Equivalent series resistance describes a method by which the resistance within a circuit, which exists because no electrical component is actually ideal, can be accurately calculated. In aluminum electrolytic capacitors, the ESR values can be high. They can, in fact, be several ohms worth of resistance. In addition to this, aluminum capacitors will increase in ESR over time and this needs to be calculated when engineering devices, as it can change the properties of the circuit. The ESR can also increase with increases in frequency.