First, you need to know the watt of your load device, such as it is 500W. Then, AC output: working time = 1500Wh * 0.85 / working power of the device. 1500Wh * 0.85 /500w=2.5 hours (rough calculated) DC output: working time =1500Wh * 0.90 / working power of the device. 1500Wh * 0.9 /500w=2.7 hours(rough calculated)