Have you thought about a battery pack/power bar for recharging over the day, independent from sunshine? Used this during trekking in the Himalaya, and I could easily recharge a G13 battery for 5 times without using up more than 60% of the battery pack capacity (Anker, 13000mAh). You can also charge you smartphone, and other devicesand recharge the power bar over night. Btw. might the 3 (or 4) batteries not be enough for a whole day?