Powering standalone atmega328P with battery and solar

I'm trying to plan out a weather station project and had a question about the best method for powering the Arduino if you're not using the full board (So I'm not using its built in linear regulator).

If I had an instance where everything was using peak current draw and factored in some wiggle room, I shouldn't be using more than 400mA for everything. (During hot summer days there's a fan to keep air moving and keep wasps from nesting)

I'm leaning towards using a 6v sealed lead battery and solar charger. I considered Lithium Ion but I've been reading that it can get fussy about being charged when its too cold or too hot, so sealed lead seemed like the better way to go.

So my question boils down to this. Would using a 6v battery with a fixed 5v Low Drop Out regulator be the way to go for powering my equipment, or do you think I would ultimately get better effeciency if I used a 12v battery with a fixed 5v switching regulator?

I'm concerned that if I use a 12v battery with a linear regulator I'm going to have way too much heat to dissipate (2.8 Watts) even with a heatsink and an active air flow radiation shield, on the other hand with a 6v battery I'm looking at (6-5v)*.4A = .04Watts using something like the tl750m05 I should be able run things nicely even without a heatsink (though there will be a heatsink :slight_smile: ).

Am I on the right track here? Or am I chasing after a couple mA of savings needlessly and just use the full board with a 9 or 12v seal lead battery?

If the linear regulator needs <=1V + Vout, then 6V for 5v output is fine. (83% efficient)
Switching regulators can be 90-95% efficient. But then you need higher output solar panels, or a way to boost voltage to charge the higher output battery.
Take a look at some of the regulators at www.pololu.com