You already have solar — what should you do next?
Most FIT-era homeowners are sitting on a simple £200+/year gain they have not claimed yet. The battery question is more nuanced than most installers let on. Here is the honest breakdown.
Switch your export tariff first — it costs nothing
If you are still on the Feed-in Tariff's deemed export rate, you are getting 5.25p per kWh for around half your generation. Good Energy's fixed SEG tariff currently pays 25p per kWh. For a typical 4.8 kWp household exporting roughly 1,250 kWh a year, that is the difference between £66 and £313 — a £247/year gain with no hardware change.
The catch: switching to SEG requires a smart meter, and once you have one you permanently lose the 50% deemed export assumption. You cannot go back. If your actual export is significantly above 50% of generation (common for households that are out all day), get a smart meter reading first to verify the numbers work in your favour before switching.
Your FIT generation payments are completely unaffected — you keep those regardless of which export tariff you choose. See our full SEG rate comparison for current supplier rates.
Should you add a battery? The honest maths
For a FIT-era system, the default battery retrofit is AC-coupled: a battery with its own inverter is added after the generation meter, leaving your existing solar inverter untouched. This is simpler, cheaper, and keeps your FIT generation tariff safe (no bi-directional meter needed if the battery cannot charge from the grid).
A 5 kWh AC-coupled retrofit currently costs £3,000–£4,200 installed (20% VAT applies on standalone battery retrofits — the 0% window only covers batteries bundled with new panels). The net annual gain from self-consuming ~1,500 kWh instead of exporting at FIT rates works out to roughly £230–£310/year, giving an 11–12 year payback on its own.
That payback shortens significantly if you pair the battery with a smart tariff. Octopus Agile lets you charge from the grid during cheap overnight or midday slots (sometimes 1–5p/kWh) and discharge during the 4–7pm peak. Agile requires a SMETS2 smart meter and is available alongside your existing FIT — it is an import tariff and does not affect your generation payments. The uplift from Agile alone is estimated at £400–£700/year on top of self-consumption savings.
Key rule: notify your FIT licensee in writing before installation. Submit your accreditation number, battery specs, and a system schematic. Use an MCS-certified installer. Failing to notify does not void your FIT, but it leaves you technically non-compliant.
When your inverter needs replacing — go hybrid
Most inverters last 10–15 years. If your system was installed between 2010 and 2014 under the FIT, it is likely approaching or past that window. A like-for-like string inverter replacement costs £700–£1,400 including labour for a 3–6 kWp system. Upgrading to a hybrid inverter (battery-ready) at the same time costs only £400–£600 extra.
Our strong recommendation: always go hybrid when replacing a failed inverter. If you add a battery later (DC-coupled, more efficient than AC), you avoid a second inverter swap. A combined hybrid inverter plus 9.5 kWh battery DC-coupled at time of replacement runs £5,000–£7,500 total for a typical home — significantly better value than doing it in two stages.
FIT rule to note: a replacement inverter must not exceed the original's rated output. Notify your FIT licensee before making changes. A DC-coupled hybrid will require a bi-directional meter for FIT compliance — factor this into your installer brief.
Can you add more panels?
Yes, but there is an important FIT constraint: new panels cannot feed into your FIT-registered inverter. Doing so would inflate generation readings, which constitutes overpayment. New panels must run through a separate inverter or a battery system that handles them independently (the Tesla Powerwall 3 can manage additional panels without touching the original system).
If expanding beyond 3.68 kWp total single-phase capacity, a G99 application to your DNO is required — build in 6–15 weeks for the DNO process. Four additional panels with labour typically costs £2,000–£4,500 depending on inverter requirements. See our guide to adding panels to an existing system for the full process.
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