Calculating required power to run 2 modems in kWh
I assume that you are trying to connect your equipment to a uninterruptible power supply (UPS) of some kind.

For your purposes, real power (watts) is given by P [watts] = V [volts] × I [amps] ÷ efficiency [%].

You may take the efficiency of the average wall power adapter to be 90%. (The other 10% is wasted as heat.)

As AndrejaKo mentions, you then multiply the power consumption per second (watts) by the desired running time on battery supply (hours) to obtain a figure in kilowatt hours (kWh).

Battery stored energy E [kWh] = P [watts] × desired run time [hours] ÷ 3,600 sec/hour ÷ 1,000 W/kW.

Note that the volts and amps written on the plug pack represent the maximum possible power draw of the device. The "idle" power usage is usually substantially less. You can measure this with a plug-in power meter (avail for $30 in stores.)