The objective of performance modeling is to predict the expected output of a solar system given 1) the system design, and 2) the environment in which it is operating, including the solar resource. To evaluate a performance model requires:
- simultaneously measuring the system output and the system environment, including the solar resource
- modeling system output, using the measured system environment data along with the system design
- comparing modeled to measured output on an annual, monthly, daily, hourly and/or sub-hourly basis as a function of various parameters.
The level of detail required for the environmental data, the system description and the measured system data varies depending on the objectives of the validation effort and the detail contained in the model to be evaluated. In all cases, however, the environmental and measured system data must be:
- easily retrievable
- consistently time stamped (to allow correlation of environmental and system data)
- free of flaws and inconsistency (scrubbed)
| Design Data Requirements (increasing) | Model to be validated (increasing complexity) |
| Location, orientation, tracking, STC rating | System Performance Factor (SPF) |
| Module type (γ), inverter efficiency, soiling estimate, shading estimate, dc and ac loss estimate | PVWatts |
| Mounting | CSI EPBB calculator |
| Modules/string, string/array, inverter number and type, shading details, row spacing | System Advisor Model (SAM) |
| Wiring lengths, module orientation, wiring configuration, monthly soiling, ac losses | PVsyst |
| Environmental Data Options | Impact |
| TMY | Compare to expected SPF, subject to interannual variability |
| Satellite | Subscription fee, no local sensors, not well validated, likely adequate for monthly comparison |
| Reference Module* | Validate PV performance models without any other environmental sensors, but not irradiance or module temperature models |
| Plane of array*and weather sensors | Validate PV performance, module temp models |
| Global horizontal irradiance* and weather sensors | Adds uncertainty in translation models |
| GHI*, DNI* and weather sensors | Full comparison to TMY dataset, High Cost |
*regular cleaning desirable