tl;dr: The OnGrid Tool is now free and open source. Files can be downloaded here.
In the early 2000s, PV systems were fringe and expensive. The PV-system purchase was typically about power security, environmental contribution, and cool tech.
Big rebates were offered in some areas with high electric rates and recently-introduced net metering... For the first time, there was a serious financial argument for the PV-system purchase.
Andy Black — selling PV systems in the Bay Area — recognized this. He wrote a related article for the PV-advocacy organization NorCal Solar (a later version of the article is here). He started teaching day-long PV financial classes at the national solar conferences and thru training organizations.
Off-the-shelf software wasn't available* to model and present this financial-investment opportunity.
Andy built out a spreadsheet to help him do this (successfully). In 2006, he made the spreadsheet — called the OnGrid Tool — available to others (thru a paid subscription).
The spreadsheet locally replicated the industry-standard PVWatts online PV production calculator. Andy took the financial modeling seriously. Example: For commercial systems, the financial timeline included the $ value lost from the reduced utility-electricity-expense tax deduction.
By 2007, several hundred PV installation companies had paid subscriptions. ...All around the country. The top-five national installer Sungevity integrated it into their backend. OnGrid pulled utility-tariff-book and incentive-program data together for users (and kept it current).
In 2007, I (Mike Bishop) took over core OnGrid Tool development — and continued that work thru 2017. I took it from a "polished turd" (Andy's term) to pretty-darn-professional software (as far as internet-connected spreadsheets go).
Jessica Yip joined, to handle the deeply-tedious work of tracking utility rate plans and incentive programs. Evan Nicoles joined to support users with crisp thinking and next-level emotional maturity. A few others made much-appreciated technical contributions.
By the mid 2010s, there were over a dozen web-based PV-system financial-software options available. Including Energy Toolbase, that offered interval-data based PV and battery modeling.
In 2015, I got ownership of OnGrid assets thru seller financing. With big stupid enthusiasm, I threw the long pass... going hard for a few years on a rebuilt-from-the-ground-up interval-data-based OnGrid Tool. I didn't make the right decisions and didn't manage cash flow well (and, to be fair, a big OnGrid revenue source — Sungevity — went bankrupt). OnGrid assets went back to Andy, and the new version never saw sunlight.
From 2018 thru 2025, Andy supported the remaining die-hard OnGrid Tool users. During this time, no significant improvements were made.
In late 2025, Andy offered the OnGrid assets to Slow Power / me... hoping the remaining users would get continued access. I took him up on this.
Andy's farewell to users is here.
My follow-up email to active users is here.
While the OnGrid Tool isn't a platform to build on in the relatively-complex year-2026 paradigm... by golly, it's still pretty neat. And still useful for simple preliminary modeling. I like how fast it recalculates results... right on the computer (not on some far-away server). I like how navigable it all is.
I decided to make the OnGrid Tool a free-and-open-source resource for the PV industry. To simplify, I removed the companion OnGrid Sky data-sharing web app. To make it a little more relevant, I added basic inputs and modeling for markets without net metering (specifically: indicate if no net metering >> input the % of summer and winter PV production that serves the site directly, and input the $ export rate). ...Basic battery modeling can be done thru these new inputs.
The open unlocked OnGrid Tool spreadsheet and supporting spreadsheets are available here.
If nothing else — especially if you once critically depended on the OnGrid Tool — it might be interesting to roam around the backend.
If the free open-source OnGrid Tool helps you, or if it doesn't but you'd like to support my energy work... thanks so much...
Here's my Give page.
I'll outro with the OnGrid logo as of 2015, made with love by Mike's buddy (before AI was a thing):
* The one other PV-system financial modeling software made available around the same time... was from Clean Power Research.