To start, Dwayne Reid has an excellent point, an 18650 with 4900 mAh capacity is incredible, and at $1.30 per battery it is truly an amazing deal. If it is legitimate, then there are a lot of people who would like to know where you are getting these 18650 batteries. So there is the very real possibility that the company selling these batteries is over estimating the capacity and if this is the case, then the safety of these batteries will be suspect as well - be careful!
Having said that, your question is still a good one.
When using the 3.7 volt nominal 18650 cells, typically 3 are used in series to get a nominal 11.1 volts and 12.6 volts fully charged. There are some solar charge controllers that use 4 - 18650 for a nominal 14.8 and a fully charged 16.8 volts. Which one you use will depend on your solar charge controller and then your inverter (if you use one), and the voltage that your appliances run on if you don't use an inverter.
Balancing issues: Over time, with repeated charges and discharges, it is likely that one or more of the 3 cells in series that make up the 12 volts (the 3 cells in series might be 3 'blocks' of 100 - 18650 batteries first conneced in parallel then wired in series) will become unbalanced. This means that one or more of the cells are of different voltages during the charge and discharge. This usually happens because of different internal resistance between the different cells. The charger will charge the battery to 12.6 volts, but one cell might be 4.2, one cell 4.1, and one cell is 4.3. The total voltage is normal, but the individual voltages are unbalanced and this can be harmful to the individual cells and even dangerous, especially to the overcharged cell. A balance charger will monitor the individual voltages of each cell while charging and ensure that at the end of the charge, all voltages are the same.
Calculations: To be accurate, you should use the nominal voltage which for the 18650 is 3.7 volts per cell or 11.1 volts for the 3 cell 12 volt battery. Also, assuming that you are using 100 of the 18650 cells and they are 4900 mAh.
4.9 Ah x 100 = 490 Ah per 100 - 3.7 volt 'block'.
3.7 x 3 cells = 11.1 volts. 11.1 volts x 490 Ah = 5439 Wh.
Hope this helps!
Please don't forget, 100 - 18650 cells in parallel is a lot of energy and you could certainly cause a lot of damage if it short circuited.