PVWatts Calculator

[Hamish]

Hi,

FYI I've been playing around with the global NREL PVWatts Calculator,
for December it got within better than 5% of my actual generation,
with me just making educated guesses for the inputs. Of particular
interest may be the weather (high/low cloud) datasets, which seem to
be nicely documented. Met data granularity seems to be on the order of
several km.

https://pvwatts.nrel.gov/


Help > About begins:

"""
About PVWatts

The PVWatts® Calculator estimates the electricity generation of a
grid-connected photovoltaic (PV) system using hourly solar resource
data for locations throughout the world. It allows homeowners, small
building owners, installers, and manufacturers to develop preliminary
production estimates of potential PV installations.

PVWatts is also available as part of NREL's desktop application System
Advisor Model (SAM), where it can be used with SAM's financial models.
For software developers, it is available as a web API on the NREL
Developer Network, or as part of the SAM Simulation Core (SSC)
Software Development Kit, which includes the PySAM Python package.

Current Version: 8.5.2
Release Date: September 25, 2025
PVWatts API Version: 8.0

Information for Developers: The current version of PVWatts is based on
the PVWatts V8 API, which runs Version 275 of the SAM Simulation Core
(SSC) pvwattsv8 compute module with weather data from the PSM V3 TMY
tmy-2020 and PSM V3 Himawari TMY tmy-2020 datasets from the National
Solar Radiation Database (NSRDB).
[...]
"""

e.g. weather data link, a nice page describing the API, it's worth a quick look:
https://developer.nrel.gov/docs/solar/nsrdb/himawari7-download/


have fun,
Hamish

[Cameron Shorter]

Hey Hamish,

Thanks for sharing. I've had a look, and adding to your comments:

Pros:

* Results from PVWatts for my house looked to be within ~ 5% to 10% (eyeballing the last few months), so it looks to be about right.

* I think that both OpenSolar and SunSpot have used PVWatts for prior inspiration (not sure to what level).

* Seems PVWatts has very mature and trusted energy calculation models if you pass in: lat/long, roof tilt, asmith, roof size.

* PVWatts has access to global weather datasets, which could be a good starting point, but not as good as NIWA NZ weather datasets.

* Code and datasets are open source, and has a public API, so we could adopt their algorithms.

* Apparently has an active community we could reach out to.

Cons: (Our opportunity)

* PVWatts doesn't calculate shading, roof tilt, asmith, building footprints, which we are doing with GRASS.

I've added a writeup to a list of related solar tools

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