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Solar Panel Installation - System size

January 11, 2021

If you have made the decision to consider powering your facility via solar panels on your roof/land, the next logical question to ask is – ‘what size should the system be?

Firstly, assuming space is not an issue, it depends on your ultimate goal; are you –

Note that these questions are not mutually exclusive but identifying your ultimate goal is part of deciding on the correct system size.

Installing solar panels for financial reasons

If you are installing panels to for financial reasons, you will want to size your system to maximise self-consumption of the solar energy that is generated. 

The reason for this is that the amount you buy power for (import) and the amount you sell power for (export) are wildly different; the average price to import power is c.11-14p per kW/h wheres the price you are paid for exporting power is c.3-5p per kW/h.

With this in mind having a higher self-consumption rate will shave your off the time it takes for the solar system to pay for itself.

As a rough rule-of-thumb we tend to size arrays so that they have a minimum ‘utilisation rate’ of 80% (80% of the power consumed on site, 20% exported to the grid). The reason for this is that whilst the value of the utilised power is fairly easy to predict – energy prices are only going one-way, after all – the export tariff is typically guaranteed for only 12 months at a time. As a result, whilst we do want to build in some headroom for future expansion/energy storage, we do not want to rely too heavily on the export rate to justify the business case as this payment is likely to reduce overtime as the grid is overwhelmed with small-scale generators looking to export their power at the same time.

How is it calculated?

In order to determine the optimum size array to maximise return on investment (ROI) we must analyse your demand for the past 12 months/a typical year; analysing data for a full 12 months allows us to identify any patterns in your usage profile.
For most large organisations – although not necessarily immediately obvious from your bill – your electricity demand is measured in half-hourly increments, and can be easily downloaded as .csv file. The data, commonly referred to as ‘half-hourly data’ or HHD, shows the sites demand for each half-hourly period over 365 days.
Once we have this data, we then look at the limiting factor of the site – the roof/land. As soon as we have determined the largest system that can fit in this area, we model the theoretical output of the system over a 12 month period.
With these two sets of data, and using simple mathematics, we overlay one set of data with the other to determine how much of the solar energy will be consumed on site, and how much will be exported to the grid. If the utilisation rate is lower than 80% we simply start reducing the size of system until we reach the desired point.

Installing solar panels to reduce your carbon footprint

If this is your main driver, you will need to decide the impact you wish to make; do you simply want to make a significant dent in your carbon footprint or do you wish to use solar panels to achieve ‘net zero?’
The difference between the two is as follows –
  • A significant dent would see you install the largest system you can but it is usually limited by the space/budget available for solar panels, as well as the time it takes for the system to pay for itself
  • In order to achieve ‘net zero’ you would have to install a solar system capable of producing the same amount of electricity as your facility uses – even if that means generating more power than you are capable of using. 

How is it calculated?

As is the case with designing a system to maximise ROI, it all starts with HHD.
If your target is to make a significant dent in your carbon emissions, we must understand what that ‘significant dent’ looks like – for example, do you have a certain amount of carbon you wish to offset?
Assuming there is a target, we convert whatever that might be into kW/h’s using the Government’s latest carbon footprint conversion tool – https://www.gov.uk/government/publications/greenhouse-gas-reporting-conversion-factors-2020. This tool illustrates the current carbon footprint of the UK’s electricity supply.
Once we have this figure it is simply a case of designing an array that generates the equivalent kW/h.
If, however, your target is to achieve net zero, we design a system that generates the same number of kW/h as you consume.
Note, however, that for each of these aims, you may end up installing a solar system that exports more to the grid than you can consume on site which will ultimately dilute the ROI.
As with most things in life there is a balance to be struck; one that balances the economics with the environmental impact. BeBa have decades of experience designing systems and we welcome the opportunity to talk to you about your system.