How many Watt-hours can an electric eel produce in a day?
This research states that an eel whose size is 90 cm can provide 6.05 mW, which is 145 mWh every day. They say you'd need 165 eels to be able to read for 24 hours using a 1W lightbulb like the one on a phone.
An electric eel should be able to flash a 0.6W L.E.D. light for 10 milliseconds at full power every second, which would be very visible in daylight, and would light up the room.