What a 100 kW Solar System Saves a Solihull Business (Worked Example)

Green Tech Hub · · 5 min read

Quick answer

  • A south-facing roof in Solihull generates about 970 kWh per kWp a year on PVGIS, so 100 kWp makes roughly 97,000 kWh.
  • At the West Midlands 2025/26 mean of £1,233 per kW for 10 to 50 kW systems, 100 kWp is about £123,300 as an average job; larger systems on good roofs usually come in below it.
  • An office using 60% of its generation on site saves about £18,700 a year at the Q1 2026 average non-domestic price of 24.14p per kWh plus export; with staff EV charging lifting self-use to 80%, about £21,000.
  • That is a simple payback of six to seven years before the Annual Investment Allowance, and the array is a 25-year asset.
  • Solihull's stock is offices and business parks, not foundries: Birmingham Business Park alone has 2.4 million sq ft and around 10,000 employees, which is why the car park matters as much as the roof.
  • Birmingham Airport's 6.8 MW solar farm on the borough's boundary supplies at least 20% of the airport's electricity, the local precedent at scale.

Solihull is not the West Midlands stereotype. Where the Black Country runs on foundries and sheds, Solihull runs on offices, business parks and distribution units, and that changes the arithmetic of a solar system in one specific way: the car park matters almost as much as the roof. This worked example takes a 100 kWp array on a Solihull office or business-park unit and runs the numbers on the official figures only.

Step one: what the roof makes

PVGIS, the European Commission’s yield model, puts a 4 kWp south-facing array at 35 degrees in Solihull at 3,881 kWh a year with 14% system losses, which is 970 kWh per kWp. A 100 kWp array therefore generates roughly 97,000 kWh a year. Flat and near-flat office roofs are usually mounted at a shallower pitch on ballasted frames to protect the membrane, which trims the figure slightly; a profiled steel deck on a distribution unit at Monkspath or Blythe Valley is closer to the model.

Step two: what it costs

The Department for Energy Security and Net Zero records the total installed cost of every MCS-certified system. For the West Midlands in 2025/26 the mean was £1,233 per kW for 10 to 50 kW systems across 1,181 installations. DESNZ publishes no band above 50 kW because the jobs vary too much to average, but cost per kW keeps falling with size, so £123,300 for 100 kWp is a ceiling for a straightforward roof rather than a quote. What moves it is the roof: ballasted non-penetrating mounting on single-ply, structural strengthening on an older frame, or a long cable run to the switchroom. The structural report and the G99 application to the network operator are in our fixed price; ask whether they are in anyone else’s.

Step three: what it saves

DESNZ’s quarterly energy prices put the average non-domestic electricity price in the first quarter of 2026 at 24.14p per kWh including the Climate Change Levy, which self-generated electricity does not attract. The variable is self-consumption.

An office using 60% on site. 58,200 kWh displaces grid power at 24.14p, about £14,050 a year. The 38,800 kWh exported at a business export rate of around 12p adds about £4,650. Total: roughly £18,700 a year, a simple payback of about 6.6 years on the average cost.

The same office with staff EV charging. Business parks are where cars sit under the array for eight or nine hours across the generating window: around 10,000 people work at Birmingham Business Park and roughly 3,500 at Blythe Valley. If workplace charging lifts self-use to 80%, 77,600 kWh displaces grid power, about £18,730, and 19,400 kWh exported adds about £2,330. Total: roughly £21,060 a year, a payback of about 5.9 years. Every unit put into a car instead of the grid is worth the full tariff rather than the export rate, which is why we design EV charging and solar together on business parks rather than one after the other.

After tax. The capital cost qualifies for the Annual Investment Allowance at 100% up to £1m, so for a £123,300 system the whole cost is set against profits in year one; above the cap, companies can use the 50% first-year allowance. Solar is special-rate plant and is excluded from full expensing. In England, renewable plant is also left out of a building’s rateable value until 31 March 2035.

Why Solihull is a good place to do this

The borough’s stock is weighted to offices and business parks: Birmingham Business Park covers 148 acres and 2.4 million sq ft with more than 150 businesses; Blythe Valley Park holds over 25 companies in 122 acres off the A34; Monkspath Business Park comprises 42 warehouse units totalling 375,000 sq ft with a minimum 6 m eaves. JLR’s Lode Lane plant, around 300 acres and more than 10,000 people, anchors a supply chain that increasingly has to answer Scope 3 questions from its customer, and metered on-site generation with MCS documentation is a clean answer.

There is precedent at scale on the borough’s boundary. Birmingham Airport commissioned a 6.8 MW solar farm of more than 12,000 ground-mounted panels at a cost of £9.7m, supplying at least 20% of the airport’s electricity and saving around 1,285 tonnes of CO2 a year. And the policy direction is set: Solihull Metropolitan Borough Council recognised a climate emergency in October 2019, committing the borough to net zero by 2041 and its own operations to 2030, and its Local Plan attaches significant weight to low-carbon and renewable energy schemes.

The lease question

Most Solihull business-park occupiers lease rather than own, and that stalls more schemes than roof condition does. A tenant with six years left on a ten-year lease is buying a 25-year asset; a landlord who installs it is spending capital on a bill they do not pay. The workable structures are a landlord-funded system recovered through the service charge or a rent uplift, a licence to alter with an agreed dilapidations position, or a funded arrangement where a third party owns the array and sells you the output. We will say which fits your lease before anyone climbs on the roof; the detail is on our commercial solar Solihull page, and the commercial solar West Midlands page covers the wider region.

Sources

Questions this guide answers

Our office empties at six. Does solar still work?

Yes, because weekday office demand overlaps the generating window almost exactly: base load runs roughly 07:00 to 19:00 Monday to Friday, and comfort cooling, the biggest summer load, peaks with the same irradiance that peaks the array. The honest caveat is weekends and holidays, when the building exports. Size the array to the weekday load rather than the roof, and the self-use share stays high.

What size system fits a business-park unit?

A 375,000 sq ft estate like Monkspath holds units whose roofs take anywhere from 50 to 300 kWp; a single office building at Blythe Valley or Birmingham Business Park typically supports 50 to 150 kWp once plant, access routes and edge setbacks are deducted. We model the usable area in CAD before quoting a kWp figure.

Can we claim capital allowances on a solar system in Solihull?

Yes. Solar is special-rate plant, so it is excluded from full expensing, but the Annual Investment Allowance covers 100% of the cost in year one up to £1m, which is the whole cost for most Solihull arrays; above the cap, companies can use the 50% first-year allowance. Renewable plant is also left out of rateable value in England until 31 March 2035.

Do we need planning permission on a business park?

Usually not. Rooftop solar on commercial buildings in England is largely permitted development under Part 14 of the General Permitted Development Order, and the previous 1 MW rooftop cap was removed in November 2023. Prior approval for siting and appearance can still apply, listed buildings and conservation areas are handled separately, and some estates have covenants or landlord consents that sit outside planning altogether.

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