Fox ESS battery — an honest review after 17 months

When I was choosing a battery I found plenty of spec sheets and almost no real-world long-term data. So here’s mine.

I’ve been running a Fox ESS AIO H3 10.3kWh with a hybrid inverter since January 2025, paired with 8 × AIKO 455W panels, Octopus Agile, and Predbat running inside Home Assistant.

What it does day-to-day

Summer pattern:

• Solar starts charging the battery from around 7–8am
• Battery hits 100% by early afternoon on a clear day
• Remainder of solar goes to export (paid via SEG)
• Battery discharges from early evening through to midnight
• Grid import for the day: close to zero

Winter pattern:

• Solar generates a fraction of summer output
• Predbat charges the battery overnight on cheap Agile slots (typically 1–3am, often under 8p)
• Battery covers morning peak, then any solar covers the rest
• Battery discharged again in the evening before overnight charge cycle restarts

On a good system, you’re rarely paying peak-rate electricity. The battery either came from free solar or cheap overnight Agile.

The Fox ESS hardware itself

What I like:

• The hybrid inverter handles both solar and battery in one unit — cleaner installation than bolt-on batteries
• Home Assistant integration via Modbus works well once set up
• 10.3kWh is a practical size — enough to carry an average house through the evening and into an overnight charge window
• The app is basic but does the job for monitoring

What’s less impressive:

• The Fox ESS app isn’t as polished as some rivals
• Initial setup required a competent installer — not a DIY job
• Documentation is sometimes unclear for Predbat integration

Reliability: 17 months, zero issues. It just runs.

The Predbat integration

This is where the Fox ESS setup becomes genuinely powerful. Predbat is a Home Assistant add-on that reads:

• Tomorrow’s Agile prices (half-hourly)
• Solar generation forecast (from Solcast)
• Current battery state

It then plans every charge and discharge slot for the next 48 hours. On a typical winter night, it’ll schedule a 2-hour charge window at 1–3am when Agile is cheapest, and protect the battery from discharging until evening peak prices drop.

Setting it up takes a weekend and is worth every minute. Once it’s running, it’s hands-off.

Actual performance numbers

17 months of data:

• Total grid import: 1,954 kWh
• Estimated generation: ~4,145 kWh
• Average monthly import: ~115 kWh (vs UK average of ~240 kWh)
• Lowest month: 3.8 kWh (May 2025)
• Agile saving vs standard tariff: ~£313 on that imported electricity

My overnight charging pattern — the big midnight–3am import spike — represents Predbat filling the battery at cheap rates. That “import” is working capital, not waste.

Would I buy it again?

Yes. The Fox ESS with Predbat + Agile is the combination that makes a solar install genuinely transformative rather than just marginally useful. Without the battery you’re still grid-dependent in the evenings. Without Agile you’re not getting the most from the battery. Together they’re greater than the sum of their parts.

The market rate for a system like mine (solar + Fox ESS battery + installation) is around £8,000–£9,000. Based on my real saving rate of £79/month, that payback period is roughly 8–9 years — and panels carry a 30-year performance warranty.

If you’re considering a similar setup in South Wales, I’m happy to share more detail and connect you with my installer.