producing.

In each case you will be shown how to work out these calculations manually so you can understand why these calculations are needed and why you would want to get them as accurate as possible. You will also be shown a Keep It Simple option which will help you to make quick calculations and give you access to tools that will make things faster and easier for those less scientifically minded.

1. Your Household's Power Requirements Power consumption for your site needs to be measured in watt-hours (Wh). You can get this information from your existing electricity bill or, if moving into a new house, estimate it for yourself. The wattage of household appliances can be found on the bottom of the appliance; on the power supply; in the product manual or you can measure them with the use of a watt meter.

A watt meter is a device that you plug into the wall socket and then plug the appliance into the meter. The amount of power the device is using will be displayed on the LCD and is the most accurate way to gauge the amount of power being consumed. You can list everything from living room lighting to charging mobile phones and don't forget that if you have gas central heating the control and the pump will require electricity too.

2. Your Location's Solar Irradiance

There are two main factors required when calculating solar energy:

The number of hours of sunlight

The strength of the sunlight

This combined information is referred to as solar irradiance or insolation and is described as watts per square metre (W/m2) or kilowatthoursper square metre over a day (kWh/m2/day). The wattage of photovoltaic solar panels is given on the number of watts of power they are expected to generate and based on a solar irradiance of 1,000 watts per square metre. This reading is what you would expect at solar noon in mid-summer and not an average daily achievement. Solar irradiance varies from place to place and, on top of that, changes throughout the year have to be factored in also.

Once you know the solar irradiance for your location (the number of kilowatt-hours {kWh} per square metre per day) that figure needs to be multiplied by the wattage of the chosen solar panels and will give an idea of the amount of energy you can expect your solar panels to provide on a daily basis.

Another benefit to knowing the solar irradiance values of your location throughout the year is so that you can see the best angle for the panels in summer and winter and an average can be calculated for the best all year round tilt for fixed roof mounted panels. Taking into consideration the angle and direction of the roof, the panel mountings can be adjusted to provide the optimum tilt for your solar panels. Fortunately, NASAs weather satellites have been monitoring solar irradiance around the globe for decades and following are the solar radiance values for London.

Keeping Solar Irradiance Values Simple

An average December day in London, a solar array lying flat gets the equivalent of 0.6 hours of midday sun (36 minutes). In contrast, an average June day in London will see the equivalent of 4.86 hours of midday sun (4 hoursand 50 minutes). Panels in an upright position of 90O would get 1.01 hours in December - that's just over an hour - and only 2.80 in August (2 hours 48 minutes). So this should illustrate how important the angles of the solarpanels are when it comes to generating power.

If a tilt of 22O gets the best winter results and a tilt of 54O gets the best summer results, we know that if we take an average of 38O this will get the best overall results across the whole year and that's what we're looking for (not the maximum power output EVERY month).

Keeping Optimum Tilt For Fixed Solar Panels Simple

So, put another way, to get the optimum tilt for fixed solar panels, for all the year round power generation, the calculation is: 90O – your latitude = optimum fixed year round setting So London would calculate as: 90O – 51.5O = 38.5O.

Based On Solar Irradiance, How Much Power Will Your Solar Panels Generate?

Based on the solar irradiance figures for your location, you can calculate how much power your solar panel will give you per day on a monthly basis. This is done by multiplying the monthly solar irradiance figure by the manufacturer's stated wattage of the panel:

Solar Irradiance x Panel Wattage = Watt-Hours Per Day

For example, if we take the December reading of the panel tilted for optimum all year round power and multiply it by 20-watt solar panel we can calculate the following:

1.05 x 20w =21 Wh of energy per day through December

You can use solar irradiance to give you a guide to how much capacity you will need from your solar array too. This will be approximate at this stage because there will be certain other factors to take into consideration in order to calculate the exact capacity required, like your site's peculiarities, which we'll be covered in the next section of this chapter.

Meanwhile, you can calculate a ball-park requirement for your solar system that will help you to cost it too. Take the figure you calculated for your total number of watt hours per day and divide it by the solar irradiance figure for the worst month.

3. Your Site's Peculiarities

There will be factors peculiar to your site that will affect the output and income capabilities of your solar system. Some of these things may be unchangeable like the size of your roof, others will be changeable like a tall garden tree that's overshadowing your panels and blocking out too much sunlight but could be kept trimmed back in order to overcome the problem.

There will also be things that are unchangeable but, once you know they will obstruct you from getting the best from your solar panels in the format you'd planned, you can find an alternative solution. For instance, a small roof may not be able to accommodate the panels you had planned on using, as you cannot fit enough of them to generate the power you require. However, you could use less panels of a more powerful system. The smaller installation required may go some way to meet the extra expense.

In order to identify these peculiarities and assess what needs to be done about them, a thorough survey needs to take place.

Some Tools You May Need To Do Your Survey

Spirit level
Tape measure

Your solar PV system installer should do a thorough survey of your site before proceeding. The following will provide you with an understanding of what is required to get the maximum effect from your system. For those who like the scientific side of things and would like to have a go at surveying their site for themselves, here are some tips and tools for going about it simply and inexpensively.

What You Will Be Looking For

You will be looking to answer these two questions:

What will be the most suitable position for my solar array?

The answer to this question will tell you if the solar energy system you're thinking of installing is viable or not and, if it is,how to get the most from it.

What are the obstacles and will they block out too much sunlight?

This is the most important question, as obstacles blocking out sunlight can dramatically reduce the efficiency of your system. The answer relies on knowing where the sun will be positioned in the sky at different times of the year in respect to your exact location. It's because the sun is much lower in the sky during the winter months that the solar array can experience more obstruction from surrounding features. Shadows from nearby buildings may only occur in winter time but you need to know so you can estimate the problem and find a solution. Perhaps it's a case of mounting the panels higher on the roof than originally anticipated.

The Importance Of Sunlight And Shade

Although solar panel systems do not require constant direct sunlight to operate, obstructions to the sun's light - like trees, buildings and even telegraph poles - can all make a difference to getting the most from your solar energy system. As the sun ‘moves' across the sky during the course of the day and year, the obstacles blocking its light will change and it's important to know what those obstructions are and how those changes will affect the power that can be generated in order to take the necessary course of action.

There are ways of plotting the sun's path across the sky and checking shade. It changes throughout the year so calculations can't be based on one day's findings but if you've never done a survey like this before, choosing a sunny day will help you get a better idea of the things you need to look out for.

Taking Note Of The Obvious First

Does the roof appear to have good access to sunlight and what obvious problems, if any, can be observed? Check there are no obvious obstructions like buildings, telegraph poles and trees that will block the sunlight from the roof. Having solar panels installed is a long term investment so include trees that are too small to worry about right now but could be a problem in a few years time. Don't forget to include other things closer to home like dormer windows, skylights and chimney stacks, that will either obstruct the sun or limit the positioning of the solar array.

IMPORTANT: It's useful to have access to the roof to check its suitability but do be aware of the dangers. Many deaths and injuries are caused through falling from roofs.

You know that the best position for the panels is south facing so which part of the roof, front or back seems the best for that? Use the compass to check and remember angled supports can be constructed at the mounting stage to ensure panels are positioned correctly.

Similarly, if you have a flat roof, the panels can be fitted using angled mountings for best results. Also take into consideration that panels can be mounted on walls at an appropriate angle without protruding too far from the wall as well as being mounted on a post or on the ground.

The way the roof is facing is one consideration but the pitch or angle of the roof is something different and needs to be taken into account also. The angle of the roof can be calculated with a protractor placed at the base of the roof rafter underneath the roof itself. Professionals use an inexpensive tool called a Roof Angle Finder or Magnetic Polycast Protractor which can be bought from builders' merchants. All you do is press the tool up against the roof rafters and you will be given a reading of the roof angle in degrees.

The structure of the roof also needs to be checked to see if it's strong enough to take the weight of the solar array and that's a tricky one. If you have no experience in building structures, it's going to be very difficult for you to judge accurately. If in doubt, get advice from a builder or architect - although the installer will be able to assess this for you when the time comes.

Solar panels individually only weigh about 15 to 20 kilograms, which is only about 33-44 pounds, but when multiple panels are combined within an aluminium frame the combined weight can make a significant difference. As the panels are going to be in situ for some time it's important that they are well supported.

When you're thinking about positioning and getting the maximum efficiency from your system, maintenance should not be overlooked. You may want to think about the possibility of mounting the panels at the lowest point of the roof, providing it doesn't compromise their effectiveness in any way.

This would make them more accessible for you if you decide to clean them yourself and most window cleaners use telescopic window cleaning kits these days and will be willing to clean them from time to time. Rain usually keeps the panels quite clean but a wipe over every few months can prevent the build up of grime and keep the panels working to maximum levels of efficiency.

Whatever you decide, you will need to get a measurement of the roof space available for your solar array. To get a really good visual idea, some people use cut-outs made from cardboard boxes to represent the panels and place them on the roof in position. Not only does this provide a visual but it could also help to identify any obstacles that may need consideration.