What a good solar quote looks like

What the prescribed estimate contains

Four sections. We describe them rather than reproducing the table, which is MCS’s.

A. Installation data — installed capacity in kWp at standard test conditions, orientation in degrees from south, inclination in degrees from horizontal, and the postcode region.

B. Performance calculations — the kWh/kWp figure (Kk) taken from the appropriate table, the shade factor (SF), and the estimated annual output, calculated as kWp × Kk × SF.

C. Estimated self-consumption, PV only — the assumed occupancy archetype (home all day / home half day / out all day), the assumed annual electricity consumption, the assumed annual generation, the expected self-consumption in kWh, and grid independence as a percentage.

D. The same again with battery storage, where one is being installed — assumed usable capacity, expected self-consumption, and grid independence. Omitted where there is no battery, and both C and D may be omitted where the occupancy archetype is unknown, such as on a new build.

The warning that must come with it

MIS 3002 requires the estimate to be accompanied by a specified note, which begins:

Important Note: The performance of solar PV systems is impossible to predict with certainty due to the variability in the amount of solar radiation (sunlight) from location to location and from year to year. This estimate is based upon the standard MCS procedure is given as guidance only for the first year of generation. It should not be considered as a guarantee of performance.

and continues, on self-consumption, that “there are a number of external factors that can have a significant effect on the amount of energy that is self-consumed so this figure should not be considered as a guarantee”.

Two additional notes are required in particular circumstances, and both are worth looking for:

• where the shade factor is below 1.00, a statement about the accuracy of the shading method used
• where the site was evaluated remotely, a note that estimated values were used for orientation, inclination or shading and that “actual performance may be significantly lower or higher”

That last one tells you whether anybody actually looked at your roof.

The rest of the pack

Section 4.2.1 requires this to be communicated in writing at or before the point the contract is awarded:

• the performance estimate
• manufacturer’s datasheet for the modules
• manufacturer’s datasheet for the inverter
• datasheet for any module-level power electronics
• datasheet for any battery
• a drawing with the proposed module layout
• the sunpath diagram used to calculate the shade factor, where SF is below 1.00 and the MCS method was used
• any other requirements stipulated by RECC, if applicable

The module layout drawing and the sunpath diagram are the two most often missing, and they are the two that show whether the design is specific to your roof.

Reading the numbers

kWp is capacity. kWh is what you get. If those are confusing, start with kWp vs kWh.

Check the implied Kk. Divide the estimated annual output by the kWp, then divide again by the shade factor if one is applied. Compare the result against our dataset for that location, pitch and orientation. At 35° facing south the UK range is 692 to 1,042 kWh per kWp; anything implying more than about 1,050 is above everything we model.

Check the self-consumption figure against MGD 003. For a 2,500–2,999 kWh household with a 3,600–3,899 kWh array, MGD 003 gives 23% home all day, 19% in half the day, 14% out all day. If a quote assumes far more without a battery, ask which table it came from. Our self-consumption page sets this out — including that our own calculators were wrong about it until we read the guidance note.

Check the occupancy archetype matches your life. It is an input, and it is the one most likely to have been chosen optimistically.

Watch for money figures with no rates attached. A saving needs an import rate and an export rate. Both should be stated, both should be current, and the export rate should be one you can actually get — see our SEG table.

Red flags

• a single annual saving figure with no assumptions
• no shade factor on a visibly shaded roof
• no orientation or pitch stated
• self-consumption well above MGD 003 with no battery
• an export rate you cannot find in the market
• the pack promised “after you sign”
• a remote assessment presented as a survey
• payback quoted without the cost assumptions that produce it

None of these is proof of anything. All of them are questions worth asking, and a good installer will have answers.

If the installer is not MCS certified

None of this is required of them, because MIS 3002 binds MCS contractors.

That is a real difference and worth weighing. You can still ask for the same information — orientation, pitch, Kk, shade factor, self-consumption assumptions — and a competent non-certified installer should be able to produce it. But you have no standard to point at, and you will not be able to claim SEG or most grants. See what MCS is.