The sound power of sources is a key input to room acoustic simulations. However, existing measurement methods are generally unpractical as they require specific equipment, for instance a reverberation room in ISO 3743. A method is proposed to find the sound power spectrum of sources for room acoustic simulations, based on in situ sound pressure level (SPL) measurements at known locations. The measured data is fitted to an ODEON simulation of the measurement space via an optimization algorithm. The method requires an accurate numerical model of the environment. A notable advantage is the possibility of estimating multiple sources at once with the same set of SPL measurements. The proposed method is investigated in a dry room in which up to three sources are active simultaneously. It is found that at least three receivers per source are required to obtain robust estimations, and it is beneficial to place them in the vicinity of the sources. The accuracy of the estimation also depends on the relative level between the sources, as a too strong source will mask a weaker one. Finally, dry environments are more suited to measuring multiple sources, due to larger spatial variations in SPL.