Many farms that fitted solar in the 2010s now have spare roof space on newer buildings, an inverter at the end of its warranty, or a lot of daytime output going to the grid at export rates. Each of those points to an upgrade — an extension, a replacement inverter or a battery. Each also has rules attached that a like-for-like repair does not.
1. Tell the network operator first
Grid connections are approved for a specific installation. A system rated up to 16 A per phase (3.68 kW on a single-phase supply) connects under the simple G98 notify-after route; anything larger needs the operator's G99 approval before it is connected. The operator looks at everything that can feed power onto its network — panels, inverters and batteries — so an extension or a battery can move an installation from G98 into G99, or need a revised G99 approval with a new export limit. Ask before you order the kit, not after it is on the roof. Our guide to farm solar grid connections covers the process.
2. Planning: Class J covers alterations too
In England, Class J of Part 14 of the General Permitted Development Order permits "the installation, alteration or replacement" of solar PV on the roof of a non-domestic building — so adding panels to a farm shed, or swapping failed ones, is normally permitted development within the same limits: no more than 0.2 m proud of a pitched roof, no more than 1 m above a flat one, and at least 1 m from the roof edge. One threshold matters for extensions: equipment above microgeneration size (50 kW) falls under Class J(c), which needs the council's prior approval on design and external appearance — especially glare — decided within 56 days. Listed buildings are excluded. See planning permission for farm solar for the full rules and the position in Scotland, Wales and Northern Ireland.
3. Replacing an inverter
Inverters usually carry shorter warranties than panels, so on an older system they are often the first major component to be replaced. The replacement must suit the array's voltage and current, and it must be a type the network operator accepts — the Energy Networks Association keeps a register of type-tested equipment. A like-for-like change still needs recording with the operator if the rating changes. It is also the natural moment to add better monitoring; see what farm solar monitoring should show.
4. Adding a battery
A battery lets a farm store daytime surplus and use it in the evening instead of exporting it. Whether that pays depends on how much you export now, the price you pay for imported electricity and the export rate you receive — so size it from half-hourly meter data, not a brochure. Two practical points:
- VAT: on business premises a battery carries 20% VAT, which a VAT-registered farm business normally recovers. The 0% energy-saving materials rate covers only residential accommodation and buildings used for relevant charitable purposes — so it can apply to a battery for the farmhouse, not to one serving the farm business.
- Grid: as above, the battery's inverter counts towards the size the network operator approves.
More detail is on our page on battery storage for farms.
5. Export payments and tax
If you are paid for exported electricity under the Smart Export Guarantee, tell your supplier before you change the system — the contract is for a registered installation, and an extension may need to meet the same certification requirements as the original. A capital upgrade is new expenditure for tax purposes; see capital allowances on solar panels for how the reliefs work.
6. Replacing damaged panels
Storm-damaged, cracked or hot-spotting panels can be replaced individually. Because panels in a string share one current, a replacement with very different electrical characteristics can hold back the whole string, so installers match replacements to the existing modules or re-design the string. Check the original panel's warranty terms first — and record the change against the commissioning documents.
System upgrades — frequently asked questions
Do I need permission to add panels to an existing farm solar system?
Usually not planning permission: Class J permits the installation, alteration or replacement of solar PV on a farm building roof, within the same limits (0.2 m on a pitched roof, 1 m from the edge). But if the change takes the system above 50 kW, Class J's prior-approval step applies, and the network operator must approve any increase that takes the system past the G98 limit of 16 A per phase.
Does adding a battery need network operator approval?
The network operator looks at everything that can feed power onto its network, and a battery's inverter counts. Adding one can take an installation past the 16 A-per-phase G98 limit and into a G99 application, so check before ordering.
Is there VAT on a battery for a farm solar system?
On business premises, yes — at 20%, which a VAT-registered farm business normally recovers. The 0% energy-saving materials rate applies only to residential accommodation and buildings used for relevant charitable purposes, so it can cover a battery for the farmhouse but not one serving the farm business.