Commercial solar calculator

Outline your roof, add your postcode and unit price. Results update as you go, for buying outright or a Power Purchase Agreement.

1. Your roof or site

Enter the length and width of each roof in metres. Add as many buildings as you need.

Mount type

2. Location

Site postcode

Add a postcode for local sunshine data, or pick a region below.

Region

3. Energy use

Unit price you pay (p/kWh)

Your current all-in unit rate, before VAT.

Solar used on site80%

80% suits most sites busy through the working week. Warehouses, farms and sites closed at weekends are often 40 to 70%. The rest is exported at 5p/kWh.

Not sure? Measure it from your half-hourly meter data with our free analyser. It runs in your browser and sends the figure back here. Why it matters.

4. Options

Add battery storage

Stores solar you would otherwise export.

System size

Your results will appear here

Draw or enter at least one roof to see your figures.

System size and generation
Savings, payback and carbon over 25 years
Buying outright compared with a PPA
A PDF report to share

Working out your figures

Contract –

Estimated saving in year 1

–

Payback–

Net benefit over 25 years–

Your net position over 25 years

Your system

Roof area– m²

Panels– × 510W

System size– kWp

Local solar yield–

Generation in year 1– kWh

Generation over 25 years– kWh

Buying outright

System cost–

Savings over 25 years–

Net benefit–

Return on investment–%

Payback– years

Carbon avoided over 25 years– t CO₂

Savings year by year

Assumptions and limits

More on the method in how the calculator works.

This is an indicative estimate

It is based on the details you entered and typical assumptions. For a site design with firm costs and savings, speak to Kinetic Energy Consulting.

Paul Johnston

pjohnston663@gmail.com

Questions about the calculator

How do I use the calculator?

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.

How accurate are the results?

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.

What does "solar used on site" mean?

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

What is the difference between buying outright and a PPA?

Buying gives the highest return over the system's life because you keep all the savings. With a Power Purchase Agreement a funder pays for, owns and maintains the system, and you buy its electricity at an agreed rate below your grid price. The calculator shows both. See our funding guide.

Does the calculator include battery storage?

Yes, as an option. It sizes a battery from the solar you would otherwise export and shows the extra saving over a 10-year battery life. A battery adds most where a site exports a lot of its solar.

Do the figures include VAT?

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