A single 400 watt solar panel would make your off-grid solar system more compact. However, there are not very many single 400 watt solar panels being manufactured yet. That is not to say that there are none. I found one that is manufactured by LG. However, I am yet to locate online sources for them. So if your search for the best 400 watt solar panel is for a single panel, then come back soon. Or reach out to LG.
Now that you are here may I suggest that you check out some single 300 watt solar panels and single 200 watt solar panels that you can combine to form your off-grid solar sytem.
In this post, I will provide a list of 400 watt solar panel kits.