SolarNexus integrates NREL's PVWatts (pvwatts.nrel.gov) for PV system production estimates. You can verify SolarNexus production estimates with PVWatts output, but you must match all of the assumptions (i.e. weather station, STC rating and module orientation per array, all losses, inverter efficiencies).

About the Input Parameters

The PV system output modeling parameters are accessible on the Services screen via a link just below the Arrays. The default for these values is set by an Administrator on the Project Settings screen.

The following items describe how particular SolarNexus system definition parameters map to PVWatts inputs:

Installation Type

SolarNexus automatically submits the correct type of installation for each array as follows:

• If install type = "parallel to roof" or "ballasted tilt", then PV Watts array type = "fixed (roof mount)"
• If install type = "tilt-up roof", or "ground mount", or "pole mount", then PV Watts array type = "fixed (open rack)"

Module Type

PVWatts allows you to specify from 3 options: Thin Film, Standard, or Premium. These essentially correspond to module efficiencies. SolarNexus automatically selects based on the product specifications data in the Public Catalog, as follows:

• If module cell type = "Thin Film", OR efficiency < 14%, then module_type=2 (Thin Film)
• Else if module efficiency > 17.5%, OR temp_coeff_pmax is between -0.394 and -0.279, then module_type=1 (Premium)
• Else module_type=0 (Standard)

Automatic Loss Adjustments for Micro-Inverters

When your system has micro-inverters specified in the equipment, SolarNexus auto adjusts system losses as follows:

• Mismatch derate automatically adjusts to 0%
• DC wire losses automatically adjusts to 0%
• System availability automatically increases by 0.3%. So a 0.5% default loss in availability would auto-adjust to 0.2%. The micro-inverter's distributed architecture eliminates the single point of failure of a central inverter, therefore a failure only takes out a single module rather than a whole system.
• Applies a "Shade Mitigation Factor" (SMF) to shade losses. See more about shade mitigation factor below.

Automatic Loss Adjustments for DC Optimizers

• Mismatch loss automatically adjusts to 0%
• DC optimizer efficiency is included with PV Watts "Inverter efficiency." SolarNexus inputs product of the efficiencies. For example, inverter efficiency = 98% and optimizer efficiency = 98.8%, resulting efficiency passed to PV Watts = 0.98*0.988 = 0.968, or 96.8% efficiency.
• Applies "Shade Mitigation Factor" (SMF) to shade losses. See more about shade mitigation factor below.

Shade Mitigation Factor

Both micro-inverters and DC optimizers have some benefit in reducing the effects of shade upon system production. This effect is known as a "shade mitigation factor" or SMF. NREL has conducted comparison field tests on systems using no MLPE, systems using DC optimizers, and systems using micro-inverters. They have found a range of SMF values from about 0.25 to 0.4, and recommend a value of 0.33 as a default.

SolarNexus auto-populates the PVWatts default value of 0.33 when your account is created. For example, if the annual "solar access", or the unshaded amount is 90%, then this is how the SMF is applied. NREL calls the solar access value 'R', so in our example, R = 90%. We apply the SMF to get a revised shade derate. The revised derate = R + (1 - R) * SMF. So in our example where solar access = 90%, 0.9 + (1 - 0.9) * 0.33 = 0.933. Solar access would increase from 90% to 93.3%.

Setting a Default SMF for Your Account

You can set your own default value for the SMF that is automatically used in any given PV production estimate. Go to Administration > Project Settings > PV Output Parameters.

You should set your account's default SMF based on the type of module level power electronics you most frequently use. We recommend using the values cited by Enphase and SolarEdge in their technical bulletins on this subject (see links below).

NOTE that if you design on roofs that are heavily shaded, you may need to adjust these values for individual arrays as described below.

SMF Limits - Heavily Shaded Arrays

The ability of module level electronics to mitigate shade has limits. NREL's tests only prove mitigation being effective up to about 25% of shading. Its simply impossible to retain sunlight that just doesn't exist.

As shown in the screenshot below, SolarNexus will flag any arrays that are heavily shaded (have solar access estimates of 75% or less).

If you have any heavily shaded arrays in your design, SolarNexus STRONGLY recommends lowering the shade mitigation factor used on that array. You can use the following guideline: