AWblain: Nice to see someone being accurate about the energy picture, there are a few incorrect posts and referenced posts above in which HP/Sec is used
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Horsepower seconds is Hp x Sec OR HP.Sec NOT Hp/sec
Hp is Power, ie energy per time - so Hp times time is an amount of energy.
EXAMPLE: 20000Hp.sec means you can consume energy at the rate of 200Hp for 100seconds.
It's a abstract but useful unit for energy since most people have a feel for how many horse power they need to acheive a mode of flight. If you know the Hp you need and you know how many Hp.sec there are in your HEIGHT, SPEED and RRPM then you know how many Seconds of that Hp delivery each of those 'stores of energy' can provide.
Interesting Factoid:
For every 330lbs you weigh the VSI is calibrated in units equivalent to Horsepower - so if you weigh 3300lbs then 1 unit on the VSI is 10Hp
(assumes VSI calibrated in sensible units ie 100fpm)