Like all appliances, different air purifiers use different amounts of energy for operation. Unlike most appliances, air purifiers run continuously, so you may want to consider your utility bill before buying an air purifier. (If only volts and amps are listed, simply multiply the two: volts x amps = watts.) Typical HEPA air purifiers can use anywhere from 50 watts on low to 200 watts on high. For comparison sake, a typical lamp uses about 60 watts, while a typical computer uses about 365 watts. Therefore, while its wise to consider energy usage, most air purifiers will not create a significant difference on your electric bill.
