In amateur radio, QRP operation means transmitting at reduced power levels. The term QRP derives from the Q code where "QRP?" was used to mean, "shall I lower my power?".
The maximum allowable transmitter ouput power one can use and still be generally recognized as "operating QRP" is five watts. In CW, this corresponds to an input power of five watts, but using single sideband modes it corresponds to ten watts input. QRPers are known to use even less than five watts, sometimes operating with as little as 100 milliwatts or even less. Commercial transceivers designed to operate at or near QRP power levels have been available for many years, but some QRPers prefer to design and build their own equipment, either from kits or from scratch. On the other hand, many of the larger, more powerful commercial transceivers permit the operator to lower their output level to QRP levels.
Most amateur radio operators use between 50 and 120 watts of power. However this isn't always necessary. When it's not, doing so wastes power, increases the likelihood of causing interference to nearby televisions, radios, and telephones and, for United States' amateurs, violates an FCC Part 97 rule which states that one must use, "the minimum power necessary to carry out the desired communications."
Communicating using QRP can be difficult since the QRPer must face the same challenges of radio propagation faced by amateurs using higher power levels, but with the inherent disadvantages associated with having a weaker signal on the receiving end, all other things being equal. QRP aficionados try to make up for this through more efficient antenna systems and enhanced operating skills.
QRP is especially popular with CW operators and those using the newer digital modes. PSK31 is a highly efficient, narrow-band mode that is very suitable to QRP operation. QRPers have their own awards, contests, clubs, and conventions.