Introducing SunSolve P90: A Free Solar Yield Forecast Uncertainty API

We are excited to share SunSolve P90, our contribution to the growing effort around better uncertainty quantification in Energy Yield Assessments (EYAs). This free Python API for calculating solar yield forecast uncertainty builds on years of research and community discussions about improving confidence in our forecasts.

Building on a Decade of Physics-Based Simulation

SunSolve P90 draws on more than 10 years of work with the solar industry developing physics-based simulation tools. Our foundation in SunSolve Power for solar cell and module design, and SunSolve Yield for utility-scale energy modeling, has given us the experience to bring a pragmatic and diligent approach to uncertainty analysis.

This new tool wouldn’t exist without extensive consultation with the industry. We ran a roundtable discussion at PVPMC 2024, surveyed practitioners at ACP Resource and Tech, and have been learning from colleagues working on similar challenges. Their insights about what uncertainty quantification needs to accomplish in practice have been fundamental to shaping this work.

The Challenge We are All Facing

An Energy Yield Assessment (EYA) is one of the most important documents in a solar project, yet uncertainty quantification remains challenging for our industry.

Banks and other investors don’t just want to hear “this project will generate 100 GWh per year.” They need to understand the uncertainty around that estimate. What confidence intervals can you provide? What’s the P50 to P90 ratio? Without proper quantification, you can’t demonstrate the value of improvements or trade-offs in project design.

While there has been valuable research and some existing tools addressing uncertainty quantification, many EYAs still rely heavily on single point estimates or simplified approaches for P90. This leaves stakeholders to make multi-million dollar decisions without complete information about forecast confidence—a challenge we are all working to address.

A New Foundation for Confidence

With support from ARENA, we have developed SunSolve P90, what we believe is the next critical step toward improving investor confidence in EYAs: a tool that makes it practical to account for the complexity of uncertainty in solar forecasting.

Our approach tackles three fundamental challenges that current methods struggle with:

1. Capturing all sources of uncertainty – from weather variability and degradation to availability and operational factors.
2. Accounting for asymmetrical sources of uncertainty - Simple gaussian distributions fail to represent the true shape of input distributions such as availability, curtailment and diffuse irradiance fraction.
3. Combining uncertainties correctly – accounting for their codependencies rather than treating them as independent.

Monte Carlo: A Proven Approach Comes to Solar

The finance industry has been using Monte Carlo simulation to quantify uncertainty since the 1960s because the concept is both powerful and elegantly simple.

Instead of generating a single forecast and then trying to estimate its uncertainty, we generate thousands of forecasts — each representing a plausible combination of weather patterns, degradation rates, availability scenarios, and other variables. The result is a distribution that naturally captures the complex interactions between different uncertainty sources.

While running tens of thousands of simulations for an EYA would normally be very time consuming. Our new approach can handle this complexity within minutes.

An Invitation to Collaborate

We believe rigorous uncertainty analysis should be accessible to everyone in the solar industry — which is why we are releasing this as a free tool. But more importantly, we see this as an opportunity to work together as an industry.

We are eager to collaborate with researchers, tool developers, and practitioners already working on uncertainty quantification. If you are developing approaches to similar problems, we wouldd love to explore how SunSolve P90 might support or integrate with your work.

There are several practical ways you can collaborate with us:

• Using the tool on real projects and sharing your experiences — what worked, what didn’t, and what could be improved;
• Contributing to a shared library of default uncertainty distributions for factors like availability, degradation, soiling, and more;
• Helping us refine the approach based on what works in practice across different project types and regions.

We would also welcome the industry’s input on establishing open, reproducible standards for how we report uncertainty in energy yield assessments. By sharing methodologies, validating approaches against real project data, and building common frameworks together, we can create a more transparent and trustworthy foundation for project financing — one that benefits everyone.

Meet us at ACP Peak

We will be presenting our work at ACP Peak 2025 and would love to hear about your experiences with uncertainty quantification. Whether you’re already working on similar tools, looking to get started, or interested in collaborative approaches, we are eager to connect and learn from each other.

Can’t make it to the conference? We will be posting detailed getting-started guides and documentation for SunSolve P90 on this website. Reach out to us if you’d like to be notified when the API launches and new resources become available.

The path to better forecasts starts with honest uncertainty quantification. We are excited to share this tool with the community and to work together on building the trust that our industry needs to thrive.

Acknowledgments

Thanks to the PVPMC for supporting the original panel discussion that helped guide this project, and to everyone who participated in our industry survey at ACP Resource and Tech 2024. We are also grateful for the encouragement and feedback from colleagues across industry and academia—too many to name individually — whose insights have been invaluable in shaping this work.

This work was supported by funding from the Australian Renewable Energy Agency (ARENA). The views expressed herein are not necessarily the views of the Australian Government, and the Australian Government does not accept responsibility for any information or advice contained herein.