Solar & Battery ROI: Agile vs the Price Cap

In July 2025 I installed solar panels and a battery in a one-bed flat in south London. Small panels, a 6.7 kWh LiFePO4 battery, and a Victron charge controller. Total spend: £2,203.

Every kilowatt-hour the system produces or shifts is one I don't have to buy at the Ofgem price cap. This page tracks — live, from Home Assistant — exactly how much I've saved so far, and which part of the kit is doing the most work.

What It Cost — Purchase records

The core system went in on 15 July 2025 (panels, battery, charge controller). Two more panels were bolted on in November once the summer yield curve tailed off. Only the panels are outside — mounted in the garden on bespoke frames I built from salvaged wooden pallets. The battery, MPPT and inverter live inside on a board.

What It Produced — Home Assistant

Live, lifetime totals. The system has been running for 305 days since commissioning in July 2025.

May 2026 So Far — Octopus half-hourly

The lifetime rollup further down spans 285 days; this is the snapshot for the current month. Real Octopus half-hourly imports over 1–16 May 2026 (16 days) priced two ways: what I actually paid (Agile + battery time-shifting + solar) versus what the same kWh would have cost on the Ofgem default tariff cap (25.77p/kWh, consumption-weighted across the install period). All cap comparisons later in the post use this same weighted rate.

This month I've paid £10.56 in unit costs against £13.04 on cap — about 81% of what the same demand would have cost a cap-tariff household. After standing charges, the bottom-line bill is £20.04 vs £20.56, a saving of £0.52 (3%). My current tariff (Octopus Agile, daily standing 59.27p) charges more standing than the cap (44.29p), so a small chunk of the unit-cost win is given back on the standing line — but the plunge wins on the unit side more than make up for it.

Actual Bill vs Price Cap — Octopus half-hourly

This is the ground-truth answer, attributed three ways: what I actually paid Octopus, what I'd have paid on Agile without the battery (time-shift removed, charging removed, loads priced at the Agile rate in force when they happened), and what the Ofgem default tariff cap would have charged for the same household demand. Plunge credits when rates went negative are baked into the actual column.

Plunge credit so far: £6.04 of negative-rate electricity where Octopus paid me to consume — 287 half-hours since install. Not life-changing money, but a satisfying number; I catch them when the battery has headroom.

What It's Saved — And When It Pays Back — HA + Ofgem cap

Priced against the weighted cap (defined up top). The daily chart below uses the actual cap rate in force on each day; the lifetime rollup here uses the weighted average.

Of the £191.11 saved vs cap so far: £41.05 from solar, £83.97 from the battery time-shift, and £103.26 from Agile pricing the imports that bypassed the battery below cap. That sums to £228.28 on units alone; the gap to the headline £191.11 above is about £37.17 handed back on the standing-charge line, where Agile (55.79p/day) costs more than the cap (44.29p/day). Agile is brilliant — the plunges alone more than cover the slightly steeper standing charge, and the half-hourly pricing lets the planner cherry-pick the cheapest slots every cycle.

If the current rate holds, combined lifetime net profit is £3,519 — about 2.6× the original £2,203 back. The cap has usually drifted up, which pushes that higher; if Agile rates tighten or the pack ages faster than rated, it shrinks.

Why Agile, Not a Fixed Tariff — Octopus live

The price cap is what most households pay on a standard variable tariff. Agile lets me buy electricity at the half-hourly wholesale price, which on a typical week is much cheaper than cap — and occasionally goes negative when wind and solar flood the grid. A battery turns those cheap hours into stored energy for the expensive ones.

At an effective rate of 12.9p/kWh, electricity here costs about half of the price cap (25.77p). That difference is where the ROI lives — Agile does most of the heavy lifting; the battery amplifies it; solar adds free kWh on sunny days.

Plunge Response — Octopus + HA

Two networks here: the main grid (what Octopus bills for, measured at the smart meter) and my microgrid, a 800 W Victron inverter fed from the battery. Microgrid runs the low-draw always-on stuff — TVs, computers, networking kit, desk electronics, uplighters, e-bike charging. Fridge, freezer, oven and washing machine stay on the main grid on purpose: I sized the inverter small to keep compressor and heating-element spikes off the battery, and Agile on a sunny, windy day often beats what overnight rates cost in a winter Dunkelflaute — the dark, still spells when neither solar nor wind shows up and wholesale prices climb. When nothing heavy is running, Home Assistant's daily_power_min (lowest main-grid pull seen each day) routinely bottoms out near 42 W — fridge on idle plus whatever else is quietly ticking over. Then Agile goes negative, and the main-grid pull goes up, on purpose.

That's roughly 6.3× the normal main-grid draw, concentrated in the exact half-hours when Octopus is paying me to consume. Most of those kWh get banked in the battery for later; the rest runs anything else I can sensibly time-shift. On a fixed tariff this mechanic simply doesn't exist.

The response scales with how negative the price goes. Deeper plunges pull me (and the dryer, and the washing machine) into action more aggressively:

The biggest single-day response was 5 April 2026: 30 plunge half-hours, 28.7 kWh drawn while the rate sat as low as -12.1p/kWh, with a peak half-hour pulling 4.7 kW sustained. Tumble dryer, washing machine, and the rest of the kitchen, all running at once — and Octopus paid me to do it.

The Takeaway

• Agile is the foundation. Without a half-hourly tariff, the battery has nothing cheap to soak up.
• The battery makes Agile workable. It charges in the cheapest slots and discharges in the dearest — sometimes that's overnight, sometimes a sunny midday. The gap is small per kWh but runs every day.
• Small solar, real impact. 700 W on salvaged-pallet frames in the garden displaces grid imports any time the sun is up, and tops the battery up for free on the good days.
• The price cap isn't my benchmark — it's the comparison point. For me, Agile + battery is the lived rate. Cap-vs-mine is what shows the gap most households are leaving on the table.