Should I carry extra batteries in the field or bring a solar charging unit for a 30 day trek in Nepal?

If space is not a critical issue. You should probably bring a few extra batteries as well as a solar charging unit. That way, if you ever run out of battery in your camera while on the field, you can swap batteries and charge the empty one on the go.

If having a solar charger, you should note the limitations of law of energy conservation.

Sunshine at best produces 1000 watts / square meter. However, best single junction solar panels are around 20% efficient so you get out 200 watts / square meter. Add 95% efficient charger (you need DC-DC converter somewhere, you can't just plug a raw solar cell into a raw battery) you get 190 watt-hours of charge per square meter per hour.

A 20 cm x 20 cm square solar panel thus provides 7.6 watt-hours of charge per hour. A Canon LP-E6NH battery is 15.762 watt-hours. To charge it at 95% battery charging efficiency (which is a different thing than DC-DC converter efficiency) you need 16.592 watt-hours of energy. Your 20 cm x 20 cm solar panel produces it in 2 hours 11 minutes. So, not even 2 hours is enough to charge the battery!

If you think a smaller solar cell let's say 15 cm x 15 cm is enough, it needs 3 hours 53 minutes of charging.

To make matters worse, this requires the solar cell placed at an optimal angle towards the sun. Most likely, the sun is at 45 degree angle on the average and your cell at 90 degree angle (you lay it flat on the ground) so the charging time is multiplied by 1/sin(45*pi/180) or the square root of 2. So your 15 cm x 15 cm panel now charges the battery in about 5 and half hours. That's nearly half of the average sunlight hours of 12 hours (of course some seasons have more and some seasons have less sunlight hours).

Is that fast enough for you? Most likely you won't want to spend half of your sunlight hours staying standstill charging one camera battery!

Also note that as is well-known among photographers, clouds can reduce the exposure by 2 stops. This means energy is one fourth of what it is without clouds. So the 5 and half hours in cloudy weather is 22 hours. The trouble is, there's no 22 sunlight hours in a single day.

So, if selecting a solar charger, please do make sure the panel is large enough that the charging time is acceptable. I would say 20 cm x 20 cm is minimum, and even then you can only reasonably use it during sunny hours unless you consider it acceptable a charge takes the whole day. Clouds? No way.

Because you need to consider that there can easily be 5 consecutive cloudy days, you most probably need not just the solar charger but also few spare batteries.

Depends on the camera. If a DSLR, you can get around 1000 shots per charge. If a mirrorless or some other camera that eats batteries, then 500 shots per charge, perhaps less. I can get by with two battery packs (one in camera, one spare) for 4 weeks in the bush (I don't shoot video). If shooting video, that really drains batteries

I would take extra batteries. I have found when traveling in remote locations electronics break and cables are easily lost and damaged. You could take a solar charger with at least 2 camera batteries and a charger battery pack. The charger battery pack is what you charge on the solar panel. Use the charger battery pack to charge your camera batteries at night.

Using a broader concept of cameras, carrying film and a camera that can operate without batteries is also an option.

Small rangefinder cameras with excellent fixed lenses are widely available in the used market for less than the price of a few on brand rechargeable batteries.

It’s not an option for everyone of course. And film will need hand inspection at airports, but otherwise it might make logistical sense. Less fiddling about in camp at the end of a long day. And less to fail.

My vote is for a solar panel. Assuming you're going during tourist season you'll have bright beautiful days with lots of sun. Sure it will take all day to charge your phone though.

Additionally I would highly recommend bringing a light socket to electrical plug adapter (like this: https://www.amazon.com/GE-Polarized-Adapter-Install-54276/dp/B002DN6QX2). Many tea houses have electricity, but don't have outlets in the guest rooms. They do however have light sockets which you can turn into an outlet to charge your devices. This worked great for me last time I was there and helped to save a few bucks rather than paying to have my phone charged in a common area and risking someone walking away with it.

I trekked for 14 days in nepal. I also bought a solar charger and extra batteries. Buy the highest wattage solar charger you can find and bring like 4 extra batteries. You can get away with charging 2 batteries at the same time.

Also note many tea houses along trekking routes have access to electrical outlets. You will need an adapter though. Below is a picture of my setup/trip:

https://www.facebook.com/media/set/?set=a.10106203663824786&type=3

https://www.facebook.com/photo?fbid=10106203664787856&set=a.10106203663824786