Sure this would work. It looks like the digispark uses about 38ma in it\'s default state, but most of this is probably the power LED! If you cut that off and use the CLKPR variable to set the clock speed down to 1mhz or so, the power consumption I would think could get as low as 0.1ma. I\'m not sure if the power regulator wastes any power when not in use?
Anyway you should be able to power a digispark off a quite small solar cell with a couple of nimh batteries getting recharged by the solar cell. So long as the batteries keep providing it 4.2-6.0 volts it should be happy! Need to be careful not to give it more than 6.0v though, ideally no more than 5.5v, or go through the regulator otherwise.
For radios I\'d recommend using 433.92mhz OOK transmitters unless you need to send lots of data. They\'re really neat little things and they only use power when transmitting, and only a very very small amount. They\'re the sort used for cheap wireless doorbell buttons and those little home weather station gadgets.
The digispark can also be put to sleep, where it will only use about 0.01 to 0.001ma until either a timer (the watchdog) wakes it up, or you can setup a pin to wake it up when it\'s state changes. This makes it easy to do stuff like have it go to sleep and wake up every ten seconds to take a little analog sample and write it to memory or transmit it or something like that.
For farm, depends how flat the land is how well a transmitter will work. You might find it easier to transmit for instance over the fence - connect one wire to ground (literally, the earth below) and one wire to a conductive metal part of your fence system, then you can transfer data along that wire by completing the circuit through the earth itself. Some telephones used to be installed using this technique out in the australian country!
