We extracted 386 W average power from a moving slab glass laser with a pumped area of 300 cm2. The total efficiency was 3.1 %. The beam was linearly polarized and had constant divergencies of 3 mrad in the small dimension (6 mm) and 13 mrad in the large dimension (20 mm), independent of the average input power. The repetition rate was 10 pps. A new method to measure the thermal load of the laser glass is presented. We found that 6.6 % of the electrical input power is dissipated as heat in the glass slab, which is of the same magnitude as the storage efficiency.