Getting energy from power and time.

[magni-mar]

When trying to calculate energy from multiplying power and time, as in: energy = power * time.

I get the following error:

error: no match for ‘operator*’ (operand types are ‘Watt’ {aka ‘units::quantity<units::isq::si::dim_power, units::isq::si::watt, double>’} and ‘std::chrono::seconds’ {aka ‘std::chrono::duration<long int>’})
   13 |     Joule motor{motor_power_at_melting * mix_time};
      |                 ~~~~~~~~~~~~~~~~~~~~~~ ^ ~~~~~~~~
      |                 |                        |
      |                 |                        std::chrono::seconds {aka std::chrono::duration<long int>}
      |                 Watt {aka units::quantity<units::isq::si::dim_power, units::isq::si::watt, double>}

for the following code:

#include <iostream>
#include <chrono>
#include <units/isq/si/energy.h>
#include <units/isq/si/power.h>

using Watt = units::aliases::isq::si::power::W<double>;
using std::chrono::seconds;
using Joule = units::aliases::isq::si::energy::J<double>;

auto main() -> int {
    Watt motor_power_at_melting{1000};
    seconds mix_time{10};
    Joule motor{motor_power_at_melting * mix_time};
}

How can I multiply power with time without accessing the raw numbers?

[mpusz]

You can't directly divide mp_units::quantity by std::chrono::seconds. Those are different libraries and the division of such variables will not produce power (in fact it is an invalid operation by default).

In order to make it work you have to convert std::chrono::seconds to a proper quantity type in mp-units. I notice you are still on the V1 of the library but here is the solution in the V2: https://godbolt.org/z/5qTsfq7MY. Similar stuff can be easily achieved in the previous versions as well.

[magni-mar]

I missed the type that covers seconds in mp units.