9.1 Calculating Maximum Array Capacity

Quick Answer: Maximum array capacity is not roof area divided by module area. Handbook outcome 4.1.4 wants module dimensions, available area, spacing between modules, and spacing around the perimeter. Total kWp is the output of each panel multiplied by the number of panels that will actually fit after those spacings. On a domestic pitched roof, MCS installation practice in MIS 3002 clause 5.9.7 keeps modules 400 mm from any roof edge unless extra wind-uplift measures are taken. Use that 400 mm figure when you count modules. Do not treat it as a City & Guilds exam-fee fact printed in the 2922 handbook.

Why 4.1.4 is a packing calculation

A sales drawing that colours the whole rectangle and writes "32 m² × 215 W/m² = 6.9 kWp" is not a design. Modules are rigid rectangles. They meet neighbouring modules at gaps, they stop short of verges, eaves, ridges, hips, and valleys, and they must leave room for clamps, rails, and a person who may later need to reach a connector or a firefighting position. The handbook therefore lists four inputs, not one:

• Module dimensions — length and width of the glass-and-frame unit you intend to buy, taken from the datasheet, not from a round-number memory.
• Available area — the roof plane rectangle (or the usable planes if the roof is cut by hips, valleys, chimneys, or roof lights), after you have subtracted the parts you cannot cover.
• Spacing between modules — the gap the mounting system needs between frames, commonly around 20 mm on a rail-and-clamp kit, but always the figure in the mounting instructions.
• Spacing around the perimeter — the clear band at every roof edge, plus any extra corridors you elect to leave for access or fire service.

Then:

Total kWp = (output of each panel in kW) × (number of panels that will fit).

If fourteen 430 W modules will not pack, you do not have 6.02 kWp. You have whatever smaller integer actually fits. That is the number you take into stringing, inverter sizing, and the customer's quote.

Orientation is a first-class input

A 1722 mm × 1134 mm module is not a square. Portrait (long side up the slope, 1134 mm along the eaves) and landscape (long side along the eaves, 1134 mm up the slope) produce different counts on the same rectangle. You must run both layouts unless the mounting kit, the roof furniture, or the string plan forces one orientation. Mixing orientations on a tiny garage roof to chase one extra module is poor practice: clamp types, rail spans, and drainage paths are designed for a consistent layout.

Convert millimetres to metres before you add gaps: 1.722 m by 1.134 m. Never add a 20 mm gap to a millimetre dimension and then treat the sum as metres.

Perimeter spacing: MCS 5.9.7 as capacity practice

On a domestic pitched roof, MCS installation practice (MIS 3002, clause 5.9.7) is that solar PV modules should not be mounted within 400 mm of any edge unless specific measures are taken to:

• resist the increased wind-uplift forces in the edge zone, through additional fixings and, where necessary, additional roof timbers for those fixings
• keep ridge tiles secure
• keep rainwater run-off patterns from being wrecked
• keep the roof covering able to do its job at the verge and eaves

That 400 mm band is MCS installation practice that a 2922 candidate should know sits with capacity, because it removes a strip from every side of the rectangle before you count modules. It is not a City & Guilds exam-fee fact, and this guide does not claim the 2922 handbook prints 400 mm as an assessment charge or as a numbered handbook constant. If a customer or a salesperson wants modules hard against the bargeboard "to get the kWp up," the capacity conversation and the wind-uplift conversation are the same conversation.

If you do enter the edge zone, you have not found free kilowatts. You have accepted extra structural and weatherproofing work: more fixings, possibly extra timber, a check that ridge and verge details remain secure, and evidence that the kit is still within its wind design. Until that work is specified, do not count the edge strip as available area.

On the worked rectangle below, 400 mm off every side turns 8.0 m × 4.0 m into 7.2 m × 3.2 m. That single subtraction is usually the difference between a brochure yield and a layout that will clamp.

Gaps between modules

A 20 mm gap between frames is typical for many rail systems so clamps can land, glass can expand, and water can shed. The gap belongs between modules, not as a hidden extra 20 mm around the outside of the array — the outside is already the perimeter rule. For n modules in a row, you have (n − 1) gaps.

Along a usable length L:

n × module side + (n − 1) × gap ≤ L

Solve for the largest integer n. Then multiply the two directions. That product is the module count. Only then multiply by watts.

Worked example: 430 W modules on an 8.0 m × 4.0 m roof

Given: 1722 mm × 1134 mm 430 W modules; roof plane 8.0 m × 4.0 m; 20 mm gaps; 400 mm domestic edge all round.

Usable rectangle: (8.0 − 0.40 − 0.40) by (4.0 − 0.40 − 0.40) = 7.2 m × 3.2 m.

Trap 1 — divide the gross area. Module area = 1.722 × 1.134 = 1.953 m². Gross roof = 32.0 m². 32.0 / 1.953 ≈ 16.4, so a careless quote writes 16 modules × 430 W = 6.88 kWp. That count ignores edges, gaps, and the fact that leftover millimetres in both directions do not combine into extra whole modules.

Trap 2 — divide the usable area. 7.2 × 3.2 = 23.04 m². 23.04 / 1.953 ≈ 11.8, so the next careless quote writes 11 modules = 4.73 kWp. Still wrong. Area does not pack rectangles.

Portrait (1134 mm along the 7.2 m eaves direction; 1722 mm up the 3.2 m slope):

• Along 7.2 m: n × 1.134 + (n − 1) × 0.020 ≤ 7.2 → n × 1.154 ≤ 7.22 → n ≤ 6.25 → 6. Check: 6 × 1.134 + 5 × 0.020 = 6.904 m ≤ 7.2 m. Seven need 8.058 m, which exceeds even the original 8.0 m eaves.
• Up 3.2 m: m × 1.722 + (m − 1) × 0.020 ≤ 3.2 → m × 1.742 ≤ 3.22 → m ≤ 1.85 → 1. Two modules need 3.464 m.
• Count = 6 × 1 = 6 modules = 2.58 kWp.

Landscape (1722 mm along the eaves; 1134 mm up the slope):

• Along 7.2 m: n × 1.722 + (n − 1) × 0.020 ≤ 7.2 → n × 1.742 ≤ 7.22 → n ≤ 4.14 → 4. Check: 4 × 1.722 + 3 × 0.020 = 6.948 m ≤ 7.2 m.
• Up 3.2 m: m × 1.134 + (m − 1) × 0.020 ≤ 3.2 → m × 1.154 ≤ 3.22 → m ≤ 2.79 → 2. Three need 3.442 m.
• Count = 4 × 2 = 8 modules = 3.44 kWp.

Landscape wins on this roof. The honest maximum for this module on this rectangle, with 20 mm gaps and a 400 mm edge, is 3.44 kWp, not 6.88 kWp and not 4.73 kWp.

Even if someone ignores the 400 mm edge and packs the full 8.0 m × 4.0 m with 20 mm gaps, landscape gives 4 × 3 = 12 modules = 5.16 kWp and portrait gives 6 × 2 = 12 modules = 5.16 kWp. Still short of 6 kWp. The customer's kilowatt target is not a physical layout.

What reduces the count

Fire access corridors on larger arrays

A garage rectangle with a 400 mm edge already leaves a slender perimeter. On larger arrays — farm roofs, commercial sheds, long terraces of modules — MCS installation practice is that module layout should allow access for maintenance and emergency services. The larger the installation, the more care is required. Industry practice, including MCS notes on layout, is to consider:

• an access walkway around the perimeter of the array
• breaking a large carpet into smaller blocks with access corridors between them
• permanent protection over fragile roof elements such as roof lights, so nobody steps through plastic while reaching a string
• whether permanent access ladders or other planned access are needed at all

Those corridors are not a substitute for the domestic 400 mm wind-edge practice, and they are not a copy of overseas fire-code 36-inch rules. They are a layout reduction: every corridor is roof you do not cover. On a small domestic garage they may add nothing beyond the edge band. On a warehouse they can remove whole rows. Count them before you promise kWp.

Do not tell a fire officer that "the 400 mm MCS edge is your ventilation strip" on a 200-module roof. State what you actually left: perimeter walkway, block size, corridor width, and how a crew reaches the covering.

Scenario: customer wants 6 kWp on a small garage

The customer has an 8.0 m × 4.0 m garage roof and a quote in their head for 6 kWp. At 430 W, 6 kWp needs 6 000 / 430 ≈ 14 modules (14 × 430 W = 6.02 kWp).

Walk the conversation in this order:

1. Fourteen will not pack. With 20 mm gaps and a 400 mm edge, landscape holds eight modules. Portrait holds six. Even filling the whole 8.0 m × 4.0 m with 20 mm gaps and no edge band holds twelve. Fourteen is not on this roof.
2. Raising wattage in the same frame (for example a 500 W module of similar 1722 mm × 1134 mm outline) still only multiplies the modules that fit. Eight × 500 W is 4.00 kWp. Still not 6 kWp.
3. Entering the 400 mm edge without extra wind-uplift measures is not a free upgrade. If extra fixings, timber, ridge security, and run-off checks are designed and evidenced, you still have to re-run the linear pack; you do not jump from eight modules to fourteen.
4. Honest options: use the house roof; a second plane; a ground mount or carport with its own structure, planning, and connection assessment; or reset the kWp target to the 3.44 kWp landscape pack (or the real pack after you survey chimneys and roof lights).

The 2922 skill is to protect the customer from a kWp number that cannot be bolted down. Area division is how that number gets into the quote. Linear packing is how it gets taken out.