Cambridge Solar Simulator

Solar panels, a home battery and Kia PV5 charging, simulated in half-hour steps for a whole year. The battery is planned ahead like Predbat to buy cheap and sell dear.

Simulating 17,520 half-hours…

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Working out the payback for your roof.

Roof & panels

Panels

Roof width (along gutter)m Roof height (up the slope)m Edge clearancem Panel powerW Panel lengthm Panel widthm Roof pitchdeg Facing Shading & extra losses%

Base yield is about 1,010 kWh per kWp per year, from PVGIS-style monthly figures for Cambridge (south, 35°). Many installers use 0.3–0.4 m edge clearance. At 0.3 m this roof takes one row of 12 panels instead of two rows of 8.

Battery & inverter

Usable battery capacity

Hybrid inverter Export limit Max charge rateC Round-trip efficiency% Cycle wear costp/kWh Capacity fade%/yr

Battery power is the smaller of the inverter rating and capacity × C-rate. The planner ignores any trade that earns less than the cycle wear cost, as Predbat does. Above 3.68 kW of export you need G99 approval from the network operator (UK Power Networks in Cambridge).

Home & Kia PV5

Home use (no EV)kWh/yr Daily pattern EV mileagemi/yr EV efficiencymi/kWh ChargerkW Charging losses% Car home in the daydays/wk Charge the car from surplus solar on those days (Zappi/Ohme solar mode). This usually earns less than exporting when export pays more than your overnight rate.

Tariffs

Installed costs

Scaffold, labour, MCS£ Panels & mounting£/kWp Hybrid inverter£ Battery per kWh£/kWh Battery fit-out£ Home Assistant box, bird mesh£ Inverter replacement yearyr Replacement cost£

Defaults are typical 2026 UK installed prices. Solar panels and batteries have 0% VAT until 31 March 2027. The EV charger is left out because you need it either way.

Money over time

Measure savings against

Years to modelyr Savings account rate% Import price rise%/yr Export price rise%/yr Panel degradation%/yr Price cap unit ratep/kWh Price cap standingp/day

The savings rate is treated as the money's opportunity cost. It sets the NPV and the discounted payback. The price cap figures are approximate for the Eastern region in late 2026, so check them against Ofgem.

Weather year

Sunny and grey days are drawn at random around the Cambridge monthly averages. Try another year to see how much the results move.

Tariffs compared

Year-one bills for this setup on each tariff. Click a row to use that tariff in the charts below.

Cumulative position

Money ahead or behind after paying for the system, year by year.

Panels × battery

Every combination on the selected tariff.

A day with the planner

What the battery planner does in each half-hour on the selected tariff.

Monthly bills

Year one on the selected tariff, against the price cap with no solar. A bar below zero is a month in credit.

How it works

The year runs in 17,520 half-hour steps on UK clock time, including BST. Solar output comes from the sun's position over Cambridge, scaled to monthly averages, with random sunny and grey days.
The battery planner uses dynamic programming over the whole year with perfect forecasts. Predbat works from 24–48 h solar and price forecasts, so expect real results a few percent lower.
The car charges in the cheapest half-hours between 18:00 and 08:00, or from surplus solar if you tick that option. The house battery never charges the car when grid power is cheaper.
Agile prices are synthetic. They follow Octopus's published formula (2.2 × wholesale + 12p from 16:00–19:00, capped at 100p; export 0.95 × wholesale + 5.93p peak uplift) applied to a modelled wholesale price that dips on sunny middays.
Tariff rates are approximate figures for late 2026. Edit them under Tariffs to match current quotes.
Panel output falls with degradation each year. Battery savings fall with capacity fade. Import and export prices rise at their own rates.