guarledes

I write and maintain everything on Home Energy Math. I am a software developer, not a solar installer, not a broker and not a financial adviser, and I have no commercial relationship with anyone in the industry this site covers.

Why I built this

The 30% federal residential solar credit under IRC §25D expired for systems placed in service after 31 December 2025. Months later, most calculators online were still applying it. That is not a small mistake: on a typical purchase it is several thousand dollars of savings that no longer exist, and it is enough to make a losing investment look like a winning one. I could not find a tool that had it right and showed its working, so I wrote one.

How I work

• Primary sources only. Generation comes from NREL PVWatts, electricity prices and per-utility figures from EIA filings, incentives from the administering agency rather than a summary of it. Every page names its source and the date it was checked.
• The model is testable. The financial code takes numbers in and gives numbers out with no network calls in the middle, so every published figure is reproducible in a unit test rather than asserted.
• I publish the uncomfortable answer. Where the numbers say a purchase does not pay back, the page says so. Sites funded by installer referrals cannot do that; this one has no lead form and no phone field anywhere in the codebase.
• I say what I do not know. Data with weak provenance is withheld rather than published with a caveat in small print.

What is here

A model covering cash, loan, lease and PPA on 2026 rules; 50 state pages; 507 individual utility pages built from EIA Form 2024 filings; a monthly tracker of what electricity actually costs in every state; and the price per kWh that third-party solar providers report to the federal government, which is not published anywhere else.

Corrections and contact

If a number here is wrong I would rather hear it than leave it up. Email [email protected].

Elsewhere: github.com/guarledes