Commercial solar questions, answered

Yes. There is no sign-up, and you can download a PDF report of your results.

Find your site on the map and outline each roof or car park, or type in building dimensions. Add your postcode, the unit price you pay and how much of the solar you expect to use on site. Results update as you go, for buying outright or a Power Purchase Agreement.

They are a well-grounded first estimate, not a design. Generation uses published SAP 10.3 sunshine data for your postcode and a typical layout, so a south-facing roof may do better and a shaded one worse. A site survey and your half-hourly meter data give a firm figure. See how the calculator works.

Solar output varies by about a third across the UK. Your postcode puts you in the right one of 21 SAP sunshine regions, from around 780 kWh per kWp a year in Shetland to over 1,050 in Southern England.

It is the share of the electricity your panels generate that your site uses at the time, rather than exporting. It matters because solar you use is worth your full unit price, while exported solar earns around 5p per kWh. The default is 80%.

About 50 kWp, which needs roughly 350 m² of roof. Smaller systems are better served by domestic-style quotes.

Yes. Draw as many outlines as you need on the map, each with its own type, or add several buildings by dimensions. The calculator combines them into one system.

No. All costs are before VAT. Commercial installations are charged at 20%, which VAT-registered organisations can usually reclaim.

For a site that uses most of its solar, the calculator typically shows payback of around four to seven years when buying outright. Payback is longer where less solar is used on site, or where unit prices are low. Read more about how long commercial solar takes to pay back.

In the UK, a typical commercial layout generates roughly 800 to 1,050 kWh per kWp of panels a year, depending on location. A 250 kWp system at the UK average generates around 224,000 kWh in its first year. Read more about how much electricity commercial solar generates.

The calculator models 25 years to stay prudent, but a well looked-after system keeps generating well beyond that. Panels typically carry 25 to 30-year performance warranties and lose around 0.4% of output a year after the first year.

Not always. A battery helps when you export a lot of your solar, for example at weekends or on lightly used sites, by storing it for later. Where you already use most of your solar, a battery adds cost for little extra saving. Try both in the calculator.

Very little: an annual inspection, occasional cleaning and monitoring of output. Inverters usually need replacing once during the system's life, which the calculator allows for in year 13.

The calculator uses 0.196 kg of CO₂ for each kWh of grid electricity your solar replaces. A 500 kWp system at UK average sunshine avoids around 88 tonnes a year.

A contract where a funder pays for, installs, owns and maintains solar on your site, and you buy the electricity it generates at an agreed rate, usually below your grid price, for 10 to 25 years. See our funding guide.

Buying gives the highest return over the system's life because you keep all the savings. A PPA needs no capital and the funder handles maintenance. The calculator shows both side by side for your site.

Solar panels are special rate assets. Companies can claim a 50% first-year allowance, with the rest written down at 6% a year. Alternatively, most businesses can use the Annual Investment Allowance for 100% relief on up to £1 million a year. Speak to your accountant.

No. Full expensing gives 100% relief on main rate assets, and solar panels are special rate. Companies can claim the 50% first-year allowance instead, or use the Annual Investment Allowance.

Not in England until at least 31 March 2035. On-site renewable generation and batteries used with it are exempt from 1 April 2022 to 31 March 2035.

The Public Sector Decarbonisation Scheme is not taking new rounds. Great British Energy is now the main source, including its £1 billion Local Power Plan and its Solar Partnerships Scheme for schools and NHS sites.

In England, rooftop solar on non-domestic buildings is usually permitted development, with no capacity limit since December 2023. Listed buildings, some protected areas and larger ground-mounted arrays are the main exceptions. See planning and grid connection.

Yes. Almost every commercial system needs a G99 application and approval from your Distribution Network Operator before it is connected.

For a typical rooftop system, allow around 12 weeks for a connection offer. It can take longer where the local network is constrained.

A cap on how much electricity your system can send to the grid, enforced by a control system. DNOs sometimes offer one where the network is constrained. It costs little if your site uses most of its own solar.

Yes, with your landlord's consent. Your lease should run longer than the payback period, or the PPA term, and you should agree who owns the system at the end.

A straightforward rooftop project usually takes five to seven months from enquiry to switch-on, of which installation itself is often around four weeks. The grid connection is usually the longest step.

Usually very little. Most work happens on the roof, and the final connection needs only a short, planned shutdown of the affected supply.

Replace or overclad it before installing solar. Removing and refitting panels later is expensive, so a structural survey should confirm the roof will last as long as the system.

Yes, but output is much lower. UK systems produce most of their energy between April and September, which is already reflected in the annual figures the calculator uses. Read more about generation through the year.

Not on its own. Standard grid-connected systems shut down when the grid fails, for safety. Backup needs a battery and controls designed for it.