Solar economics pitfalls: payback is not a promise
How to test models for hidden assumptions about prices, generation, export, degradation and costs.
Reviewed 2026-09-24 · 6 min read
Separate facts from forecasts
A model should distinguish measured inputs from assumptions. Annual demand may be known, but future import prices, export income, generation, battery behaviour and maintenance are forecasts. Ask the date and source of each and request low, central and high cases.
Payback is sensitive to timing. Two homes with equal annual demand can have different outcomes if one uses electricity in daylight and the other imports at night. Batteries alter timing but add losses and replacement considerations.
Ask for the calculation period and whether the model assumes a constant tariff, inflation, degradation or equipment replacement. A nominal payback in future pounds is not the same as a discounted return today. For a business, the finance team may need to test accounting treatment, tax and cost of capital separately.
Check whether the system is sized to maximise output or to match demand. More panels can increase generation but may also increase export, require a different connection or add marginal cost. A smaller system can be sensible in one property and poor value in another; the model should show the reasoning.
Find omitted costs
Check surveys, roof repairs, scaffolding, DNO work, metering, monitoring subscriptions, maintenance, insurance, finance, VAT and eventual replacement. Businesses should add legal, access, downtime and end-of-term costs. Do not treat an illustrative export rate as guaranteed future income.
Ask what happens when a warranty expires, a monitoring platform closes or an inverter fails. Include access and labour, not only the replacement box. If a battery is included, test the business case with a replacement allowance and the warranty’s throughput or end-capacity condition.
For a commercial site, include roof lease payments, landlord approvals, security, business interruption and the cost of buying electricity during an outage. A quoted annual saving can look attractive simply because these costs were left outside the spreadsheet.
Use meaningful measures
Ask for annual generation, direct use, export, import reduction, cost and payback separately. For business decisions, consider discounted cash flow and sensitivity testing with an adviser. A good spreadsheet cannot fix an unsuitable roof, missing permission or a constrained connection.
Use a scenario table rather than a single bar chart. Change generation, demand, import price, export income, downtime, maintenance and capital cost one at a time, then in a combined downside case. Record which assumptions are contractual and which are the modeller’s judgement.
Economic value is only one decision criterion. A business may value resilience, carbon reporting or a predictable supply contract; a household may value lower grid dependence. State the objective before choosing the metric, and avoid describing an illustrative outcome as a guaranteed saving.
Finally, ask what would make the recommendation change. If a roof survey finds repairs, if the DNO limits export or if demand falls, a good adviser should be able to show the revised option. A model that cannot be updated is a sales illustration rather than a decision tool.
Common questions
Can an installer guarantee savings?
Outcomes depend on weather, use, equipment, tariffs and export. Treat them as estimates unless a separate contract defines a measurable guarantee.
What is the biggest payback mistake?
Accepting one forecast without checking timing, assumptions, omitted costs, degradation and sensitivity.
Should I use the quoted tariff?
Only as a dated scenario; tariffs and eligibility can change.