On average, what percentage of heart disease can an electrocardiogram detect on its own?

I don't think ..: .... a specific percentage is known, but a resting electrocardiogram is often normal even when some form heart disease is present. It very occasionally reveals that a person has had a heart attack, but didn't know it. It detects a heart attack in progress. It can detect heart rhythm problems some of which may be associated with a heart disease. Other tests (stress tests and echo) are often needed.

ECG: The ECG is neither highly sensitive nor highly specific overall. It is the important test in identifying rhythm disorders, very helpful in ischemic and hypertrophic heart disease and less so in some others. There is such a large spectrum of diseases that I don't think an accurate estimation can be put forward.