400 watts might be enough to run a couple of LED lights adequate to light up the barn. That's only 3.3A at 120V with maximum output.

back when I used to be in wind we'd do a site survey for a year or more to determine the viability of a wind farm. Basically we measured the wind in the area for the survey.

A $600 battery pack isn't likely to last the night much less a day or more of insufficient wind.

His heaters ought to have a data plate somewhere showing the draw it needs. Add them up and then you can get a better idea of what you need.

Don't forget, either way you go it might be considered a business expense so you might get a tax write off.