Your winter sun
"Sun hours" means hours of strong, overhead-strength sun. Size solar for your darkest month, not the yearly average, and winter won't catch you out.
Where will your panels be? Pick whichever's easiest.
Massachusetts: about 2.7 sun hours in December
That's the darkest month, for panels tilted toward the sun at the same angle as your latitude. Lying flat on a van or bus roof: about 1.5. The brightest month gets about 5.3.
In December, a 400-watt panel there makes about 0.8 kWh a day. For a tiny house using 7 kWh a day, that's about 9 panels.
Source: NASA POWER long-term averages at Massachusetts's center of population. Every state, every month
The whole country, in its darkest month
The yellower the square, the more winter sun. The Northwest and the Great Lakes get about a third of what the desert Southwest does, so the same home needs far more panels there.
Go deeper: why the darkest month
- Summer sun can be double winter sun. Size for the average and your batteries run dry in December.
- Northern and cloudy places (the Northwest, the Great Lakes, New England) have the darkest winters.
- These numbers are planning estimates for your region. Your exact roof or site can differ with shade and tilt.