100kW Solar System UK: Cost, Output & Payback (2026)

This guide covers everything that figure buys you, panels, roof space, annual output, payback and who this size actually suits.

100kW System at a Glance

Who a 100kW System Suits

Warehouses, factories and distribution centres with large, unshaded roofs and heavy daytime electricity use. At commercial rates of 25–30p/kWh with 70–90% self-consumption, real-world payback is typically 3–5 years, and the Annual Investment Allowance lets businesses deduct 100% of the cost from taxable profits in year one. VAT-registered businesses reclaim the 20% VAT.

Cost Breakdown

The headline £72,500 price for a 100kW system covers:

• Panels: 200–370 modules from a tier-1 manufacturer
• Inverter: string or hybrid inverter matched to system size
• Mounting & cabling: roof hooks, rails, DC/AC cabling, isolators
• Installation labour: typically 1–3 days for residential, longer for larger commercial
• DNO application: G98 notification or G99 approval, see below
• Commissioning & MCS certificate: required for SEG registration and 0% VAT

See our solar battery cost guide if you are considering adding storage. For a 100kW system, expect £60,000+ for a matched battery.

How Much Electricity Will It Produce?

A 100kW system in the UK produces around 92,686 kWh per year. Daily and monthly output varies with season, roughly 70% of generation arrives between April and September. Regional variation is significant:

• Southern England: upper end of the range (92,686 kWh)
• Midlands & Wales: ~90–95% of that figure
• Northern England: ~85–90%
• Scotland & Northern Ireland: ~80–85%

For context, the UK average household uses around 2,900 kWh per year, so a 100kW system produces 32.0× the typical home's electricity needs.

Savings & Payback

Savings come from two sources:

1. Self-consumption: ~£9,146/year avoiding imported electricity at ~24–25p/kWh
2. SEG export income: ~£8,342/year from surplus sold back to the grid

Over 25 years, the typical panel warranty, total savings reach roughly £437,200. That gives a payback period of 3–5 years at current prices. Electricity prices have risen faster than inflation historically, so real-world payback is often shorter than the modelled figure.

Pairing the system with a time-of-use tariff like Octopus Flux can lift export earnings significantly, see our best Octopus tariffs for solar guide.

DNO Approval (G98 vs G99)

G99 approval always required, with a connection study at this scale. Expect 8–12 weeks, possible export limits, and a three-phase (often dedicated) supply. Make DNO approval a contract milestone.

For full detail on the application process, timelines, and the nine UK DNO regions, see our DNO application guide.

Common Mistakes at This Size

• Signing before G99 approval, at 100kW a refused or export-limited connection can render the design uneconomic; make approval a contract milestone
• Skipping the structural survey, a 100kW array adds 12–20 kg/m² plus wind loading, and many older industrial roofs need strengthening first
• Not claiming the Annual Investment Allowance, 100% year-one relief is worth around 25% of the project cost to a company paying the main corporation tax rate
• Comparing quotes without an O&M plan, at this scale annual monitoring, inspection and cleaning costs materially affect lifetime ROI
• Ignoring half-hourly metering and supply contract impacts, some suppliers reprice commercial contracts when on-site generation is added

Frequently Asked Questions

Compare Other System Sizes

Get a Real Quote for Your Home

The easiest way to benchmark a 100kW system for your property is to compare quotes from two or three MCS-certified installers in your area. Pricing varies with roof complexity, panel brand, and whether you need scaffolding.