Thanks for the quick - and complete - reply. I've been doing a lot of reading today and that's the best solution I've read.
I've seen plenty of 12v solar controllers, so I should be able to use one of those for the panel to battery connection, then go with the step down for the ESP8266.
The solenoid I'm using is from Sparkfun and is one of the 'normally closed' models. I've already left the water on twice this year in excess of a full day. $51 fine each time. So this is going to save me money!!
Thanks again for the suggestion.