The easiest way to accurately measure up to 26v is to use the INA219 voltage & current sensor chips.
There is a library for this chip already available for the Photon & Electron.
You can make adjustments to the code and resistor to measure more current if needed.
Measuring AC power is not as easy from what I have seen so far but you should check out this project if your interest in doing that since there is a Particle version of it on this forum also.
