If you plug in your electrical heater in for 15.0 minutes and then in the process used 16,000 j how much power did you use

         Power  =  (energy)  /  (time)

BUT ... it has to be joules and seconds.

       (15 minutes) x (60 sec/min) = 900 seconds

             Power  =  (energy)  /  (time)

                       =  (16,000 J)  /  (900 seconds)

                       =     (16,000 / 900)        joule/sec

                       =     (17 and 7/9) watts.

That's not much for a 'heater'.

A typical SMALL heater pours out like 1200 watts or 1500 watts.

A company that sells heaters that only put out 17 watts
won't be able to sell many of them, because they're 
pretty useless as heaters.

Maybe they could be used as aquarium heaters, or as
hamster-cage heaters.

Try b? sorry if i’m incorrect