8.3 Estimating Annual Yield

8.3 Estimating annual yield

Quick Answer: Handbook v1.3 learning outcome 4.1.2 asks how you estimate yearly generation: with performance-calculation software or tools, and with MCS requirements where they apply. For an MCS-style estimate, MIS 3002 Appendix B is blunt: annual alternating-current output (kWh/year) = kWp × Kk × SF. kWp is the sum of the STC nameplate figures printed on the module labels. Kk is kilowatt-hours per kilowatt-peak from the postcode-zone tables (Climate-SAF-PVGIS data × 0.8). SF is the shading factor from section 8.2. A teaching example with realistic made-up table values is 4.3 kWp, Kk = 850 kWh/kWp, SF = 0.94 → 4.3 × 850 × 0.94 = 3435.7 kWh/year. Proprietary software may add a second figure, but it must not outrank the MCS estimate and needs caution wording if it is much higher.

Independent OpenExamPrep teaching uses the published Appendix B method and the handbook 4.1.2 information list. OpenExamPrep does not issue MCS certificates and does not replace the live tables on the MCS website.

Two approaches, one exam expectation

4.1.2 is not "pick a nice yearly number." It is "show the approach." On a non-MCS conversation you might use manufacturer software, a commercial design package or a spreadsheet, provided you can describe the irradiance files and the shade method. On a job that needs an MCS performance estimate, Appendix B is the standard method. Extra software is allowed as an additional estimate only if you:

• clearly describe and justify the approach and the factors used,
• give that extra figure no greater prominence than the MCS table result, and
• add the warning text that the extra figure should be treated with caution if it is significantly better than the standard method.

A salesperson who quotes 50% above the MCS product and hides the table result has failed that rule, not "found a better climate."

The Appendix B arithmetic

Work in this order so an assessor can reconstruct the sheet:

1. Sum kWp from every module's STC data-plate watt-peak. Ten 430 W modules are 4.3 kWp, not "about 4 kW" and not the inverter's AC rating.
2. Read postcode zone from the MCS map and zone table (the same family of zones as SAP).
3. Measure pitch from horizontal and round to the nearest 1°.
4. Measure azimuth relative to due south and round to the nearest 5° (south 0°, south-east or south-west 45°, east or west 90°).
5. Look up Kk in that zone's table for that pitch and azimuth. Do not apply another 0.8 by hand; the published Kk already includes Climate-SAF-PVGIS × 0.8.
6. Determine SF (1.00 if obviously clear; otherwise MGD 005 NS method or 1 − estimated fractional loss).
7. Multiply kWp × Kk × SF.

Worked example (teaching numbers, not a live table cell):

• kWp = 4.3
• Kk = 850 kWh/kWp (a realistic made-up look-up for this chapter)
• SF = 0.94 (for example NS = 6 → 1 − 0.06 = 0.94)

First, 4.3 × 850 = 3655 kWh if the site were unshaded at that Kk.

Then 3655 × 0.94 = 3435.7 kWh/year.

Write the product as 3435.7 kWh/year (or the integer your centre's sheet format uses) and keep the three factors visible. If a colleague only writes "about 3,400 kWh," the assessor cannot see whether Kk or SF was dropped.

A 50% higher sales figure on the same system would be 3435.7 × 1.5 ≈ 5154 kWh/year. That gap is exactly the situation Appendix B warning text is for: keep 3435.7 kWh as the MCS estimate, explain the software assumptions, and do not lead the quote with 5154 kWh as if it replaced the table.

Information the handbook expects you to be able to state

Outcome 4.1.2 is as much a documentation outcome as a calculator outcome. Be ready to list:

• Total generation (kWh/year) = kWp × Kk × SF and the three numbers you used.
• Model and manufacturer of the modules and of the inverters (the estimate is for a named kit, not "a typical panel").
• Nominal array power (the kWp you summed).
• Array orientation and pitch (the rounded Appendix B values).
• Site location (address / postcode zone, not "southern England").
• Details of irradiance files if you used software (which dataset, which year range, which transposition model). For a pure MCS table estimate, the irradiance story is the published Kk source: Climate-SAF-PVGIS × 0.8.
• Description of shade methods (MGD 005 NS count, clear-horizon SF = 1.00, or a defended alternative).
• Financial assumptions if the pack includes payback, export tariff, self-consumption fraction, inflation or discount rate. An energy number and a money number are different claims. If you did not do finance, say so; do not let a payback slide inherit an unexplained tariff.

MCS also wants the pre-sale table in a recognisable shape: installed kWp (STC), orientation in degrees from south, inclination from horizontal, postcode region, Kk from the table, SF, and estimated annual output as the product. If the site was only evaluated remotely, Appendix B requires extra wording that orientation, inclination or shading were estimated and that actual performance may be significantly lower or higher.

In practice: A salesperson emails a 3-D render that claims about 5,150 kWh/year for the 4.3 kWp garage array. Your Appendix B sheet, using Kk = 850 kWh/kWp and SF = 0.94, is 3435.7 kWh/year. You do not average 5150 and 3436. You do not delete the MCS line because the render looks expensive. You print 3435.7 kWh as the MCS estimate, attach the software figure as a secondary estimate with the factors named, and include the caution that a much higher figure should be treated with care. If the render used a different pitch, ignored the chimney, or skipped the 0.8 already inside Kk, that is a method problem, not proof that the table is "out of date."

What the product is, and what it is not

The Appendix B product is an estimated annual alternating-current yield for customer information and MCS documentation. It is not a guarantee, not a winter-month forecast, and not a self-consumption figure. Self-consumption, if quoted, is a separate calculation with occupancy and (if storage is involved) a clear statement about electrical-energy-storage certification. Do not let a yearly kWh line quietly become "this is what the bills will fall by."

If two roof planes exist, estimate each plane (its own kWp, pitch, azimuth, SF) and add the kilowatt-hours. One blended heading with one SF is how 4.1.2 documentation fails even when the arithmetic looks tidy.

The exam will try to skip a factor, swap inverter kW for module kWp, or let a glossy software number outrank the table. Slow down: nameplates → zone → pitch → azimuth → Kk → SF → multiply → show the working → keep extra software in its place.