Everyone tells you solar is worth it. But installers have an obvious reason to say that, and most online calculators use best-case assumptions. I wanted real data.

So after getting my system installed in January 2025, I connected Home Assistant to every sensor and tracked every watt — generated, imported, exported. This is what 17 months of real data looks like.

My setup

8 × AIKO 455W N-Type ABC panels — full black, 23.3% efficiency, south-facing
Fox ESS 10.3kWh battery with hybrid inverter
3kW inverter, 3 strings
Octopus Agile tariff + Predbat for automated smart charging
Location: Whitchurch, Cardiff (CF14)

Cardiff isn’t the sunniest place. Does that matter?

Yes, but less than you’d think. Cardiff gets around 1,600 hours of sunshine per year — less than Spain, more than most people assume. The real question isn’t how sunny it is but how much you can offset from the grid.

My system generated an estimated 4,145 kWh in 17 months. On a purely solar-only system that would still be useful. But paired with a battery that stores daytime generation and a smart tariff that charges overnight at cheap rates, the impact is multiplied.

The actual numbers

My grid import over 17 months: 1,954 kWh total.

That sounds like a lot until you compare it to the UK average of around 4,800 kWh per year (roughly 6,800 kWh over 17 months). My import is about 71% lower than average.

Monthly import ranged from:

3.8 kWh in May (almost entirely solar-powered)
259 kWh in December (Predbat charging overnight on Agile)

The winter months look higher but that’s actually Predbat deliberately importing cheap overnight electricity — often at 5–8p/kWh — to fill the battery before the morning peak. That’s not waste; it’s arbitrage.

What it saved

Over 17 months:

Solar self-use vs buying at 28p: ~£836
Export earnings (SEG at ~15p): ~£174
Agile smart charging saving vs standard tariff: ~£313

Total recovered: ~£1,337 on a £8,500 install.

At typical market rates (£8,000–£9,000 for a similar system), that’s still a very strong return — roughly £79/month once everything is running, with a payback period of around 8–9 years and then 20+ years of free electricity after that.

The Octopus Agile effect

This was the part I underestimated. On a standard flat tariff, the battery adds value but you’re still paying 28p for any grid import. On Agile, my overnight charging averages around 6–8p. On the best nights it goes negative — the grid pays me to use electricity.

Predbat handles all of this automatically. It reads the next 48 hours of Agile prices, checks the solar forecast, and plans every charge and discharge slot. I haven’t manually touched it since setup.

Is it worth it in Cardiff?

For me: absolutely. The combination of solar + battery + Agile means my electricity costs are a fraction of what they were. Cardiff’s weather isn’t perfect but it’s more than adequate, and the smart tariff + battery makes up for anything the sun doesn’t cover.

The key questions are:

South or south-west facing roof? Essential. East-west split works but reduces generation.
No significant shading? Trees and chimneys kill output.
Can you switch to Agile? Without a smart tariff, battery payback slows significantly.
Budget for the battery? The panels alone pay back faster, but the battery + Agile combo is what makes it transformative.

I’m happy to share my full data and put you in touch with The Solar House — the team who fitted my own system. Get in touch here — no obligation, just honest advice from someone who’s done it.