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Questions about a calculation, a correction to a formula, or a suggestion for a calculator that is missing — all of it is welcome.

Before you write

If the question is about a specific result, it helps to include the numbers you entered and the figure you expected. Most reported errors turn out to be a difference in assumptions — a temperature derating, a depth-of-discharge limit, a different regional convention for cable sizing — and those are much easier to check with the inputs in front of us.

What we cannot do

We do not design systems, quote for installations or recommend specific brands. For anything that will be wired into a building, the right person is a qualified installer working to your local regulations.

Suggesting a calculator

The list grows from what people actually ask for. If you have found yourself doing the same solar or energy sum by hand more than twice, it probably belongs here. Tell us the inputs you start from and the answer you need, and it will go on the list.

Meanwhile, the existing tools are grouped by job on the home page, and the blog covers the wider background.

Reporting a problem with a calculator

Every tool on this site does its arithmetic in the browser, which means a wrong answer is either a wrong formula or a wrong assumption — and both are fixable. The useful report contains three things: which calculator, what you entered, and what figure you were expecting instead. With those, a discrepancy usually resolves in minutes.

Differences between countries account for a large share of reports. Cable sizing conventions, the standard safety factors applied to string protection, and the assumed depth of discharge for a given battery chemistry all vary by market. Where a calculator follows one convention, the page says which one.

Questions the calculators cannot answer

Some things need a site visit rather than a formula. Shading is the obvious one: two roofs with identical orientation and identical arrays can differ by a third in annual yield because of a single tree. Roof structure, existing wiring, meter arrangements and grid connection rules are all in the same category.

For those questions the right answer is a local installer who can stand on the roof. What the calculators do is tell you whether the conversation is worth having, and roughly what size of system to discuss.