Upgrading a developer-fitted solar system

Start by working out what is actually limiting you

Three different complaints have three different fixes, and buying the wrong one is the common mistake.

“I generate plenty but still buy electricity in the evening.” The panels are fine; the problem is timing. That is a battery question, and the value depends on the gap between your import price and your export rate.

“The roof is half empty and the system is small.” That is a panels question, provided the inverter can take them.

“Output flatlines on sunny days.” That is clipping — the array is producing more than the inverter can convert. That is an inverter question, and it is worth quantifying before spending: clipping on a handful of summer afternoons costs very little, and a system deliberately designed that way is not faulty. See export limiting for how limits are applied and why.

Your monitoring data answers all three. If you do not have access to it, get it — that is part of the handover pack.

The number that decides the paperwork

G98 covers fully type-tested micro-generators up to and including 16 A per phase, which the Energy Networks Association converts to 3.68 kW single-phase and 11.04 kW three-phase. Inside that limit you connect and notify within 28 days. Above it, G99 applies: apply, wait, then install.

Two things follow that people get wrong:

The limit is on registered capacity, not panel capacity. A 5 kWp array behind a 3.68 kW inverter has a registered capacity of 3.68 kW. So adding panels alone may not move you into G99 at all.

A change is a modification, and modifications are notifiable. You cannot quietly increase capacity because the original notification exists. Your installer normally handles this, but it is your installation and worth confirming.

Batteries on a new build

Storage is not required by the standard, so on most new builds it is a retrofit — with two saving graces: the wiring is new and documented, and the consumer unit is modern.

What to check:

• Space and a route. Garage, utility, loft — with the cable run available.
• A spare way in the consumer unit.
• Whether the inverter is hybrid or whether an AC-coupled battery is needed. Both work; they cost differently.
• Fire safety and siting. Approved Document L points to Approved Documents B and P for storage, and says on-site generation and storage should be “an appropriate size for the site”. Manufacturers’ instructions govern siting.
• Your tariff. A battery on an import-export tariff with cheap overnight rates earns in two ways rather than one — see SEG tariffs.

If the house is still being built, ask for ducting to a battery location now. It is close to free at first fix.

Replacing the inverter

The inverter is the component most likely to need replacing first, and on a developer-fitted system it is the component most likely to have been chosen on price.

When it fails, that is the moment to reconsider sizing rather than to like-for-like replace out of habit — particularly if the array was oversized behind it.

Approved Document L is explicit that where renewable technology such as a photovoltaic array is replaced, the new system should meet the standards then applicable. Replacement is regulated work, not a free swap.

Keep the paperwork. A new inverter changes what your handover pack describes, and a future buyer’s conveyancer will ask.

Constraints particular to new estates

Three that catch people on new developments specifically:

Covenants in the transfer. Restrictions on external alterations are routine on modern estates and are enforceable by the developer or a management company. Read the transfer before assuming the roof is yours to alter — covenants.

Management company consent. Where a management company holds common parts or estate roads, its consent regime may extend further than you expect.

Planning is usually not the obstacle. For most houses, roof-mounted solar is permitted development subject to the conditions — see permitted development and the 200 mm rule. Conservation area or listed status changes that, and some new developments sit inside designations.

Warranties: three different clocks

Ask about all three, in writing, before work starts:

The specific risk worth naming: if a leak appears near new penetrations, you want it obvious who is responsible. Photographs before and after, and a written scope, are cheap insurance.

The certification question, before you commit

If you rely on SEG export payments, settle the certification position before the work rather than after.

Ofgem’s guidance sets out what generators need for SEG, and suppliers ask for the MCS certificate for the installation. How a supplier treats added capacity on an existing arrangement is where practice varies, and where we would want your own supplier’s answer in writing.

Ask three questions: does the upgrade require a new or amended MCS certificate; will my existing SEG arrangement continue to cover the installation; and is my export meter adequate for the new capacity.