Is commercial solar on a flat roof worth it?
The honest maths
Commercial solar is worth it when it saves you more than it costs, and on a flat roof that turns on one number: self-consumption — the share of the generation you use on site instead of exporting. A business that runs its lighting, machinery, refrigeration or servers through the day uses most of what the roof produces, and every unit self-consumed is a unit you no longer buy at the full commercial rate. That is why a commercial system with a strong daytime load typically pays back in the region of six to eight years, against a panel life measured in decades. A system that would export most of its output pays back more slowly, because export rates under the Smart Export Guarantee are lower than the price you pay for grid power.
A flat-roof array is an asset, not an expense: it is eligible for capital allowances via the Annual Investment Allowance (solar is special-rate plant, so the route is the AIA, not full expensing), and it can be funded from capital, through asset finance or a lease, or under a power purchase agreement with no upfront cost. A typical system is around £600 to £900 per installed kWp.
The common objections, answered honestly
Will it damage or leak the roof?
Not with a penetration-free ballasted system, which never pierces the membrane. Where a mechanical fixing is unavoidable it is sealed to the manufacturer's specification. The roof warranty stays intact.
Can the deck take the weight?
A structural engineer confirms the deck's residual capacity for the roughly 15 to 25 kg/m² an array adds, before design. If it cannot, a lighter system or strengthening is the answer — and sometimes solar is not viable, in which case we say so.
What if we move or the company changes?
A rooftop array adds value and lower running costs to the building, which transfers with it. On a lease, we design around the term and the landlord-tenant split so the payback lands inside your tenure, or use a PPA so a third party carries the asset.
Is the payback figure realistic?
We build it from your actual half-hourly consumption, not an inflated headline. We would rather quote a slower, defensible payback than an optimistic one you later find was fiction.
When solar is NOT worth it
Being honest cuts both ways. Solar on a flat roof is not worth it, or not yet, when:
- the roof membrane is life-expired and must be renewed first — do the roof, then the solar, or you pay to lift the array twice;
- daytime electrical demand is very low, so most generation would be exported cheaply rather than self-consumed;
- the deck genuinely cannot carry an array and strengthening is uneconomic;
- the building is due to be sold or vacated before the payback period, with no lease or PPA structure to capture the value.
In every one of those cases we will tell you plainly, because a system that does not pay is not a sale worth making.
Find out if your flat roof is worth it
Responds within one working day
- 1. Free desk feasibility from your meter data and roof, no obligation.
- 2. Site survey and a fixed-price proposal, itemised in writing.
- 3. Install and aftercare by MCS-certified engineers.
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- NICEIC
- RECC
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