# Is the net power of an open system always equal to zero?

Are the following statements, line of reasoning all correct? I'm particularly interested in the last bullet.

• For a closed system energy is conserved and constant.

• For an open system energy is not necessarily conserved but is
constant. In other words if we account for the work done on the
system, the work done by the system (energy losses and gains due to
work) and the energy stored in the system, then the net energy is
constant.

• The power is the time rate of change of energy.

• Therefore the net power (over any interval of time) for an open system is equal to zero.

Any caveats?

• Would it be better to say "isolated" instead of "closed" system? In thermodynamic lingo, closed usually just means no mass transfer; but doesn't restrict energy. – JMac Aug 21 '18 at 19:12
• what is the difference between conserved and constant? – Adam Aug 21 '18 at 20:13
• If by net power you mean mechanical power (turning a turbine for example) then it is not necessarily zero for an open system. – Deep Aug 22 '18 at 6:25