Notes

How these tools work, and why their answers are often less encouraging than the ones you will find elsewhere. Each piece is meant to be worth reading whether or not you ever open the tool it belongs to.

There is a reason these exist rather than a page of feature bullets. A calculator asks you to trust a number, and the only honest basis for that trust is being able to check the working. So the working is written down: the formula, the assumptions behind it, where each figure came from, and what the model cannot see.

Two of these pieces document occasions when the tools here were wrong. The solar model assumed electricity prices would rise 3.5% a year when the EIA's own data says 2.9%, and it charged nothing at all for upkeep, as most solar calculators still do. Both errors flattered solar, both were corrected, and both are described in full rather than quietly fixed. A site arguing that other people's tools are optimistic has no standing to hide its own corrections.

None of this is financial advice and none of it is written by a licensed adviser. It is arithmetic, its sources, and its limits - the parts you could check yourself if you had the afternoon.

If you only read one, read the first. The incentive it describes - that a free tool paid by lead generation is paid more when the answer is encouraging - explains most of what is wrong with the category, and it gives you four questions you can put to any calculator in about a minute. That is more useful than trusting this site instead of another one.

Corrections are welcome and get acted on. If a figure here is wrong, or a source has moved on since it was checked, saying so is the most useful message this site can receive.