I would look into an LCD display system.
That alone could drop your power requirements so low that a set of good AA battery's could power it for a few years or use a simple solar panel too.
Or use a micro amp power source taken from the voltage dividers used for the voltage sensing circuits could easily power it as well.
Over all your design would be competing against a large market of self powered industrial metering devices presently available.
So far self powered LCD based voltmeters, amp meters, frequency meters, temperature gages, pressure gages, flow meters, timers, hour meters, and power meters are already available and competitively priced as well.