Ways to maximize battery life
If you use more or bigger batteries, then you can increase the battery life.

The important thing here is the energy of the batteries. Your device uses 5W of power, which means that it uses 5Wh of energy every hour of operation.

If you use a battery that has 10Wh of energy capacity, then in theory, you device would work for two hours (in practice a bit less due to various inefficiencies).

A DC-DC converter can help here, that is, it can help you use more batteries in series to have more total energy. You can also connect the device (using a Dc-DC converter) to a car battery (~840Wh) and have it work for ~168 hours.

The DC-DC converter changes the voltage and current, but laves power constant, so, it can take 12V/!A and produce 1V/12A. In practice, due to inefficiencies, the converter will always take more power than it outputs, the difference being used to heat up the converter itself.

However, you have to use a switching DC-DC converter, and not a linear regulator such as a 7805. The linear regulator takes as much current from the power source as the load needs (it would use 12V 1A (12W) to produce 5V 1A (5W) dissipating the rest (7W)), therefore it dissipates much more power as heat.

A switching converter is usually 70% to 95% efficient.