How Our Solar and Battery Estimates Work

Annual solar model

Let G be AC generation, L household demand and D directly consumed solar, all in kWh/year. D must be between zero and the lesser of G and L. Import/export rates convert pence per kWh to GBP once at the calculation boundary. Installed transaction totals are integer pence; energy is rounded only for display.

export = G - D
grid import = L - D
bill reduction = D × import rate
export receipts = (G - D) × metered export rate
annual solar benefit = bill reduction + receipts - additional solar costs
simple payback = installed cost / annual benefit

Zero or negative annual benefit has no finite payback. Zero capital cost can mean an existing system, not a normal free purchase. Annual totals do not establish timing. Standing charges remain payable.

Additional solar-charged battery

S is otherwise-exported AC solar diverted to storage; eta is AC household energy delivered per kWh diverted. The year begins and ends with equal stored energy. S cannot exceed solar exports; delivery cannot exceed remaining demand or usable capacity times assumed equivalent discharge cycles.

delivery = S × eta
losses = S - delivery
export with battery = G - D - S
grid import with battery = L - D - delivery
additional benefit = delivery × import rate - S × export rate - additional battery costs
battery simple payback = additional installed battery cost / additional benefit

Deemed FIT and unknown export-payment arrangements need verified agreement inputs before a financial judgement. No-payment scenarios use zero export receipts on both sides. Existing solar cost is sunk for retrofit. Capacity and cycles are bounds, not proof of seasonal energy availability.

Versioned assumptions and limits

Model solar-ac-1.0.0; assumption set illustrative-2026-11-01. The first release replaces unrelated formulas and arbitrary battery boosts with conserved-energy annual scenarios. Direct-use and shifted-energy values are explicit estimates, not measured load curves. Grid charging, dynamic trading, battery exports and finance are excluded.

Sensitivity changes one input at a time. Constant-year cash flow includes year-zero capital, entered annual costs and no tariff escalation, degradation or replacement. It is not a lifetime forecast. The first non-negative year-end balance is separate from fractional simple payback.

• example-generation: 3600 kWh/year. illustrative; Synthetic worked example, not a regional generation estimate. Excludes: No roof, orientation or weather prediction. Checked 2026-11-01.
• example-import: 30 p/kWh. illustrative; Editable scenario rate; not a supplier offer. Excludes: Standing charges excluded. Checked 2026-11-01.
• example-export: 15 p/kWh. illustrative; Editable metered-export scenario, not a current tariff. Excludes: Eligibility and FIT not assumed. Checked 2026-11-01.
• example-efficiency: 0.9 fraction. illustrative; AC solar diverted to AC household delivery. Excludes: Not a product specification. Checked 2026-11-01.
• powerwall-capacity: 13.5 kWh. manufacturer; Published energy capacity. Excludes: Installed price and backup scope. Checked 2026-11-01.
• module-wattage: 450 Wp. manufacturer; Example nameplate power. Excludes: Not annual generation. Checked 2026-11-01.
• example-consumption: 4000 kWh/year. illustrative; Synthetic household annual demand. Excludes: Not measured household data, a supplier offer or an equipment promise. Checked 2026-11-01.
• example-direct: 0.4 fraction. illustrative; Direct solar share before storage. Excludes: Not measured household data, a supplier offer or an equipment promise. Checked 2026-11-01.
• example-solar-cost: 6000 GBP. illustrative; Synthetic installed solar-only cash total. Excludes: Not measured household data, a supplier offer or an equipment promise. Checked 2026-11-01.
• example-battery-cost: 3000 GBP. illustrative; Synthetic additional installed battery total. Excludes: Not measured household data, a supplier offer or an equipment promise. Checked 2026-11-01.
• example-shifted: 1000 kWh/year. illustrative; Assumed otherwise-exported AC solar diverted to storage. Excludes: Not measured household data, a supplier offer or an equipment promise. Checked 2026-11-01.
• example-usable: 5 kWh. illustrative; Assumed accessible storage; not a product specification. Excludes: Not measured household data, a supplier offer or an equipment promise. Checked 2026-11-01.
• example-cycles: 250 cycles/year. illustrative; Assumed equivalent discharge cycles; throughput bound only. Excludes: Not measured household data, a supplier offer or an equipment promise. Checked 2026-11-01.
• example-horizon: 10 years. illustrative; Selected battery scenario horizon, not useful life or warranty. Excludes: Not measured household data, a supplier offer or an equipment promise. Checked 2026-11-01.
• example-opex: 0 GBP/year. illustrative; Exposed no-additional-operating-cost scenario. Excludes: Not measured household data, a supplier offer or an equipment promise. Checked 2026-11-01.

Selected tariff record

This register explains structures rather than ranking the whole market. Eligibility and prices need checking with the supplier before a decision. Supplier rates are not calculator defaults.