Most UK solar guides tell you sunshine hours and panel efficiency decide your payback. They don't — not primarily. Your electricity tariff structure decides more of your payback than your roof does. Two identical solar installations on two identical roofs can have paybacks years apart, purely because of the rates the two households pay for electricity. This guide covers the methodology that actually drives the numbers, not just the headline claims.
The one rule that decides almost everything
Every solar and battery decision — whether to store energy, export it, or use it directly — comes down to a single comparison:
If your export rate is higher than the rate you'd otherwise pay for the thing you're trying to power, export. If it's lower, use the energy yourself.
This sounds obvious once stated, but it's routinely got backwards. A common piece of advice is "divert your spare solar into your immersion heater instead of exporting it." That's only true if your export rate is lower than the cost of heating water another way. On many current UK tariffs, export rates (commonly 4p–15p/kWh depending on supplier) can sit above the cost of heating water via gas (about 10p per kWh of heat once boiler efficiency is factored in: 8.4p gas through an 85%-efficient boiler, the calculator's own assumption) — in which case exporting and paying for gas separately is the better financial move, even though it feels counterintuitive to "waste" free solar.
The same rule governs whether a battery should charge from solar or from a cheap overnight tariff rate, whether an EV should charge from solar surplus or its own off-peak window, and whether a heat pump's daytime running is worth it. One rule, applied consistently, replaces most of the generic advice you'll find elsewhere.
Sizing: what actually fits your roof
Most online solar calculators overstate how many panels fit on a given roof area. A common assumption is roughly 1.7m² per panel — but that figure describes the panel itself, not a real installation. Real installs need edge clearance (UK practice typically keeps panels 0.3–0.5m from roof edges) and symmetric spacing, which meaningfully reduces the panel count a simple area-division formula suggests.
Practical example: a 20m² south-facing roof face might mathematically fit 11–12 panels by area alone, but once edge clearance is applied, 6 panels in a clean, symmetric 3×2 layout is often the realistic number. That's not a rounding error — it roughly halves the system size a naive calculation would suggest, which cascades into every downstream figure: cost, generation, savings, payback.
If you're using an online calculator, check whether its panel-count estimate accounts for edge clearance. If it doesn't, treat the output as an upper bound, not a real estimate.
Self-consumption: the number that gets guessed, not measured
"Self-consumption rate" — the share of generated solar you use directly rather than export — is one of the most commonly fudged inputs in solar calculators. A flat 30–40% assumption is common, but it's frequently applied incorrectly: some tools let solar "cover" nighttime usage, which is physically impossible without a battery.
A more defensible approach models self-consumption against your actual daytime usage pattern, capping direct solar use at what your home could plausibly draw during generation hours — not your total daily consumption. Getting this wrong tends to inflate solar savings estimates by a meaningful margin, since a big share of most households' usage happens in the evening, when solar isn't generating regardless of system size.
Battery: sized for what, exactly?
The most common battery-sizing mistake is matching the battery to total daily household usage. That's the wrong target. A battery's job — assuming you're on a tariff with a peak/off-peak spread — is to cover your peak-rate usage specifically, charged cheaply overnight, not your entire day's electricity.
Why this matters: off-peak usage is already cheap. There's nothing to "arbitrage" by storing energy to cover hours you're already paying a low rate for. A battery sized to your peak-rate daily usage (plus modest headroom for degradation over its working life) typically captures the large majority of the available saving. Sizing significantly larger chases a shrinking marginal return — you're paying for capacity that increasingly sits unused.
A genuine exception: if you want backup power resilience (covering an outage) rather than pure bill-saving, a larger battery is a legitimate choice — but it should be understood as a resilience decision, not a financial one, since the extra capacity beyond peak-rate coverage rarely pays for itself on savings alone.
VAT and grants: what's actually current
As of this guide's review date:
- 0% VAT applies to solar panel and battery installations in the UK, currently scheduled to run until 31 March 2027.
- Solar panels and batteries are not covered by the Boiler Upgrade Scheme — that scheme (worth £7,500) is for low-carbon heating like heat pumps, not solar or battery storage. Don't conflate the two; this is a common point of confusion.
- Additional regional schemes exist (see our county-level pages for what's specifically available in your area) — eligibility and scheme details change, so treat any specific figure as worth confirming before you commit.
What actually moves your payback
In rough order of impact:
- Your tariff's peak/off-peak spread (or lack of one) — a wide spread makes batteries far more valuable; a narrow or flat tariff makes solar-only the stronger option
- Roof orientation and real usable area (post-edge-clearance, not the naive figure)
- Install cost per kWp — this varies meaningfully between installers for comparable specification; get multiple quotes
- Export rate relative to your usage rates — determines whether diverting or exporting wins, as above
- Whether you're chasing pure ROI or resilience — these lead to genuinely different "correct" system sizes
- How fast energy prices rise — every kWh your panels or battery save is worth more as prices go up. The calculator assumes 3% a year (energy has risen faster than general inflation); at today's prices for 25 years, a typical solar and battery payback comes out about a year and a half longer. You can change the assumption on the first step
Common mistakes worth avoiding
- Sizing a battery to total usage instead of peak-rate usage — inflates the "optimal" size and the price tag with it
- Assuming diverting solar always beats exporting — check your actual export rate against the rate you'd otherwise pay first
- Ignoring degradation and replacement costs in payback calculations — a headline "6-year payback" that doesn't account for a mid-life inverter or battery replacement is overstating the case
- Using panel-count estimates that don't account for edge clearance — inflates system size, cost estimates, and savings together
Our free calculator applies this methodology — including edge-clearance-corrected sizing, peak-rate battery sizing, and the export-vs-divert rule — to your actual roof, tariff, and usage.