And we'll see if it really pays off for him. So far, unless you're on an island and have to ship in your diesel fuel, solar doesn't make economic sense without massive subsidies. (I'm pretty sure even a billionaire like Musk would blanch at backing solar without FITs, RECs, PPAs, etc.)
I work in solar, and it's a technology I'd really like to see succeed, but we're quite some ways away, and in a few important ways, we're slipping backwards - It takes at *least* 20-25 years to make back your investment. But current solar cells (even good ones) will begin to rapidly degrade at that time -(to about 80% output, falling off a cliff to single digits within around another 5 years. So even if everything goes your way, you've got only about five years of positive and rapidly decreasing power production before you have to replace the whole thing and start over.
The race to cheap Chinese panels now has panels lasting fewer than 10 years before delaminating and coming apart (leaching toxic heavy metals in the process...) - if that happens to even a few percent of the panels there's no way you can *ever* break even. Add in big outstanding questions about the lifespan of other expensive components such as inverters and wiring, and it's a good bet that only the most attentive operators of solar plants will ever make thier money back. (On the DC wiring issue, the prevalent PV industry practice of grounding the negative leg effectively *designs* for galvanic corrosion of the wiring, resulting in little more than hollow straws in a few years if things get a little damp - 300-1200 VDC *will* do that!)
Lastly, you don't get much power out of solar on an areal basis - a good figure for perfect siting, etc. puts the max power per panel/year at only a few dozen dollars worth of electricity. (Heck, there's less than 1000 W/m^2 there to start with on a clear day and a LOT less than that if there are *any* clouds, and after conversion and transmission losses, you're down to only a little over 10% of that.)
Solar is starting to make sense in limited cases, but it will be probably at least another decade or two before putting solar panels everywhere makes economic sense - especially in very distributed environments like residential rooftops, where no one is really going to be monitoring or maintaining the system. that's one advantage of Musk's approach - he tends to be focused more on larger sites that he can make sure are performing (or at least not sucking too bad...)