Electric hot water tank performance, how to check?

We had a new 80 gallon electric water heater installed to replace our 50 gallon propane tank. The setting was initially around 120, but the water came out of the tap at 100, so over a few days we've turned it up to 140 and now get water at around 115 out of the tap.
I'd like to verify that this is really operating correctly and giving us the specified 78 gallons of hot water during the first hour....because I'm afraid it isn't. We won't often need 80 gallons of 120 degree water, but I'd like to know this unit can provide what I paid for. Can someone explain how to do that?


Put a 5gal bucket in the tub and run the water for 1 minute to determine the GPM flow rate. (Should be about 4gpm), then run the water and time how long it takes before the water is getting cold. Multiply the time by the gpm rate.