ArrayBudget is an independent site that sizes solar arrays, battery banks, DC cable and backup generators.
It exists because solar quotes arrive as a single number with the assumptions stripped out, and the assumptions — sun hours, depth of discharge, round-trip efficiency — are what decide whether the system actually covers your load.
Where the numbers come from
- the NREL PVWatts default derate factor of 0.77
- NEC Chapter 9 Table 8 for conductor resistance and NEC 310.16 for ampacity
- manufacturer depth-of-discharge and round-trip efficiency figures by battery chemistry
Every page states the source and the assumptions behind its figures, because a number without its assumptions is not much use. Where a derate is an industry rule of thumb rather than a measured figure, the page says which it is.
Two derates, not one
Battery sizing needs two derates, not one — depth of discharge and round-trip efficiency compound, and dividing by depth of discharge alone leaves a lead-acid bank about a fifth short.
What this site does not do
We do not tell anyone how to wire a generator into a house. That is a transfer switch, it is an electrician's job, and back-feeding a wall socket can kill a lineman restoring your power.
Who runs it
One person, an engineer by training, doing this outside work hours. No editorial team. What should earn your trust is that every derate and every sun-hour figure names where it comes from, so you can check it against your own quote.
The sizing runs as code over every array, bank and cable combination on the site, so a wrong constant would be wrong across hundreds of pages at once and get spotted. When a figure turns out wrong it is corrected, and the change is noted.
How it is paid for
ArrayBudget is free to read and funded by display advertising. No sponsorships and no affiliate links — no installer or panel brand has any say in what appears here.
Getting in touch
Corrections and questions go to [email protected]. Corrections are more useful than anything else you could send; the editorial policy covers how they are handled.