I'm trying to make an Arduino controlled irrigation timer. My idea is to use an Arduino pro mini and a cheap solenoid valve, wired to a TIP120.
However, from my calculations, it's impossible to run it on batteries for long. From what I've seen they draw a considerable amount of power. (the one I'm using is similar to this one https://www.adafruit.com/product/997). If i power it with 9V, it draws 240mA, which is 2.16W. And if i power it for 30 minutes, it draws 1 watt-hour. That is way too much to use with batteries.
How do battery powered controllers work then? (like this one http://www.ebay.com/itm/Home-Water-Timer-Garden-Irrigation-Timer-Controller-Set-Water-Programs-/231932984347?hash=item3600487c1b:g:HAcAAOSw9ZdXKbTA)