The 3-bed house is where UK solar’s numbers are most settled: it’s the country’s most common home, the industry’s default quote, and the size the whole market prices around. That’s good news for you — the ranges below are tight, well-evidenced, and the traps are known. Here’s the honest version of all of it.

The right size: why 4kW is the 3-bed default (and when 5kW wins)

Sizing is a usage calculation, not a bedroom count — bedrooms are just the industry’s shorthand for it. A typical 3-bed household uses 2,700–3,400kWh a year; a 4kW system generates ~3,400kWh — around 1.2× typical usage, the sweet spot where you cover a large share of your own consumption and export genuine surplus rather than most of your output. In panels: modern residential panels run 400–450W, so 4kW means 10–12 panels on ~20–24m² of roof — comfortable on a standard semi’s main pitch. Step up to 5kW (13–15 panels) if you work from home, run electric showers hard, or expect an EV within a couple of years. And a quietly useful technicality: 4kW paired with a standard 3.68kW inverter fits within G98 grid rules — no advance network approval, no weeks of waiting — one of several reasons it’s the size installers quote first and fit fastest. (Our 2-bed and 4-bed guides bracket this one if your home sits either side.)

2026 costs: what £6,000–£8,000 buys

Those are complete prices: panels, inverter, mounting, scaffolding, labour, certification, grid paperwork — and 0% VAT, already included, running until at least March 2027. The market average for a 4kW install sits near £7,500, with the South East priciest and the North cheapest. What moves an individual quote: roof complexity and access (scaffolding on a steep slate roof isn’t scaffolding on a shallow concrete-tile semi), panel tier, inverter brand, and occasional electrical extras like a consumer-unit upgrade (£300–£800). The practical takeaway hasn’t changed in a decade: identical roofs get quotes £1,000+ apart, so compare two or three — it’s the highest-paid ten minutes in this whole project.

What you’ll generate and save: the self-consumption lever

Your 3,400kWh of annual generation is worth different amounts depending on where it goes: used at home, each kWh saves ~25p; exported, it earns ~12p on a decent SEG tariff. That 2:1 ratio is the entire savings game. A typical out-all-day household self-consumes ~45% and lands around £500–£650 a year (the Energy Saving Trust’s typical estimate sits right in this band); a home-all-day household shifting appliances into daylight pushes £700–£800; add a battery and 70–85% self-consumption takes the total benefit toward £900+. The free wins come first: dishwasher and washing machine on daytime timers, car and immersion heater drinking surplus sunshine. Working from home, so often framed as an energy cost, is one of solar’s best multipliers — you’re simply there to use what the roof makes.

Payback, honestly: 8–11 years — then the good part

A £7,000 system saving £600–£800 a year pays back in eight to eleven years. You’ll meet 6-year claims in the wild; they lean on optimistic self-consumption and generation assumptions, so treat them as the best case rather than the plan. What genuinely accelerates you toward the front of the range: daytime usage, an EV on surplus solar, a heat pump (which also transforms the case for going bigger), southern sunshine. What drags: exporting most of your output at half its home value. The part that deserves more attention than it gets is what happens after payback: panels last 25–30 years, so a system square at year ten delivers 15–20 further years of largely free electricity — a five-figure lifetime return on cautious assumptions — with one pencilled-in mid-life bill, the inverter replacement (£500–£1,500) somewhere in years 10–15. Solar on a 3-bed isn’t a get-rich scheme; it’s a boring, reliable one — the best kind.

The battery question: right idea, right order

Panels first, battery second — and battery for the right reason. A 5kWh battery lifts self-consumption from ~45% to 70–85%, storing lunchtime generation for the evening — but on solar-shifting alone its own payback runs 10–14 years, which is why the modern case rests on the second trick: charging cheap overnight (7–8p on tariffs like Intelligent Octopus Go) to run the house all day — a saving that works every day of the year, December included. If your evenings are busy and you’ll pair the battery with a time-of-use tariff, the combined system earns its £9,000–£11,500 keep; if you’re rarely home and would run it purely as summer storage, spend the difference elsewhere. Size it to your actual evening usage — our battery sizing guide does the arithmetic, and the 4kW-with-battery guide covers this exact combination in full.

Terrace, semi, detached: what changes

The semi — Britain’s archetypal 3-bed — is the easy case: a main pitch fitting 10–12 panels, standard scaffolding, textbook install. The terrace often has one usable pitch and party-wall considerations, sometimes capping the system at 8–9 panels (3–3.5kW) — a smaller but still thoroughly worthwhile system, and worth checking the roof-capacity maths before assuming; east-west terraces split panels across both pitches surprisingly well. The detached 3-bed usually has roof to spare — the question flips from “what fits” to “what’s worth it,” and the answer is still usage-led: don’t buy an 8kW system for a 3,000kWh household unless an EV and heat pump are genuinely coming (the FAQ below does that maths). In all three cases the constant is the same: size from your bills, not your bedrooms — bedrooms got you to this page; your usage picks your system.

Frequently asked questions