How Many Watts Does a CPAP Machine Use?

Understanding the difference between watts and watt-hours is fundamental to planning any off-grid or backup power system. Watts measure instantaneous power draw - how much electricity an appliance uses at any given moment. Watt-hours measure energy consumption over time - the total electricity used during a period of operation. For example, a 100-watt light bulb running for 3 hours consumes 300 watt-hours of energy. This distinction matters because your battery capacity is measured in watt-hours (or amp-hours), while your inverter and generator must handle peak wattage. Confusing these two measurements leads to undersized systems that cannot handle your actual loads.

Many appliances have two distinct power ratings: running watts and surge (or starting) watts. Running watts represent the continuous power needed during normal operation, while surge watts describe the brief spike required when the appliance first turns on. Motors are the primary culprits here - refrigerators, air conditioners, pumps, and power tools can draw 2-7 times their running wattage during startup. This surge typically lasts only a fraction of a second, but your power source must handle it or risk tripping breakers or damaging equipment. When sizing inverters and generators, always account for the highest potential surge you might encounter, especially if multiple motor-driven appliances could start simultaneously.

Duty cycle dramatically affects actual energy consumption and is often overlooked in system planning. Few appliances run continuously - a refrigerator might have a 200-watt compressor, but that compressor only runs about 30-40% of the time as it cycles on and off to maintain temperature. This means actual consumption is closer to 60-80 watts averaged over time. Space heaters with thermostats, well pumps, and HVAC systems all exhibit similar cycling behavior. Understanding duty cycle helps you more accurately estimate daily energy needs and avoid oversizing your solar and battery system. However, be conservative in your estimates - duty cycles increase during extreme temperatures or heavy usage periods.

Measuring real-world power usage provides the most accurate data for system design. Manufacturer specifications represent ideal conditions and often understate actual consumption. A simple plug-in power meter (like a Kill-A-Watt) costs under $30 and can measure both instantaneous watts and accumulated watt-hours over time. Monitoring each appliance for 24-48 hours reveals true consumption patterns, including startup surges and duty cycles. For critical loads like medical equipment or refrigeration, this data ensures your backup system will actually meet your needs. The small investment in measurement often saves hundreds or thousands of dollars by preventing both under-sizing (system failure) and over-sizing (unnecessary expense).