Why two quotations for the same roof give different generation figures

The point of MIS 3002 prescribing a single estimation method is that it removes optimism from the comparison. Capacity multiplied by a tabulated yield multiplied by a shade factor is not a figure anyone can improve by wanting to.

So when two quotations for the same roof differ by twenty per cent, the difference is in the inputs, and there are only five of them.

1. The capacity is not the same

This is the obvious difference, and the easiest to miss when comparing totals instead of per-kWp figures. Twelve 440 W panels are 5.28 kWp; twelve 400 W panels are 4.8. Panel size alone explains nine of those twenty points.

Compare kWh per kWp, not kWh. If those agree, the systems are being estimated identically and only the sizes differ.

2. One assumed no shading

This is the commonest cause by a distance. A shade factor of 1.00 needs no site visit and no assessment. A shade factor of 0.88 needs somebody on a ladder with a sunpath chart.

MIS 3002 allows the assessment to be skipped where there is an obvious clear horizon, and on some roofs there genuinely is. On most British roofs there is a chimney, a neighbouring house, or a tree. If one quotation carries a shade factor of less than 1.00 and the other does not, ask the second one what it assessed.

3. The orientation or pitch was recorded differently

The surveyor should measure both and round them a particular way: orientation to the nearest five degrees from due south, pitch to the nearest degree.

Getting the orientation twenty degrees wrong on a south-facing roof costs almost nothing, because the curve is flat near the top. Getting it wrong on a roof that is already south-west facing costs more. And the difference between a stated pitch of 30 and a measured pitch of 45 on an east-facing roof is worth several per cent.

Both should be on the quotation. If they are not, they were probably estimated from the pavement.

4. The postcode zone

Rare, but it happens, and it is worth checking on a boundary. Some postcode areas are split by district: BD is one zone but BD23 and BD24 are another; GU is one zone but GU30 to GU35 are another. An installer working from the area letters alone will get those wrong.

5. It did not come from the method

The remaining possibility is that a figure came from somewhere else: proprietary software, a manufacturer's tool, or a rule of thumb about how many kilowatt-hours a kilowatt makes.

Proprietary software is not forbidden. MIS 3002 allows an alternative shading methodology where it can be shown to be equivalent to or better than MGD 005. But the estimate a contractor certified by the Microgeneration Certification Scheme (MCS) gives you should be the MCS estimate. A figure from a tool that also credits better panels or a better inverter cannot be compared with it, because the MCS method applies the same 0.8 loss factor to every system in the country.

Checking for yourself

All five inputs are things you can establish. The capacity is on the quotation. The zone comes from your postcode. Pitch and orientation are properties of your roof. The shade factor is the only judgement, and even that has a published procedure behind it.

Put the same five numbers through the same table and the answer should match, within rounding. Where it does not, the gap is a question worth asking, and the answer usually explains more about the quotation than the figure did.