Cal and joule are units of energy. Force times distance. Force, of course, is mass times acceleration.
Energy = F x d = m x a x d = kg m^2 / s^2
Watt and hp are measures of power. How much energy gets used per unit time. How quickly energy gets used.
Power = energy / s = kg m^2 / s^3
Now you are concerned with kWh. Kilowatthours. Kilo is just a prefix. In unit analysis, two words next to each other (or separated by a hyphen like kilowatt-hour) are multiplied together. This is a unit of power multiplied by a unit of time. This is just energy again!
kWh is what the electric company sells you each month on the electric bill. They sell you energy. Turn on a 60W lightbulb for an hour, or a 120W light bulb for half an hour, it is all the same to them. Your electric company could sell you electricity in joules.
When my friends and I had trouble with this concept, we joked that you could measure power in "kilowatthours per hour", and that seemed to help some of us.
