Capacitance is the number on the label, but it's rarely the number that predicts failure. Two other measurements — equivalent series resistance (ESR) and leakage current — catch the degradation modes that capacitance alone misses, and production lines that skip them let weak parts through.
Capacitance — The Baseline Check
Capacitance is typically measured with a small AC test signal at a standard frequency — 120Hz is the long-established reference frequency for electrolytic capacitors, with 1kHz or 100kHz used for film and ceramic types where the datasheet specifies it. This confirms the part is broadly the value it claims to be, but a capacitor can measure correct capacitance and still be failing by the other two metrics below.
ESR — The Number That Predicts Heat and Ripple Performance
Equivalent series resistance describes how much the capacitor behaves like a resistor in series with itself when charging and discharging — essentially the internal losses that turn into heat under ripple current. ESR is typically measured at a higher frequency than capacitance (100kHz is a common reference point for electrolytics) with Kelvin (four-terminal) leads, since ESR values are often in the low milliohms and lead resistance alone would swamp the reading with a standard two-terminal connection.
Rising ESR is one of the earliest and most reliable signs of electrolyte drying or degradation in an aluminium electrolytic capacitor — often visible in an ESR trend long before capacitance itself drops out of specification. In a power supply, elevated ESR shows up as increased ripple voltage and heat at the exact capacitor generating that heat, a feedback loop that accelerates failure once it starts.
Leakage Current — What It Reveals About Dielectric Health
Leakage current testing applies the capacitor's rated DC voltage and measures the small residual current that continues to flow after the capacitor has charged — normally measured after the current has settled, commonly after 30 seconds to a few minutes depending on the capacitor's size and the test standard being followed. High leakage current indicates a compromised dielectric — pinholes, contamination, or breakdown in the oxide layer that forms the capacitor's insulating barrier — and is a direct predictor of the part failing catastrophically in service rather than degrading gracefully.
Why All Three Together
A capacitor can pass any one of these tests individually and still be defective by the others: correct capacitance with high ESR indicates a part that will run hot and fail early under ripple load; correct capacitance and ESR with high leakage indicates dielectric damage that risks a hard failure. Production-line capacitor testers that check all three in a single pass — rather than capacitance alone — catch defect modes that a capacitance-only incoming inspection simply cannot see.