Photovoltaics: Why size matters

December 2013 Download a PDF version of this article »

By analysing how much electricity is required by a business’ activities, at what time of year and even what time of day it is possible to specify PV systems that have the maximum impact in the most cost effective manner. These also provide the best return on investment figures.

Photovoltaics: Why size matters


Once the decision has been made to install PV panels on a building the next crucial step is to identify exactly how many.

In many, if not most, cases the equation is simple – a certain number of PV modules fit on the suitable available roof space and will generate enough electricity to contribute to, but not exceed, the electricity demand on site. Some times the local distribution network operator limits the number of panels that will be approved. It therefore makes sense to use as many as possible or allowable in these circumstances.

However, in many cases there may be less demand for electricity than the panels can produce. When this is the case there needs to be careful consideration of the electricity demand profile, the solar generation profile and how the optimum arrangement can be reached. There are also subsidy bands to consider; if installing the largest possible system takes you into a lower FIT band for example then it may be prudent to limit the output in order to retain the higher rate.

Simply putting as many panels as possible on a roof is often suggested, but frequently it isn’t the correct course of action.

By analysing how much electricity is required by a business’ activities, at what time of year and even what time of day it is possible to specify PV systems that have the maximum impact in the most cost effective manner. These also provide the best return on investment figures.

Solar generation profile – day by day

Solar PV systems generate different amounts of electricity depending on their situation. Time of year is clearly a key factor, with longer days and generally clearer weather conditions during the summer meaning far more light reaches their surface and more electricity is generated. This results in a broadly “bell” shaped distribution, with smaller quantities of solar generation in the winter months and the maximum in June. Weather patterns mean that some days will produce more electricity than others, resulting in spikes in the graph when days are plotted side-by-side. The chart below shows generation through the year from a PV system in 2013:

Generation from PV system in 2013

Electricity demand

Every business uses a different amount of electricity. Depending on the sector and business activity this demand will vary from day to day and it is possible to recognise trends that may be important when considering installing a PV system.

Where there is a similar amount of activity throughout the year, the amount of electricity that is consumed is often fairly standard (if the same number of electrical appliances are being used, in roughly the same fashion each day, it follows that demand will be fairly uniform).

If there is seasonal electrical equipment being used then this may not be the case. For instance if electric heating or air-conditioning are used this may mean that certain times of year have a much higher demand than others, or if the business has season-specific activity it might be that the demand is very low a lot of the time & very high for a given period. The second graph below shows an example of electrical demand for a year:

Electrical demand for a year

Optimising payback by matching generation and consumption

Maximum value – Tariffs, prices and export

Income from PV systems comes from 3 key areas:

  • The government subsidy available (FIT, ROC, CFD);
  • Savings that are made from not purchasing grid electricity; and
  • The sale of exported electricity that goes back to the grid.

The level of subsidy varies depending on the size of the system , depending on which ‘band’ the total installed capacity falls. If the available roof space points to an installation that falls just above one of these bands, it might make sense to settle for a smaller installation that receives the higher FIT rate.

FIT rate vs system size

Making a decision about whether a system should be reduced in size to fall into a different FIT band is straightforward, but it is the ratio of electricity used on site to electricity exported which needs to be managed in order to harvest the maximum value from the generated electricity.

The rate that is being paid for grid electricity is always going to be higher than the price that it can be sold back to the grid for. Essentially you are purchasing at a retail price, but you are selling at a wholesale price. When designing systems all of these factors need to be taken into account in order to specify the optimum arrangement.

Because of this, each unit of electricity used on site is more valuable than those that are exported; stopping yourself from needing to purchase a unit of electricity is better than selling one that you have generated. Striking the balance between generating the maximum that can be used on site, across the whole year, without producing too much excess electricity during peak months is where correct system sizing is essential.

Keeping the total yield figures within the overall consumption by careful design is the best way to ensure that maximum value is extracted from every kWh that a PV system generates.

Consumption vs PV yield

Solar generation profile – hour by hour

Path of the Sun

The orientation of the panels is crucial, with south facing roof surfaces being ideal because sunlight will hit them throughout the day. If the panels face east then the majority of the generation will be in the morning, if they face west then it will be in the afternoon. In some circumstances this can be an important distinction – depending when the highest demand is.

Path of the sun

The pitch of the roof or mounting system can also play a part; if there is a steep pitch this will hinder the amount of generation in summer, when the sun is high in the sky, but will improve its performance with a low winter sun. A shallow pitch will perform better in summer but will suffer in the winter since the light will be hitting the panels at an oblique angle.

Typical PV generation curve

The path of the sun through the sky means that on a clear day, for a south-facing system, the maximum generation is at noon.

In the morning there is a gradual increase in output, and after noon the output gradually decreases. This means that we again see a bell-shaped distribution of the yield.

Typical PV generation curve


In the winter months days are shorter, meaning that there is less light available for electricity generation (you can see in this graph that there is only generation from 8am until 4pm) and overcast weather periodically impacts the yield as cloud cover moves in and out of view (the peaks and troughs show where more or less light is making it through the clouds).

This example is taken from a day in mid-November. The red zone represents the total demand for electricity at this site, the orange area shows the proportion of this demand that was covered by the PV generation. The small yellow sections show times when electricity was being exported to the grid, i.e. the PV system was generating more electricity than the business required.

Mid-November PV generation

Here, 94% of the electricity generated by the PV system is used on site, so the business makes a high saving from each unit generated by the PV system – they hardly need to sell any at the low export rate.


In the summer months days are longer, giving far more time for the PV system to be active, and the sky is generally clearer, meaning there are fewer peaks and troughs in the generation profile.

The example below is the same system in July. The first thing to note is that electricity is being generated from before 6am right through to 9pm. This time the yellow section is far more prominent – indicating that a higher proportion of the electricity generated is being exported.

Mid-July PV generation

On this day 60% of the yield was used on site, with the remaining 40% being used elsewhere on the local distribution network. This is not ideal, since the lower export rate is applied to the 40%, however it is a consequence of balancing summer and winter generation profiles for maximum overall value.

Commercial PV

Solar solutions

NWT is a well resourced, established brand whose success and growth follows significant investment in PV technology development and knowledge generation. The UK team, based in Mid Sussex, includes specialists in every aspect of implementing solar energy generation systems on a commercial scale and we will implement PV systems anywhere in the UK. Our policy is to be transparent and accountable in every aspect of our performance.

Supporting our technical teams are planning experts, specialist legal and financial advisors, and a knowledgeable customer support team based in our offices in the UK.

NWT Energy advises and invests in energy generation and energy saving projects that reduce emissions and overhead for our clients, who are commercial, industrial, institutional, agricultural and government property occupiers and owners.

NWT Europe has been established for 20 years and operates within Europe, the Middle East, India and Australia. Our growth over recent years has allowed us to increase our offerings and create new employment opportunities within the countries in which we operate.

Peace of mind

The fact that we also have a UK solar installation company as part of the group puts us in a strong position because it enables us to identify potential sites, provide consultative advice on and deliver large scale commercial PV systems within a single client relationship.

Having the same group in charge of contracts, financial investment, project design, installation delivery, operation & maintenance means NWT are extremely well placed to help you take advantage of a lucrative opportunity without negotiating the risk of losing track of accountability.

Our financial backing is substantial and secure and we are ready to invest 100% in commercially viable opportunities. And because we retain a key interest in the operation of the system we only use the highest quality components and perform regular maintenance checks to ensure that everything is working as it should be.


Author: Hugo Logan, Programme Manager, NWT Energy


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