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, Mike Bishop (me) took over core OnGrid Tool development — and continued that work thru 2017. He 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, Mike got ownership of OnGrid assets thru seller financing. With big enthusiasm, he threw the long pass... going hard for a few years on a rebuilt-from-the-ground-up interval-data-based OnGrid Tool. He 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 (now he's building a new model (at least for now: just for his own PV-install company)).
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 / Mike... hoping the remaining users would get continued access. Mike took him up on this.
Andy's farewell to users is here.
Mike's 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. Mike digs how fast it recalculates results... right on the computer (not on some far-away server). He digs how navigable it all is.
Mike decided to make the OnGrid Tool a free-and-open-source resource for the PV industry. To simplify, he removed the companion OnGrid Sky data-sharing web app. To make it a little more relevant, he 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.
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.