When converting volt-amps to watts, the volt-amps equal watts if you're using DC or in-phase AC power. However, Powerstream Technologies notes this is only for DC and AC power that's in phase. AC power that's out of phase, or that has currents that don't match exactly, needs to take something called a power factor into account. This is what GeneratorGuide.net calls "the ratio of real to apparent power," or the ratio of watts to volt-amps, which are not equal when using out-of-phase AC power.
Use the same value for watts as you have for volt-amps if you're using DC power or AC power that's in phase. So, 5,000 VA would be 5,000 watts.
Find the power factor if using AC not in phase. If you can't find the power factor listed on the equipment you're using, use a default factor of 0.6. You might see this expressed as a per cent.
Multiply the volt-amps by the power factor to get the number of watts when using out-of-phase AC power. In this case, if using 0.6 for the power factor, 5,000 VA would equal 3,000 watts.