I thought you might like to see what I put together for a backpack.
12 NiMH AA batteries = 14.4 Volts. A 5W solar panel rated 0.28 A at 17.9 V to charge the battery pack. Three sets of Anderson power pole connectors connected with wire nuts.
My V3 uBITX consumes less than 2W on receive. Using this batteries-only setup, I get about 5W SSB output on 40M with the same antenna I use to get around 15W with my 13.8V, 25A power supply.
I assume current is being limited by the relatively high resistances of the AA batteries and the small wires on the battery holders.
I expect to get more RF output with the battery pack plus one of the Sun-lit solar panels but I have not tried that yet.
I have not done any testing on battery life (with or without the solar panel attached).
Everything came bargain-priced from eBay. The 5W panel was less than $10. As a Matter of fact, I have also purchased a couple of 10W solar panels for less than $13 each from eBay.
I have not tried to measure but concur with the others who have estimated transmit power consumption at around 25W.
I have been a Ham for over 50 years. It has been a long time since I have had more fun than I have been having with my little uBITX. Good SSB signal reports from all over the US, the Caribbean and South America.