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How to configure GHI and DNI weights within Solcast when extracting TMY data

To get a TGY (Typical GHI Year - where the long-term average GHI matches the TMY GHI), request a Solcast TMY with ghi_weight=1.0, dni_weight=0.0.

If you need further details, the TMY is constructed so that the weighted average:

AVERAGE(ghi_weight*GHI + dni_weight*DNI)

is equal between the TMY and the long-term Historical Timeseries average.

You can also configure the weights yourself:

• TGY (Typical GHI Year): ghi_weight=1.0, dni_weight=0.0
• TDY (Typical DNI Year): ghi_weight=0.0, dni_weight=0.0

Or you can customise the weights yourself. Generally, systems that track the sun (dual-axis solar PV, single-axis solar PV, concentrated solar power) are more sensitive to DNI (direct normal irradiance); whereas fixed-tilt solar PV systems are more sensitive to GHI (global horizontal irradiance).

Why would I customise the GHI:DNI ratio?

There are a couple of main reasons we see users want to vary the ratio of GHI and DNI weights:

• To match an existing workflow e.g. you prefer to use Typical GHI Years (TGY, 100% GHI weight) for all projects
• To better align with the relative importance of direct and global irradiance on power yield e.g. single axis tracking vs fixed tilt systems have decreasing importance of direct irradiance.

If these reasons don't apply to you or you are unsure what ratio to use we have recommendations for fixed and tracking systems noted below.

GHI:DNI Weight Recommendations:

By default, Solcast recommends

• for fixed-tilt systems: ghi_weight=0.8, dni_weight=0.2
• for tracking systems: ghi_weight=0.2, dni_weight=0.8

These weights are due to the differing relative importance of direct and global irradiance on PV power yield