Solar & Battery ROI Simulator

Generation. Annual output = array size × regional yield (MCS-style kWh per kWp) × orientation factor × shading factor, distributed across months using a typical UK solar profile (December ≈ 2.7% of annual output, May–July ≈ 12–13% each). Panel output degrades 0.5% per year.

Daily energy balance (computed per month). 38% of household consumption is assumed to fall in solar-generating hours. Direct self-use = min(daily generation, daytime consumption) × 0.85 coincidence factor. Remaining surplus charges the battery (90% usable depth, 90% round-trip efficiency, capacity fading 2%/yr) — but only up to what can usefully be discharged against evening demand, since storing energy you'll only re-export wastes round-trip losses; the rest is exported directly. Battery discharge offsets evening/overnight consumption at the peak import rate.

Winter arbitrage. On time-of-use tariffs (if enabled), remaining battery headroom charges at the off-peak rate and offsets peak-rate consumption, net of round-trip losses. This is why batteries increasingly pay back even with modest solar.

Cash flows. Year-1 saving = direct self-use and battery discharge valued at peak import rate + exports at export rate + arbitrage margin. All rates escalate with your price-inflation input. NPV discounts at your chosen rate. Battery replacement (optional) is charged in year 13.

Uncertainty (Monte Carlo, 500 runs). Each run draws price inflation from N(your input, 2%) and applies independent ±6% annual weather noise to generation. The chart band shows the 10th–90th percentile of cumulative cash flow; headline payback and NPV are medians. Real outcomes also depend on tariff availability, export-rate policy and your usage pattern — treat results as a fair central estimate, not a promise.

What this deliberately ignores. Standing charges (unchanged by solar), maintenance (~£100/yr inverter reserve would lengthen payback slightly, an inverter replacement ~£1,000 around year 12 is not included for solar-only setups), house-price effects, and VAT changes (0% on domestic solar/storage until 2027).