This is sort of an "it depends" question, and you noted that yourself in your text I believe. It will take some Q & A with yourself to come to a conclusion.
First: What is the "field" situation.
a. A literal field?
b. The side of the road?
c. A shelter in place situation with no grid mains?
d. Pure mobile/portable on the move?
Second: Based on "First", one decides what is then appropriate, and whether the situation may be fluid and require "oozing" from one situation to another.
a. A literal field: Photo-voltaic panel(s) with integrated charge controller to an appropriate battery. AGM batteries offer the capacity of a lead acid with less weight and less maintenance and less chance of losing your battery due to a tip over where you might lose the fluid. They also don't emit gasses like wet lead acid (more important for c.).
b. The side of the road: A mobile set-up with a PV charging backup is ideal, because you can run the engine if you really need to, but can choose to recharge the vehicle battery with the PV. Not ideal because the vehicle battery is not deep cycle, and you run the risk of stranding yourself until you get enough sun to let her crank again. Having a second battery, and a deep cycle one either installed with vehicle charging or carried with PV charging is a good move.
c. A shelter in place situation: This allows you to plan this all out in advance. A generator is a perfect companion to the deep cycle battery(ies) and the PV charging. Fuel-sipping, quiet, 2kW Honda generators can run for 8 hours or more on a gallon of gasoline, particularly with a low overall load.
d. Pure mobile/portable: Pretty much b. above, but if constantly changing position, mobile operation within the vehicle and it's battery makes the most sense, just so long as fuel is abundant, thus your "solution" is carrying fuel, vs. carrying batteries, PV's, etc.
This is truly one of those never-ending answers, and I guess there isn't really any sort of truly lightweight power to be had. Battery technologies improve in size, weight and capacity regularly.
If I were to be forced to choose only one technology, it would be PV and AGM (or Lithium Ion) deep cycle batteries. None of the answers are terribly convenient, but as long as you have sunshine, you can use the radio.
Third: Operating requirements/types.
a. Repeater ops that can be done with hand-helds or low power mobiles?
b. Simplex ops with HF?
These are the two major methodologies that Hams have used for decades. Using the least power to accomplish the "mission" is your task in this discussion.
Repeater ops make it easy as they are typically very low power, and if you are using ONE repeater, then the antenna is as (or more) important than the power source (and a gain antenna sort of IS a power source).
Simplex HF ops are more challenging, but one can usually move more information further, per hop, thus using potentially less power. The best of the QRP capable HF mobile/portable rigs like the Yaesus are incredibly flexible when coupled with the right software and perhaps a simple Signalink.
While this consideration doesn't speak directly to a power source, I think it certainly colors the decision making process. You could work all day and half the night with one AGM battery in repeater ops. It might require two or three for HF ops.