The watt (symbol: W) is the SI derived unit of power, equal to one joule of energy per second. A human climbing a flight of stairs is doing work at a rate of about 200 watts. An automobile engine produces mechanical energy at a rate of 25,000 watts (approximately 30 horsepower) while cruising. A typical household incandescent light bulb uses electrical energy at a rate of 40 to 100 watts, while energy-saving compact fluorescent lights, which are beginning to replace incandescent bulbs, typically consume 8 to 20 watts. Definition 1 mathrm = 1 dfrac}} = 1 dfrac cdot mathrm}} = 1 dfrac}}. In electrical terms, it follows that: 1 mathrm = 1 mathrm cdot 1 mathrm. That is, if 1 volt of potential difference is applied to a resistive load, and a current of 1 ampere flows, then 1 watt of power is dissipated.