i work in the battery industry, i get why 18650s are popular, but if your going to run solar, i suggest looking into golf cart batteries. there lead acid, deep cycle, and heavy as hell, but your not going to be moving them around (preferably) so weight is moot (imo)
they are 6 volt, but run them in series and your talking way more amp hours of run time than you would have with li-ion.
li-ion also has the problem of requiring special charging circuits, standard chargers will either over charge or undercharge, and balancing the packs can be a nightmare. but with lead acid, a standard 12v batt float charger or solar charger is plug and play.
if you have boxes of tested and rated 18650s, and the ability to weld them then by all means, use them. PLEASE dont solder li-ion batteries, not only does the void every warranty that i know of, you can overheat the cell and have a runaway cell (ie fire)
i have 2 military tank batteries the nearby military base was scrapping so i reconditioned the batteries and took them home. dude, i run my tv, laptop, modem, stereo and room lights for 2 days on my setup should i lose power (often during the winter)
if you have any questions about batteries, charging, and set ups ask and ill be happy to help any way i can.