Depending on species, age, and health, these electric rays can put out between 8-220volts. While the voltage may not seem to be something to worry about, consider the amperage. They can produce attack and defense currents up to 30amps. Less than 1 amp passing through your heart for 3 seconds can fatally disrupt heart function.
One good thing is that once they've put out a shock it takes time for them to recharge. This makes handling them (fishing or aquarium) easier than other electric species.