The Basics

Considering Solar Power?

How to size your solar solution?

The first step into planning an new solar PV solution is figuring out what size it needs to be. When I say what size I don’t mean physical size, I mean how big power-wise, measured in Watts, or more often in kilo Watts or kW. This article will describe the typical logic used to calculate domestic energy consumption and how to apply that to your solar design so you know exactly what your minimum requirements are.

I have to say that although I am about to describe how to specify a solar energy solution power value that best fits your requirements, there are certain caveats. Namely although we can attempt to forecast the weather based on statistical averages and modern technology, we cannot be certain of the daily sunlight in your location, an exceptionally large variable. Then there are other variables like the angle to the sun, shading, temperature and the animal world that can affect efficiency.
I would also like to underline that this is intended as a rough guide to a non-technical person and only scrapes the surface when it comes to designing and specifying the right solar solution.

In fact, it is very difficult and time consuming to design a solution that you can be confident will tick all the boxes, it is certainly not an exact science. We can however presume many variables on the most part will be understood, accounted for, and mitigated where at all possible. Follow a few trusted dos and don’ts and you will be fine.
Right now, you will need to gather some information and maybe do a little due diligence if you are going down the self build route..

Describe your location for installation

How large is your roof or area where you wish to install the solar panels?

If the answer is ‘unlimited’ then you are half-way towards a perfect scenario.

If the answer is something else, then you will need to get your tape measure out. Measure your roof dimensions if possible (do not go hanging off a ladder just yet!).

If your roof is pitched then it is doubtfully accessible from the outside so enter your loft area and measure from inside, where it is a little safer. Obviously, you are looking to measure the side of the roof which is South most facing.

If your roof is a flat roof then it may be safely accessible from the outside, if so, measure edge to edge. If it is not accessible, then measure the walls below and try to be as accurate as possible.

Remember whichever type of roof you have you will need to observe any obstructions like vents, flues, chimneys, or fixtures decorating the roof, the maximum spacing is around 25cm between the rear of the panel and the roof so anything over that will not accommodate a solar panel above it.

Once you have your measurements you will not only know your total surface area but also how many of a particular solar voltaic panel you will be able to fit in either direction.

Check your roof eligibility

Can my roof accommodate solar panels?​

This is important to know, not only so you know what type of mounting to opt for and what type of tile/slate etc. to buy (trust me some will be broken during installation) but also that you are designing a solution that can cope with the structural & physical stresses that come from absorbing wind and weather effects.

You may not necessarily need to do these checks to size your solar PV system, but it is good to know what you are dealing with, especially if you are going down the self-install route. You could also find some areas of your roof that you will need to address before thinking about solar.

Measure the spacing between your rafters so you can identify if your roof can accommodate a safe amount of anchor points for the expected weather behaviour in your geographical location.

Inspect the rafters for rot or damage, also ensure your felt is in good shape. Any signs of problems then get a professional roofer in for an opinion or give us a call and we can have a look as part of our site survey.

Many different mounting systems are available, your roof type will dictate what you will need to use. There are systems available for slate, tile, pantile, flat, corrugated, trapezoidal and many other types, styles and variations of roof. Also, different methods are employed to ensure the anchor points are weatherproof from lead to rubber flashing.

Know your power consumption

How much power does your household or business consume?

Sounds tricky to figure out; But you can accomplish this in a few separate ways.

You could ask your supplier to provide some historical data or download it from your supplier’s website account if you have a paperless account/online account.

You can take a reading of your meter (assuming you only have one supply) over some days and find the average daily consumption. Better still a reading over more days will give better clarity.

If all else fails, you can assume your domestic household uses around 3578 units per year (or 3578kWh) which is considered the national average [sourced from Dept of business, energy & industrial strategy].

Another good solid way to understand your power usage is to make a list of all your appliances or hard-wired devices using AC power. Look at the label on each device and record the power rating (usually shown as a value with a ‘W’ after it) i.e., an electric radiator may be 2000W. To figure out from that how many kWh that is you need to understand what a kWh is, A kWh is one kilo watt of electricity moved for one hour.

Example, I use this 2000W radiator on average 2 hours a day during the winter so; 2000W = 2kWhours, and as its used for 2 hours 2 x 2kWh = 4kWh total energy use.

I also have a 1500W spin dryer that I use for 30 minutes each day; 1500W = 1.5kWh x 0.5 = 750Wh (0.75kWh) of energy use.

Combined would see you consume 4.75kWh of electricity in total.

Remember that not all devices will consume power all the time they are plugged in, appliances like refrigerators will only use power when they are cooling, once it reaches the right temperature it shuts off, typically a fridge runs for 20 minutes in every hour.

What next?

So I now have the physical and theoretical information to base my solution on

Before we go further, I would like to state that this guide is written with a grid-tied solar solution in mind and as such in the summer when there is more power produced there is likely to be an excess, whereas in the winter there is likely to be a deficit.

Any excess will be fed back into the grid, any deficit will be offset by the grid.

If you were able to get your power consumption information from you supplier then look at what your daily consumption is, it will show you either units or kWh used. You may need to divide by the days of the month if your figures are monthly.

The way you found your daily consumption is irrelevant, what is important is the calculated daily average value, say for instance our value came out to 11.4kWh (11400Wh) then you would ideally need to produce at least that amount each day. Obviously, this value will be the daily average so in the summer you will have an excess and in the winter a deficit of solar power.

Which PV system is suitable for me?

So how do I know what size solar PV system is best for me?

Remember – You may now know what power you consume but do not forget that you are limited by your roof space or whatever space you intend to place your panels. Here is where those roof measurements will come in handy; Calculate how many panels you can fit on your roof, remember to leave a 30cm gap around the array and the edge of the roof. The average wattage per square metre in the UK is around 150W. The average panel size is approximately 1.7 m x 1m although the size usually dictates the power they can generate, the larger the panel generally means a larger output power. Also, a little gap might be needed on each row depending on the mounting system employed.

Work out what your maximum wattage is to be gained from your roof and then see how that stacks up against your consumption, if your consumption is less then, happy days! You can likely produce more power than you need. If not, then it is unlikely you will be able to produce enough power to be fully self-sufficient but enough to drastically lower your electricity bill and find you a return on your investment.

Obviously when you are calculating how many panels you need, the amount of sun drastically affects the photovoltaic process and therefor the amount you can turn into power. You can find many websites in your which will give you all applicable solar information based on your specific location.

Rather than get bogged down in the nitty gritty and science of it, I will run over a few examples which are the more common domestic installation in the UK. I say this as most domestic installations will fall under 3 or 4 similar size & build strategies. This is mostly due to roof size & limited inverter power ratings.

In the UK, the most common size solar installation in the domestic market is 3.6kW but 4kW and 6kW are popular too.

A 3kW system will theoretically invert 3000W of DC (Direct Current) Solar power in one hour, therefor 4 hours of sunlight will net 4 x 3000W = 12000W or 12kWh. Now that is in a perfect world which we are not at present… Slice another 35% off for real world inefficiencies and the DC-AC inversion, 12000- 4200W = 7800W. So, 7.8kWh per day is realistically the top end of what you will get if everything goes your way conditions-wise for a 3kW solar power system.

A 3.6kW system will theoretically invert 3600W of DC Solar power in one hour, therefor 4 hours of sunlight will net 4 x 3600W = 14400W or 14.4kWh. Now that is in a perfect world which we are not at present… Slice another 35% off for real world inefficiencies and the DC-AC inversion, 14400- 5040W =9360W. So, 9.36kWh per day is realistically the top end of what you will get if everything goes your way conditions-wise for a 3.6kW solar power system.

A 4kW system will theoretically invert 4000W of DC Solar power in one hour, therefor 4 hours of sunlight will net 4 x 4000W = 16000W or 16kWh. Now that is in a perfect world which we are not at present… Slice another 35% off for real world inefficiencies and the DC-AC inversion, 16000- 5600W = 10400W. So, 10.4kWh per day is realistically the top end of what you will get if everything goes your way conditions-wise for a 4kW solar power system.

Use the same formula to figure out exactly what you need. Like I said earlier there are so many variables and outside influences that can affect your power production but if you do the research and find yourself some solar friends like Bolido Solar then you will be sure to see some benefits from going solar.

So, that is about all you need to know to have a good understanding of what your solar PV project specification is going to look like. I have not gone into too much detail and mathematics and tried to keep it light but detailed enough to be of use.

Other considerations

Look for a managed service based around this functionality

There are even one or two small incentives like certain export schemes can be entered into which may see you gain around £70-£100 per year for exporting your excess if you qualify.

Although as the sell price is around 5 pence per unit (kWh) and the buy price is around 15 pence it is probably more cost effective in the long term to size your solution slightly smaller, export less power in the summer and buy a little more in in the winter as although you pay more to import you will pay less on the initial build and lower your time to get a return on your investment. Saying that it really does depend on your individual circumstances and lifestyle.

If you plan to use a reasonable amount of power in the evenings or are in (or want to move into) an off-grid environment, you may need to design batteries into your solar PV solution to store your excess power during the day to use at night. If that is the case then you will also need to look up some information on off-grid type solutions and the sizing of battery storage, in another guide.

You can plan to make use of other renewable power sources like hydro or wind then again that is the topic of another guide.
You can also add monitoring into your solution to keep abreast of solar PV power production or any events happening in real time from anywhere in the world. We offer a managed service based around this functionality.