We can't have people coming and changing batteries or charging LiPo's each day! So the devices need to be solar charged! Here's what I'm working on now:

SOLAR PANEL

What is the output power of the solar panel we need?

First we need to determine the total energy usage of the device in 24hr period (Watts). We can put this into the following equation as E. Whilst the output power of solar panel is P(solar). Let's supply the total time solar panel is in direct sun as 10hrs

P(solar) = E / (10*60*60)

BATTERY

Now how long do we need the battery to be able to deliver energy without solar charging? Obviously this is all kinda rough. We can go ahead and use a solar insulation map to determine more exact number of hours solar panel is in direct sun at a given time in year. So let's say anyway, we want the battery to deliver 48hrs of energy without any charging:

C(battery) = E / V*60*60

Where V = voltage we need, and C(battery) = storage capacity in Ah, and E = energy usage of device.

WIRES

To connect the solar panel to the charge controller, and the charge controller to the battery, and the charge controller to the IR illumination device, and (not shown) to the elephant deter devices, I'll use 18 AWG Gauge Electrical Wire.  

SUMMARY

I'm yet to calculate E with any accuracy.  So I can't say what output power solar panel I need. I've actually got a 30 watt solar panel at the moment which cost £40. It would be great to find I only need a lower cost 5W! As per battery, we need 12v for the IR illumination. We'll step down from that for the raspberry pi. I'm thinking of 7Ah 12v at the moment. I'm a bit worried about the cost of charge controllers really. The whole idea of these is to protect against reverse current flow from battery to solar panel (e.g. a diode), and to prevent overcharging the battery (that seems a whole lot more complicated than protecting against reverse current flow!).

N.B. I'm aware overcharge is a big issue! Especially if you've got these batteries in a forest and it is dry. We certainly don't want a fire! So off top of head re charge controllers: these will have their own micro-controller to monitor reverse current flow, temperature. Protection is going to vary according to the battery chemistry for over-charge. Like a guess you can use a regulator to prevent overcharging?

Update: 

Well, as regarding charge controllers! I've found this one: 

which actually seems to have a step-down to 5V! So I'll have a look at that since it's only £16 "

HQST 10 Amp 10A PWM Smart Solar Charge Controller 12V/24V Solar Panel Battery Regulator with LCD Display USB Port "