Best Solar Battery for Octopus Agile in 2026
The best solar battery for Octopus Agile in 2026 is whichever battery your hybrid inverter is certified against, paired with a Solis LV or HV hybrid inverter and automated by Predbat running on Home Assistant. For most UK homes that’s a Fogstar 16.1kWh + Solis 5kW LV stack. The brand matters less than getting the automation right — without Predbat-style price-aware control, you’re leaving 30–50% of the available Agile savings on the table.
Octopus Agile is the half-hourly variable tariff where electricity prices change every 30 minutes based on wholesale market conditions. Some half-hours are negative (Octopus pays you to import). Some are 35p/kWh. The point of pairing a solar battery with Agile is to shift consumption out of the expensive windows and into the cheap ones.
Done well, that pulls 30–50% off an already-low solar bill. Done badly — meaning the battery charges from solar but doesn’t respond to Agile prices — you save roughly nothing extra over a flat-tariff battery setup.
What ‘best for Agile’ actually means
It’s less about which lithium chemistry is inside the battery (almost all credible 2026 options are LiFePO₄) and more about three things:
- How fast the battery can charge and discharge. Agile half-hours are 30 minutes. If your battery can only charge at 2.5kW and you’ve got 30 minutes of negative pricing, you only get 1.25kWh free. If it charges at 6kW, you get 3kWh.
- Whether the BMS allows price-driven scheduling. Some closed-ecosystem batteries (Powerwall is the standout) only let you control charge/discharge through the manufacturer’s app. Open-ecosystem batteries (Fogstar, Pylontech via Solis) let Predbat issue charge/discharge commands directly.
- Inverter compatibility with Predbat. Predbat is the open-source Home Assistant add-on that turns Octopus Agile forecasts into battery commands. It supports a long list of inverters — Solis is on it (the default Spectrum specification), SolarEdge is partially supported.
The Spectrum default Agile spec
| Component | Default specification | Why |
|---|---|---|
| Inverter | Solis 5kW or 6kW LV hybrid | Predbat-supported, opens the BMS to price-aware control |
| Battery | Fogstar 16.1kWh 51.2V Solar Battery | Best price-per-kWh, 6kW charge/discharge headroom |
| Automation | Home Assistant + Predbat add-on | Reads Agile half-hourly prices, schedules battery accordingly |
| Smart meter | SMETS2 (existing) | Required by Octopus to settle Agile |
| Optional | Octopus Mini local data feed | Improves Predbat’s real-time forecast accuracy |
What Predbat actually does
Predbat is an open-source automation that runs locally on a Home Assistant install. Every half-hour it pulls the next 24-48 hours of Octopus Agile prices, forecasts your home’s solar generation (from PV*SOL-style models tuned to your roof) and your expected consumption (from the last few weeks of metered data), and decides whether to charge the battery from the grid, charge from solar, or discharge to cover consumption.
The result is a battery that buys electricity at 2p/kWh on a Sunday night and discharges it at 4pm Monday when Agile is at 28p/kWh. Done across a year, that’s genuinely significant. We have customers reporting Agile bills 60–80% lower than equivalent flat-rate Octopus bills.
Batteries that work well with Agile (ranked)
1. Fogstar 16.1kWh + Solis LV. Our default. Open BMS, fast charge/discharge, Predbat-controllable, best price-per-kWh. The combination most Spectrum Agile customers run.
2. Pylontech Force-H2 + Solis HV. Excellent for commercial scale Agile customers and very high-consumption homes (large families running EVs and heat pumps). HV stack handles >10kW continuous discharge.
3. Solis hybrid + Fogstar (Spectrum default specification). Was widely deployed before GivEnergy went into administration in April 2026 — no longer specified for new installs.
4. Tesla Powerwall 3. Works on Agile via Tesla’s own scheduling, but you don’t get the granularity of Predbat. Spectrum doesn’t fit Powerwall — see our comparison article for our reasoning.
5. SolarEdge + LG/RESU battery. Partial Predbat support. We’d specify this only where shading on the roof has already forced a SolarEdge optimiser decision.
Already on Agile and not seeing the savings you expected?
That’s almost always a missing automation layer, not the wrong battery. We’ll audit your existing system and quote the Predbat retrofit if it’s the right call.
Speak to Our TeamHow big should the battery be for Agile?
Sizing rule of thumb: enough capacity to cover your highest-priced four-hour Agile window with your typical evening consumption. For a 4-bed home pulling 8kWh in that window, 10–16kWh is the sweet spot. For a 5-bed home with an EV plus heat pump, 16–32kWh is realistic. Too small and you’ll drain mid-evening and pay peak Agile rates anyway. Too large and you’re tying up capital that doesn’t turn over often enough to earn back its cost.
Our solar savings calculator gives a first-pass battery size in under a minute. We refine it on site survey.
Domestic vs commercial Agile use
Domestic Octopus Agile customers have a relatively predictable load — cooking, EV charging, and (in some homes) a heat pump. Battery sizing is straightforward and Predbat handles it cleanly.
Commercial Agile-equivalent customers (Octopus Shape Shifters is the closest Octopus commercial tariff) typically have larger and more variable loads — refrigeration, milking parlours, manufacturing equipment. The same Solis-Fogstar logic applies but at HV scale with Pylontech, paired with Spectrum’s Predbat configuration tuned to the commercial demand pattern.
How to switch to Agile if you’re not already on it
- Confirm you have a SMETS2 smart meter giving half-hourly readings.
- Switch to Octopus Energy as your supplier (free, takes 21 days).
- Once with Octopus, request Octopus Agile via your account — switch is usually next-day.
- Install / commission your battery and Predbat setup before the switch goes live, so day one is already optimised.
FAQs
Do I need solar panels to benefit from Octopus Agile with a battery?
No — a battery alone can arbitrage Agile’s half-hourly prices. Charge in negative or low-price windows, discharge in the evening peak. You’ll save less than with solar (no free daytime generation), but the maths still works, particularly if you can charge an EV from cheap windows too.
Is Predbat free?
Yes — Predbat is open-source software that runs on Home Assistant, which is itself free. You’ll need a small home-server device (Raspberry Pi 4/5 or equivalent) to run Home Assistant on. Hardware cost is around £100–£200. Spectrum can supply and configure this as part of a battery install.
Will Octopus Agile still exist in five years?
Almost certainly. Half-hourly settlement is now market-mandated for domestic supply, and Agile-style variable tariffs are spreading across the market — not retreating. Even if Agile’s exact branding changed, a battery sized and automated for half-hourly arbitrage would still pay back on whatever variable tariff replaced it.
Can Powerwall 3 do Agile-aware charging without Predbat?
Yes, via Tesla’s app, which now supports Octopus tariff integration in the UK. It’s neat and easy. It just isn’t quite as aggressive at price-following as Predbat. For most Powerwall customers that gap is acceptable. For the savings-maximisers, it isn’t.
Can I export-arbitrage on Octopus Agile?
Yes — Octopus Agile Outgoing is the export-side tariff with the same half-hourly pricing dynamic. Charge the battery from solar or from cheap Agile import windows, then export when Agile Outgoing peaks. This is where Predbat really earns its keep, optimising both import and export simultaneously.
What size Fogstar do I need to run Agile properly?
For a typical UK family home, the Fogstar 16.1kWh hits the sweet spot — enough capacity to cover the 4–7pm peak window twice over, with headroom for a morning charge from solar. Heavier loads (EV plus heat pump plus large family) point to a 32kWh outdoor unit or a 48.3kWh stack.
Do I need an MCS installer to fit an Agile-optimised battery?
For the solar PV install itself, yes — MCS is required to be eligible for the Smart Export Guarantee and most retrofit grants. For a battery-only retrofit, MCS isn’t strictly mandatory but most insurers and warranties prefer it. Spectrum is MCS-certified (NIC200223) and uses the same standard for retrofit-battery work as new installs.
Related reading
- Fogstar vs Pylontech: which solar battery is right for you?
- Octopus Agile + Solar: 2026 guide to maximum savings
- What is Predbat? Solar battery optimisation explained
- How much solar battery storage do I need?
For the full Spectrum service overview see domestic battery storage.
Talk to a real Predbat-experienced installer
Spectrum has Predbat live across dozens of homes and businesses in the East Midlands. We’ll quote the battery, the inverter and the automation in one package.
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