The first wave of UK domestic solar under the Feed-in Tariff didn’t just build an industry — it built a fleet. Between roughly 2010 and 2015, hundreds of thousands of roofs were fitted with 175W–250W polycrystalline modules, largely because that’s what the panel market offered at the time, not because anyone had done a rigorous efficiency calculation. Fifteen years on, that fleet is entering a genuinely interesting phase: FiT contracts are ageing, string inverters are past or approaching their design life, and the panels themselves — while rarely “dead” — are increasingly outclassed by what a 2026 installer can put on the same roof. Repowering, in the sense of swapping ageing generation kit while keeping the array’s grid connection and (where applicable) its FiT export payments, is becoming a legitimate line of installer work rather than a niche.
Why 2026 is the inflection point
The maths behind repowering has shifted for three separate reasons converging at once.
First, the panels themselves have moved on. A 250W panel from 2011 typically measured around 1.6m² and produced roughly 156W/m². A modern N-type TOPCon or HJT panel comfortably exceeds 210W/m² and degrades at around 0.4% a year rather than the 0.5–0.8%/yr common on early polycrystalline cells — meaning a decade-old 4kW roof array, now derated to perhaps 3.6–3.7kW of genuine output, could be replaced with 6kW+ of nameplate capacity on the identical roof footprint. That’s not a marginal upgrade; it changes the entire self-consumption and export equation for the household.
Second, the inverter cliff is real and it’s now. Domestic string inverters installed in the 2011–2013 FiT boom are typically rated for 10–15 years. A huge cohort is now failing or flagging faults, and replacement costs of £500–£1,000 are landing on homeowners who assumed the system was a “fit and forget” asset. When an inverter has to come off the wall anyway, the incremental cost of also swapping the panels — rather than reusing 15-year-old glass — is often modest relative to the labour already being spent on scaffolding and isolation.
Third, battery economics have caught up. In 2011 there was no sensible domestic battery product; in 2026 a well-specified system (roughly £4,000–£8,000 installed, or a Tesla Powerwall 3 at around £8,500–£10,500 for 13.5kWh) can absorb daytime generation that a FiT-era household never had a way to use. For anyone still exporting at old FiT rates, or for anyone who’s moved onto the Smart Export Guarantee, storage materially changes what a repower is worth.
The rule that actually matters: FiT preservation
Before any installer or homeowner touches a FiT-registered array, the single most important thing to establish is whether the existing FiT contract survives the change — because it does not automatically.
Ofgem’s FiT scheme rules treat panel replacement as a potential “eligibility date” reset. In practice, adding capacity or replacing generating equipment on an accredited installation can trigger a requirement to re-register the additional capacity (sometimes at current, much lower, export-only rates under the closed-to-new-entrants FiT or via SEG instead), and in some circumstances can jeopardise the tariff on the original capacity if the installation isn’t handled as a defined extension. The scheme has provisions for like-for-like replacement of faulty inverters and even panels without loss of accreditation, but the paperwork obligation sits with the FiT licensee (the supplier paying the tariff) and needs notifying before, not after, work happens.
This is not a detail to guess at on-site. Any installer being asked to “just swap a few panels” on a FiT roof should be treating it as a compliance job first and a technical job second: confirm the MCS certificate status, check which FiT licensee the customer is with, and get written confirmation of how the modification will be treated before signing off any generation meter change. Get it wrong and a homeowner receiving, say, 15–20 years of index-linked generation tariff can lose it overnight for the sake of a panel swap that saved them a few hundred pounds a year in extra yield. Get it right, and repowering can be pitched as pure upside — more generation, same protected income stream.
For installers who want the underlying scheme mechanics rather than a summary, this is worth building into a standing customer FAQ, and it’s the kind of technical distinction that separates a proper MCS-certified installer from someone offering a quick panel swap with no compliance check.
The panel-swap economics, worked through
Take a representative FiT-era case: a south-facing roof with a 4kW system (16 panels at 250W), installed in 2012, generating perhaps 3,200–3,400 kWh/yr at UK’s typical ~850 kWh/kWp yield assumption (more in the sunny south, where 1,000+ kWh/kWp isn’t unusual). The inverter is on borrowed time. Three options exist:
Do nothing and wait for failure. Cheapest short-term, but a failed string inverter with no warning leaves the household with zero generation and a forced emergency replacement, often at a worse negotiated price than a planned job.
Like-for-like inverter swap only. Roughly £500–£1,000, keeps the existing panels and FiT status unambiguous (no capacity change, minimal re-registration risk), but leaves ageing, lower-yield panels producing at their now-derated output for another 10+ years.
Full repower — new panels plus inverter. For a 4kW-to-6kW uplift, all-in costs on an existing roof (no new mounting structure needed if rails are sound, though most installers will want to inspect and likely replace ageing aluminium rail systems from that era) typically land in a range comparable to a fresh 6kW install minus some labour efficiency — so roughly £8,000–£11,000 depending on roof access, scaffolding, and whether a battery is bundled in. Against that: 0% VAT on residential solar and battery storage applies in Great Britain until 31 March 2027, which meaningfully improves the payback maths on any repower completed before that date, since VAT reverts to the standard 5% rate afterwards.
The extra ~2.4kW of nameplate capacity, even after accounting for degradation on the new panels, could realistically add 1,500–2,000 kWh/yr of generation. At today’s typical import price of around 25p/kWh (Ofgem price cap, variable by supplier and tariff), self-consumed generation is worth roughly four to five times what SEG export pays (SEG rates vary by supplier, broadly 12–20p/kWh at the better end) — which is exactly why pairing a repower with a battery, rather than repowering panels alone, tends to be where the real payback improvement sits. Installers offering combined panel-and-battery repower quotes should be modelling both the generation uplift and the shift from export to self-consumption, not just kWp added.
Who should actually be doing this work
Repowering isn’t a like-for-like commodity job — it needs someone who understands both the electrical/structural side and the FiT compliance side. Regional installers with a strong track record on domestic retrofit are well placed for this, including firms like ecoaim.co.uk covering Central Scotland’s ageing FiT-era stock, or Premier Electrical Renewables for households wanting a combined panel-plus-battery upgrade path rather than a bare swap. For homeowners specifically chasing the numbers before committing, a proper cost breakdown — covering current battery storage pricing and a realistic payback period model — should sit alongside the FiT compliance check, because the decision genuinely hinges on both.
There’s also a maintenance dimension the trade shouldn’t skip: before recommending any full repower, an honest condition survey of the existing array often reveals that the real fault is degraded connectors, a failing isolator, or shading from vegetation growth over a decade, not the panels themselves. National O&M specialists such as Solar Maintenance Solutions exist precisely because a chunk of “my system isn’t producing what it used to” calls turn out to be a £150 fix, not a £9,000 repower. Recommending replacement before ruling out the cheap fix is bad practice and, increasingly, bad for reputation in a market where customers compare notes.
The commercial parallel
The domestic FiT fleet gets the attention because of the volume of installations, but the same repowering logic applies — arguably with a stronger business case — to the commercial rooftop estate installed under the original FiT and early Renewable Obligation Certificate schemes on warehouses, factories and agricultural buildings between 2010 and 2015. A 50kWp commercial array from that era, built on the same first-generation crystalline modules, sees a proportionally larger uplift potential simply because of roof area: commercial installs at £900–£1,200/kWp for new capacity make repowering a straightforward capital allowance and energy-cost decision for finance directors, without the FiT-preservation complexity that residential installers have to navigate line by line (large commercial FiT accreditation is subject to its own, generally stricter, modification rules, so the same “check before you touch it” principle applies, just with a different licensee process). Building owners assessing whether to repower or simply extend an ageing array should be running the numbers through a proper commercial solar finance route rather than treating it as a straight cash purchase, particularly with the 0% VAT window closing in March 2027 concentrating minds on timing.
What the trade should be doing now
For installers, the practical opportunity here is a genuine outbound campaign: identify customers on the books (or reachable via warranty records, if retained) whose systems were installed 2010–2014, lead with the inverter-cliff urgency rather than a hard sell on new panels, and build the FiT compliance check into the initial site visit as standard — not an afterthought once a quote’s been issued. Getting that sequencing wrong is how installers end up with an angry customer whose tariff got reset by a licensee they never contacted.
The FiT-era fleet isn’t going anywhere quietly. Over the next three to five years, the inverter cliff alone will force a decision on hundreds of thousands of UK roofs, whether homeowners actively seek it out or not. The installers who’ve built the compliance knowledge and the honest condition-assessment habit into their process now will be the ones capturing that work — rather than losing customers to a rushed emergency call-out and a botched FiT re-registration.