Solar panel installation cost breakdown

Understanding where the money goes in an installation does two useful things. It tells you which parts of a quote are negotiable, and it explains why the economics improve as the system gets larger.

The structural insight is this: a substantial share of the cost does not scale with system size. Once you see which parts those are, several pricing puzzles resolve themselves, including the one visible in the government’s own numbers: the DESNZ installation cost data puts the 2025/26 median at £1,780 per kilowatt for systems up to 4kW but £1,697 for 4 to 10kW systems. Bigger is cheaper per unit, and the fixed costs below are the reason.

The parts that scale with size

Panels. More capacity means more modules, roughly linearly. Panel prices have fallen a long way over the last decade, so this is a smaller share of the total than most people assume.

Mounting hardware. Rails, brackets and clamps, proportional to panel count.

Installation labour on the roof. More panels take longer to fit, though not proportionally, since setting up and packing down are fixed.

DC cabling. Modest, and proportional to array size.

That is roughly it. Everything below is largely fixed.

The parts that do not scale

Scaffolding. Priced by the property and the access, hired for the duration. A four panel system and a sixteen panel system on the same house need much the same scaffold.

The survey and design. Someone visits, measures, assesses shading and produces a specification. Same effort either way.

Certification and registration. A fixed administrative cost per installation, and the item whose absence costs you export payments, so it is fixed but not optional.

Network operator paperwork. Fixed, and it interacts with sizing: up to 3.68kW of inverter on a single phase the installer simply notifies the network operator within 28 days under G98, while larger inverters need a G99 application approved first. Some installers price and size to stay under the line; the paperwork is cheaper than the capacity it costs you.

The inverter, in steps rather than smoothly. One inverter covers a range of array sizes, so within that range adding panels adds no inverter cost at all. Cross the boundary and it steps up.

Consumer unit work and the AC side. Largely independent of array size.

Mobilisation. Getting a team and a van to your house for a day.

VAT. Zero on the whole qualifying installation until 31 March 2027, so it should be absent from the breakdown entirely, at any size.

Why this makes larger systems better value

Put illustrative numbers on it. Suppose £2,500 of a job is fixed: scaffolding, survey, certification, mobilisation, the AC side. On a 3kW system that fixed block is over £800 per kilowatt before a single panel is paid for. On a 6kW system it is around £415. The medians above show the effect in real data, and the same logic is why bedroom-table pricing misleads: the tables scale the whole price with size when only part of it scales.

Two practical conclusions:

Very small systems have poor economics. The scaffolding and paperwork dominate. This is worth knowing if you are considering a token installation: two panels ride on the same scaffold as twelve.

Undersizing is expensive. If your roof and your consumption support more capacity, the marginal panel, added to a job already happening, costs little more than the panel and the labour to clip it on, and buying it later means paying the fixed costs a second time. The worked 4kW example shows what the resulting capacity earns.

Where roof type changes the number

Labour is where your specific property shows up:

• Concrete tile is the quickest common covering to work with.
• Slate is slower and more fragile. Breakages are expected and replacement slates must be sourced.
• Clay sits between the two, and older clay is brittle.
• Metal and flat roofs need different mounting approaches entirely.
• Steep or high roofs slow everything down.
• Restricted access raises scaffolding cost and sometimes needs a permit for the pavement.

None of this appears on a price list, and all of it is why a phone quote is not a quote, and why the honest spread around the national medians is as wide as it is.

What margin looks like

Every business has overhead and profit. A national installer with a sales force, a call centre, a marketing budget and a warranty reserve carries costs a small local firm does not.

That does not make either wrong. It means you should know what you are buying: a stronger covenant behind the workmanship warranty has value, and so does a lower price. What you should not do is assume the difference is pure profit without asking, and the consumer code protections behind each firm are checkable rather than takeable on trust.

Using the breakdown when comparing

Take each quote and estimate, roughly, how much sits in fixed costs and how much in per panel costs. Then compare cost per kilowatt peak, against each other and against the published medians.

If one quote is much higher on a per kilowatt basis for a similar specification, ask directly what accounts for it. A genuine answer names something specific: a harder roof, a better inverter, per panel electronics for shading, a longer cable run, the usual suspects behind honest quote gaps. A vague answer about quality is telling you the difference is margin.

Then check the whole thing against your own model, using your consumption and your tariff rather than the savings figure that came attached. The breakdown tells you whether the price is fair; only the projection tells you whether the purchase is sound.