Energy savings

Where the benefit came from, by day

Solar and battery are separated by costing three worlds for every half hour — no panels and no battery, panels alone, then what actually happened — and taking the differences. A negative battery day is one that spent more storing energy than it gave back; it returns when the battery discharges. Export earnings sit inside the solar bar rather than beside it: the panels-alone world already sells its surplus, so that income is part of what solar is worth.

Cumulative benefit

Running total across the selected window.

Inverter versus meter

The two measure at different points in the circuit and will never agree exactly. A few percent of drift is normal; a consistent double-digit gap means one feed is wrong and the solar/battery split should be treated with suspicion. The headline figures come from the meter either way.

Return on investment

Measured across everything on record (7 Apr 2025 to 8 Nov 2026), not the range selected above — annualising a fortnight of summer would roughly double the apparent return. The yearly figure comes from the trailing twelve months, so it covers a full seasonal cycle.

£18,990 put into FTSE All-World (VWRP.L) on 7 Apr 2025 and left alone would have been worth £27,315 on 6 Nov 2026 — a gain of £8,325. The system has handed back £4,536 over the same dates. That is the real closing price of a real fund, not an assumed rate; it is an accumulating class, so dividends are in it. No dealing costs and no tax, both of which would only make the fund look worse. The energy bills are already in this, which is the part that catches people out: £4,536 is not profit, it is the bill that did not arrive. Hold the money instead and you pay that bill in full — so the two sides are the financial benefit of each world over the same dates, with the electricity accounted for on both. Gains are compared with gains: an investment still has its £18,990 at the end and this system has hardware with a finite life instead. Two things the comparison leaves out, both of which favour the panels — these savings are not taxed, and they rise with the price of electricity. And 1.6 years is far too short a run for a stock market to mean much: this is what happened, not what tends to.

Left in a savings account at 4.50% instead, it would have earned £1,377 over the same period — the risk-free version of the same question. That rate is a sustained one rather than a headline best-buy: the base rate was 3.75% in October 2026 and the accounts paying a flat rate with no stepped bonus were around 4.3%. Interest on a balance this size runs close to the personal savings allowance, and past it for a higher-rate payer, which this does not deduct.