The rated watts of an appliance usually state the max. The appliance usually will consume a bit less over the long haul. While conversing in watts seems simple enough, reality involves both voltage and current and as the DC voltage drops the DC amps will rise. This is why the above helpers use voltage and current.
If your appliance uses a wall wart and its rated voltage is in the 12VDC range, I would consider eliminating the inverter. You will get more efficiency & reliability. If your appliance is life support related (CPAP,etc.) a 140 watt inverter is way too close to max for my comfort.
