Summary

"How many panels do I need?" is the question every solar PV customer asks first, and giving a confident, wrong answer — or a vague "depends on the roof" without walking through the actual numbers — loses the quote to whichever installer does show the maths. The calculation isn't complicated, but it does need four real inputs specific to that property: the household's annual electricity consumption (or the portion of it the customer wants to offset), the roof's orientation and pitch, any shading, and the panel wattage being specified.

The output of a solar PV array depends on system size (measured in kWp — kilowatt-peak, the panels' rated output under standard test conditions) multiplied by the site's specific yield (how many kWh of real-world generation each kWp actually produces per year at that location, orientation, and pitch). Specific yield varies significantly across the UK — roughly 15-20% higher in the south of England than in Scotland for the same roof — and drops materially for anything other than a south-facing roof at an optimal pitch (roughly 30-40°), and drops further still with any shading from chimneys, trees, or neighbouring buildings.

The other number that matters and gets missed in casual conversations: self-consumption. A solar array sized to match annual consumption on paper does not mean the household uses all of that generation directly — most homes without a battery self-consume only 30-50% of solar generation (the rest is exported, currently paid via the Smart Export Guarantee), because generation peaks at midday when many households have lower demand. This affects the payback calculation and is worth explaining alongside the panel count itself.

Key Facts

Quick Reference Table

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Orientation Approx. yield relative to due south Typical impact on panel count for same output
Due south, 30-40° pitch 100% (baseline) Baseline
South-east / south-west ~90-95% +5-10% more panels
Due east / due west ~80-85% +15-20% more panels
Flat roof (optimally angled mounts) ~90-95% (mount-dependent) +5-10% more panels
North-facing ~60% or less Generally not recommended for panel placement
Partial shading (moderate) Highly variable, can be 10-30%+ loss Site survey and optimiser/microinverter assessment required

Detailed Guidance

The core sizing formula

Required kWp = Target annual demand (kWh) ÷ Site-specific yield (kWh/kWp/year)

Panel count = Required kWp ÷ Panel wattage (kW)

Always round the panel count up to the nearest whole panel, and check the resulting kWp against the roof's actual usable area (accounting for eaves clearance, obstructions like chimneys and roof windows, and any structural or fire-access setback requirements) before finalising a quote.

Worked example 1 — typical 3-bed semi, south-facing roof, no shading

A 3-bed semi in the South East with annual electricity consumption of 3,100 kWh/year (slightly above the Ofgem medium TDCV, reflecting typical modern appliance use), south-facing roof at 35° pitch, no shading.

Worked example 2 — larger detached home, east/west roof split, planning for an EV

A 4-bed detached home with annual consumption of 4,200 kWh/year, planning to add an EV doing roughly 8,000 miles/year (add ~2,000 kWh/year), no south-facing roof face available — the property has an east-facing and a west-facing pitch of similar size, no significant shading.

Worked example 3 — smaller bungalow with partial shading, targeting partial offset

A retired couple's bungalow with lower annual consumption of 2,400 kWh/year, south-facing roof at 30° pitch, but a mature tree causes shading on part of the array for a portion of the day, reducing effective yield by roughly 20% versus an unshaded equivalent roof. The customer's goal is to offset a meaningful proportion of consumption, not necessarily 100%, given budget constraints.

Why the "just cover 100% of my bill" framing misleads customers

Sizing a system to generate the same number of kWh annually as the household consumes does not mean the household's electricity bill goes to zero, because generation and consumption rarely align in time — most solar generation happens in the middle of the day, while much household consumption happens morning and evening. Explaining self-consumption percentages and the role of battery storage or shifting usage (running appliances during generation hours) is part of setting an accurate expectation, not just a nice-to-have upsell conversation. See solar battery systems for battery sizing guidance relative to a given array size.

Roof area and structural checks

Before finalising a panel count, confirm the roof has enough contiguous usable area once eaves clearance, ridge clearance, roof windows, vents, and chimneys are accounted for — a roof that looks large enough on a satellite image can lose 20-30% of usable area to these obstructions. A structural assessment of the roof's condition and loading capacity (typically 10-25 kg/m² for a mounted PV array) should also be part of the survey, particularly on older roofs or those with a history of movement. See solar panel roofing for loading figures and fixing detail.

Frequently Asked Questions

Should I always size the system to match 100% of annual consumption?

Not necessarily — this is a common default but not always the right answer. Roof area, budget, and the customer's appetite for export versus self-consumption should all factor in. A smaller system with strong self-consumption can sometimes deliver better financial payback than a larger system exporting a high proportion of its generation at a lower rate than the customer pays for imported electricity — model both scenarios where budget allows.

Does adding a battery change how many panels I need?

Not directly — panel count is driven by generation capacity, while battery size is driven by how much of that generation you want to store and use later rather than export. However, if the goal is to maximise self-sufficiency (minimise both import and export), sizing the panels and battery together as a system, rather than sizing panels alone and treating the battery as an afterthought, gives a better overall result. See solar battery systems for battery sizing methodology.

How much does shading really matter if it's only for an hour or two a day?

More than most customers expect, particularly with older string inverter systems where shading on even one panel in a string can reduce the output of the entire string, not just the shaded panel. A proper shading assessment (ideally using a tool like a Solar Pathfinder or software-based shading analysis during the site survey, not just a visual guess) should inform both the panel layout and the inverter/optimiser choice.

What panel wattage should I quote for if the customer hasn't specified a brand?

Use a realistic current-market panel wattage (400-440W is a reasonable working assumption at time of writing) for initial sizing conversations, but always finalise the actual panel count and array layout against the specific product being quoted, since wattage, physical dimensions, and efficiency vary between manufacturers and product ranges —.

Does this calculation apply the same way to flat roofs?

The same formula applies, but flat roofs need mounted frames set at an angle (typically 10-15° to balance yield against wind loading and self-shading between rows), and the spacing between rows to avoid rows shading each other reduces the effective panel density per m² of roof area compared to a pitched roof — factor this into the roof area assessment for flat-roof commercial or larger domestic installations.

Regulations & Standards