Solar Panel Payback & Savings Calculator

Solar Panels UK Guide 2024

What do I need to know about Annual Solar Generation in the UK?

A 4 kWp solar system generates approximately 3,200-4,400 kWh per year depending on location, roof angle, and shading — enough to cover 40-60% of typical household electricity consumption. Peak output occurs in May-July; December-January generates approximately 5-10% of summer output. Orientation matters: south-facing at 35° from horizontal is optimal. South-west/south-east at 35°: approximately 95% of optimal. East/west-facing: approximately 80% of optimal. North-facing: not recommended (under 50% of optimal output) — a north-facing roof rarely makes financial sense for solar unless it's the only available option and self-consumption (rather than export) is the priority. Flat roofs can use tilted mounting frames to achieve a favourable angle regardless of the roof's own orientation.

What should I know about Smart Export Guarantee (SEG)?

The Smart Export Guarantee replaced the Feed-in Tariff in 2020. Suppliers with over 150,000 customers must offer an export tariff — rates vary significantly. 2024 best rates: Octopus Energy 'Flux' or 'Agile' can exceed 15-30p/kWh during peak demand periods. Minimum SEG rate: suppliers must offer above 0p/kWh — some offer very low rates. Getting the best export rate: use a smart meter (required for SEG), compare SEG tariffs at ofgem.gov.uk, consider battery storage to store excess for evening use, since even a modest SEG payment (often 4-15p/kWh) is usually well below the 25-30p/kWh saved by using that same unit of self-generated electricity instead of buying it back from the grid in the evening.

What's the key thing to understand about Battery Storage?

Solar battery storage (e.g. Tesla Powerwall, Givenergy, SolarEdge) stores daytime solar generation for evening use — when solar panels are not generating but electricity demand is highest. Typical 10 kWh battery cost: £4,000-6,000 (installed). Benefit: a typical household imports 70% of solar generation in summer but only 20-30% with a battery — moving more self-consumption from off-peak export rates to peak rate savings. Battery payback (standalone): 10-15 years in most cases — marginal economics on its own, but the case improves substantially when paired with solar (storing free daytime generation instead of buying overnight electricity at full price) or with a cheap overnight time-of-use tariff, both of which shorten the payback period meaningfully compared with a battery bought in isolation.

Is Solar Worth It in 2026?

With electricity at 24p/kWh and good SEG rates, typical payback is 6-10 years for a 4 kWp system costing £6,000-8,000. System lifetime: 25+ years (panels degrade approximately 0.5% per year). After payback: essentially free electricity for 15+ years. ROI: approximately 8-14% annually in cash terms — better than most savings accounts and comparable to equity market returns, but illiquid. Factors reducing payback: east/west-facing roof, significant shading (trees, chimneys), low electricity consumption during daylight hours (since more of the generated electricity is exported at the lower SEG rate rather than self-consumed at the higher retail-saving rate), and a north-facing or heavily overshadowed roof. Get a site-specific quote and shading assessment before committing — headline payback figures assume a reasonably unobstructed, well-oriented roof.

What's the key thing to understand about Factors That Affect Solar Panel Payback Period?

The payback period for a home solar panel installation — how long it takes for cumulative electricity bill savings (and any export payments for surplus generation) to equal the upfront installation cost — depends on several factors beyond simply the size of the system installed. Roof orientation and pitch significantly affect generation: a south-facing roof (in the northern hemisphere) at an optimal pitch angle generates meaningfully more electricity over a year than an east- or west-facing roof of the same size, and a north-facing roof generates considerably less, which matters because panels on a suboptimal orientation take proportionally longer to pay back even at an identical installation cost. Shading from trees, chimneys, or neighbouring buildings at any point during the day can disproportionately reduce generation, since many solar panel systems are wired in a way that means a single significantly shaded panel can reduce the output of an entire string of panels, not just its own individual contribution — this makes a careful shading assessment before installation important, not just a rough visual check. How much of the generated electricity is actually used directly by the household, versus exported to the grid, also matters significantly for payback speed, since self-consumed solar electricity avoids the full retail price of electricity that would otherwise have been bought, while exported electricity is typically paid at a lower rate — this is a major reason many solar installations are now paired with a home battery or scheduled appliance use timed to coincide with generation, specifically to increase the proportion of self-consumed electricity and shorten payback.

What should I know about Solar Panel Degradation and Long-Term Performance?

Solar panels don't generate electricity at a constant rate indefinitely — their output gradually declines over time through a well-understood process called degradation, typically losing somewhere around 0.3–0.5% of their original output capacity per year, meaning a panel might still be producing somewhere in the region of 80–90% of its original output after 25 years, depending on the specific panel quality and technology. Most reputable manufacturers provide a performance warranty guaranteeing output stays above a specified percentage of the original rating for a set number of years (commonly 25 years), which gives a useful benchmark for comparing panel quality between different products and manufacturers, since not all panels degrade at the same rate. This gradual decline should be factored into a realistic payback calculation, since assuming constant output at the panel's rated capacity for the entire payback period slightly understates the true payback time compared with a calculation that accounts for gradual degradation, though the effect over a typical payback period of several years is usually modest rather than dramatic given how slow annual degradation actually is. Inverter lifespan is a separate consideration worth factoring into the total cost of ownership calculation, since most inverters (the component converting the panels' DC output into usable AC electricity) have a shorter expected working life than the panels themselves, and will likely need replacing at least once during a solar installation's full operational lifetime — this replacement cost is a genuine ongoing cost of solar ownership that a simple upfront-cost-versus-annual-saving payback calculation can sometimes overlook.

Reference: Payback Period by System Size and Household Usage

Payback period depends on how much of the generated electricity you actually use in the home versus export at the lower Smart Export Guarantee rate, so the same size system pays back faster in a household that is home during the day.

Adding a battery typically extends the simple payback period on the combined system, even though it increases total lifetime savings, because the extra battery cost is large relative to the extra electricity it lets you self-consume.

Worked Example: Payback on a 4.5kW System

A south-facing 4.5kW system in the Midlands costs £7,500 installed and is expected to generate around 3,900 kWh per year based on standard UK solar yield assumptions. The household is home for part of the day and manages to self-consume 50% of the generation directly, with the rest exported. Self-consumed electricity: 3,900 x 0.50 = 1,950 kWh, valued at a typical import rate of 26p/kWh = £507 per year in avoided purchases. Exported electricity: 1,950 kWh at a Smart Export Guarantee rate of 15p/kWh = £293 per year. Total annual benefit: £507 + £293 = £800. Simple payback: £7,500 ÷ £800 per year ≈ 9.4 years. If the household shifted more usage into daylight hours — running the washing machine and dishwasher midday — and raised self-consumption to 65%, the annual benefit rises to roughly £900, cutting payback to around 8.3 years, which illustrates why usage pattern matters as much as system size.

What should I know about Inverter Replacement Costs?

Most solar inverters carry a 10-12 year warranty and typically need replacing once during a 25-year panel lifespan, at a cost of roughly £800-£1,500 for a domestic-sized inverter. This is a real cost that sits outside the initial payback calculation but matters over the panels' full working life, since panels themselves commonly keep producing usefully for 25-30 years while the inverter is the shorter-lived component in the system.