The 7-year solar experiment

I put solar on my house in 2019 and couldn't get a straight answer to one question: is this actually paying off? So I tracked every bill for 7 years. Here's what I learned — and why I turned it into a free tool.

Why SunTally exists

The question nobody would answer

At signing, the installer hands you a single, confident payback number. Then you're on your own. The monthly utility bill is no help — it bundles everything into one figure and obscures what solar is actually doing. So I started a spreadsheet: every month, the kWh my panels produced, what I pulled from the grid, what I sent back, and exactly what I was charged for each.

What the numbers actually showed

A few things surprised me:

• The bill never went to zero — even in great solar months — because solar only offsets generation, while delivery and fixed fees roll on. Across the whole dataset, a large share of my energy charges were delivery/fixed, not generation. (More on that here.)
• "Payback" wasn't one number. Depending on what you assume you'd have paid without panels, my system looked either comfortably ahead or still behind. The honest answer was "it depends — here are both framings."
• Seasonality and one-off events (a production gap, a loan payoff) swung individual months hard. You only see the truth by looking at the whole trend, not a snapshot.

The spreadsheet didn't tell me solar was great or terrible. It told me the truth, which is more useful than either sales pitch.

Why I made it a tool

Every solar calculator I found was a sales instrument — built to make the number look good. I wanted the opposite: something that would honestly tell you when a system isn't worth it, that you could run on your own bills, and that didn't harvest your data. So I rebuilt my spreadsheet as SunTally: a free, open-source calculator that runs entirely in your browser. Your bills never leave your device — there's no server to send them to.

It does two things: project payback from a quote (for people considering solar), and build a real dashboard from your actual bills (for people who already have it and want the truth).