I use 2 15 watt units on our RV (it keeps the batteries charged on the roads if we arent using a draw (then we use a generator)
My wife has 2 units on her box trailer for her craft shows. She has seberal tools that are 24 v cordless. and the trailer battery is nice and "topped up"
They recc one 15 W solr pnel but two cant hurt for our area of the world. (Sometimes, in New ENgland we wont get full sun for days and we have to keep the generator fired up (The gennies now have a computer that senses when you are running low and it kicks on or off -like if we use the nuke and have the tv and some appliances on)