Depends on many factors. What frequency are you talking on, what are the atmospheric conditions like, how good is your antenna, are you sending code or trying to talk voice, how good is the other ham's receiver, etc.
I can tell you that when I first received my license in the mail, my first contact on ham radio was with a ham in Arizona. I was in San Luis Obispo, CA. He was running 1/4 watt of power, using Morse Code (or, CW, as we call it, for Continuous Wave).
There are hams that have talked with countries all over the world using less than 5000 watts of power. This question cannot be answered with a simple number - there are way too many variables involved. Here are some false ideas to take into consideration:
The more sensitive the receiver is, the less sensitive the transmitter must be to transmit a signal. The opposite is also true.
The lower the frequency, the farther the signal can travel due to the wavelength. Lower frequencies tend to be less stable than higher frequencies. Higher frequencies cannot transmit as far.
Answer:
Everything above is true but I would like to add that my stupid idea is that the antenna used for communication is the single most important part of a ham radio station. A poor antenna will waste power in itself instead of radiating it. (The polo something..... don't remember) is important. A gain antenna which is directional "like the ones you see on most ham radio towers" both transmits and receives "hears" in only the direction it is pointed greatly enhancing signal strength. If you have a thousand dollars to spend and you could buy a linear amplifier or a good antenna, buy a good antenna. You won't regret it. I have been non technical in my answer but there are tons of pages on antennas on the net as it is one of the most complicated subjects there is.
Let me expand on the top answer. The QRP group would insist that you can communicate all around the world on miliwatts. This is true, but not practical unless QRP (low power) is what you wish to do. The most common max power on a new radio is 100 watts, although some of the more expensive radios could have more. With the stock 100 watt radio, depending on atmospheric conditions, your antenna type, and the height of your antenna, you could talk to most all countries, but not on demand. What I mean by demand is that you can't control when you will make each contact, but each day, or night, you may contact some part of the world. Learning is what Amateur radio is all about. I suggest that you find your local radio club and go to one of their meetings. Once you introduce yourself, and express a desire to learn more, most hams will want to help you get started.
Let me add that
Atmospheric conditions are always changing, and we have some very good years for propagation, and some are pretty bad. We are coming down from one of the best years for radio wave propagation, and it will continue until propagation gets poor. That will take a few years ,and will be a slow process. As already stated, the antenna is 75% of a great station, with the radio being the other 25% It is true that you can use a linear amplifier to put out maximum allowed power, but putting that power into a poor antenna will be a big waste of time and money. The poor antenna will yield very poor reception, and even though the linear amplifiers will amplify reception as well as transmission, it can only amplify what it can hear. Very poor antennas can hear very little compared to the ideal antenna. If this all sounds like Greek to you, it is more the reason to join your local Amateur radio club and become well educated in radio use, antenna design, radio operation rules, and meet a lot of great people with the same interest. If we ever have a world war and it renders our normal communication methods useless, it will be radio Amateurs who will save the day with ingenuity that is MacGyver-ish in nature.
An opinion added by another contributor:
I strongly second the motion and support the statement that the antenna is
the most important part of the ham radio station, and if I could give it 18 more
votes then I would do that too. The reason it may sound like a stupid idea is
because it's such a simple statement of such profound wisdom. Are you trying to
tell me that a bunch of wire or metal rods or pipes has more effect on successful
communication than a big hot linear amp that goes for 5000 bucks has ? ! ?
Absolutely.
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Five thousand watts? Legal power on the amateur bands IF you have an Extra-class license is only 1500.
If you're working Morse you can communicate with Japan on 5 watts or less if you have the right antenna and good atmospherics. It's been done with less than 1 watt and a good directional antenna with director and reflector elements. Voice you'll need more power, but 100 watts and a directional antenna will talk to anywhere in the world.
The power required is(6) times (the amperes of current the radio draws when it operates) watts.
12 watts
The frequency on the radio dial doesn't tell us the power the station is using to transmit. There may be an AM radio station at any spot on the dial that is transmitting at any power level between, maybe, 10 watts and 50,000 watts. That decision is made when the radio station applies for its license, and the license to operate is granted.
Ofcom have licenced the broadcaster to radiate 100 watts ERP
A clock is not a generator so it does not have any power output. If anything the clock could consume 30 watts of power in its operation. The formula you are looking for is I = W/E, Amps = Watts/Volts.
Power is VI so 360 watts.
it has 400 watts
It costs [ 0.0005 x (power in watts used by the radio) x (cost of 1 KWh in your area) ]
while we worry about the 0.5 watts radiation of a cell phone, a walkie talkie can normally generates a radio power of up to 5 watts. which is more harmful?
One million watts
the unit which are measured for power of lights are watts....
A watt meter measures the electric power in watts from circuits. They can measure utility frequency, audio frequency power and radio frequency measurement.