QRP is primarily an abbreviation from the telegrafie with commercial radio services.
It means "Reducing you the transmitting power!" or as question formulated "Should I reduce the transmitting power?".
The term QRP is used by the hams, for thereby to refer to the special mode of its radio station.
It is the transmit mode with low power.
The question about the QRP output power seems to be a difficult question. If you ask this question to an OM or a YL with a 750 W station, then he or she will answer you (certain) that 100 W are already QRP. That I already experienced. If you ask however the owner of one of the many 100 W stations, then he will perhaps answer you that are so for instance 10 W guarantees QRP. Everything which has fewer power, is for her to transmits uselessly. You can use it at the most still as signal generators. In order to bring light in the darkness and quarrels avoid, the clubs and QRP federations agreed on following power outputs: 5 W output for CW, 10 W output PEP for telephony. These values are however no lower limit. It goes also with still fewer power. Power up to 1 W calls as QRPP. There are not few stations, which used up themselves this power level.
Perhaps a small note besides. Those rather "dimy" lamps of a bicycle deliver a power of approximately 2 to 3 W. With approximately this power you can transmit also around the world.
There are many reasons, why someone makes QRP. Perhaps times a few examples:
Many hams live in cities, in which they cannot structure large antennas. They follow therefore to their hobby in free nature. Since a QRP station is quite small in very most cases, this station fits together with a accumulator and something accessories easily into a backpack.
In addition, interferences can occur by the own QRO station in the closer area. Already sometimes around these interferences to eliminate it helps, if you reduces the power.
In addition, not least the question of cost plays an important role. A normal 100 W transceiver costs far more than one QRP station. Like that it was also not possible for me after the radio examination to put many bank notes for a station on the desk. My first station was therefore a QRP station. And I with QRP remained, since I saw that this power was sufficient.
But there are not only negative reasons. So you can become also in the vacation with a QRP station QRV, without carrying forward a half freight vehicle full with equipment. With my past journeys as amateur radio operators a small transceiver and something wire fit always also into the luggage. A mast in form of a fiber glass telescope mast for a vertical you can carry forward in almost all cases without problems also with flights (when fishing rod camouflaged).
Also the curiosity can drive someone to it. Many hams turn the power during a QSOs already times down, in order to see simple, with how much power it still goes. There amazing values come off. Sometimes this hams remains then with the adjusted power level. This is the most pleasing way.
Or the coincidence has its hand in the play. After tuning the antenna you forgot to turn upward the output regulator at the transceiver. Only after the following QSO you notices that the automatic controller is still on 5 W.
It is possible to achieve with QRP each point of the earth. Believe it calmly! I "cracked" pile ups at 15 m in ZL from DL already with 5 W and an antenna on the balcony. Many more is also not possible. It requires something other operation mode and some technical demands for the used station, but you almost always comes to the target. This was however no individual case, which is to due to particularly good propagation conditions, proves the DXCC results achieved so far by me.
Everyone can make QRP operation. Many amateur radio operators shrink from themselves to build their own transceiver. But no fear. There are also finished QRP devices and kits for it. The knowledge that you acquired for the amateur radio examination, is in most cases sufficient for the building. If you gets yourself still something addition knowledge from QRP books, technical periodicals or of friends in the club, can go actually nothing more wrong.