There are several extra factors to think about when calculate the system lifetime comparing the battery’s rated capacity with the systems energy consumption. In real systems, it is important to look at how power decay over time. In Figure 2.2, the voltage of an AA battery, depending of time, rapidly falls from the 1.5V to just 0.8V when it is drained at a 500mW. Theoretically, a system requiring 2.7 V and consuming 250mW, it would last for 100 minutes with 2 AA batteries. On the other hand, if the system operates at a voltage of only 2.0V, it would last approximately 5 times longer.