Wattage is simply calculated by multiplying voltage by amperage. Look on the back of your system, and it will say how many amps the system uses. Often, it will also provide the wattage. For instance, the back of a ViewSonic E220 is 2.5A at 120V - ~300 Watts/Hr. An average rate is 10 cents per kilowatt hour, or 3 cents per hour to leave your monitor on. or 72 cents per day. A computer system can use between 150 and 600 or more watts per hour. Together, it could cost between $20 and $60 a month.

Here is an answer I found. This answer sure seems complicated. I got the following from http://text.broadbandreports.com/faq/2404 Cost of Running SETI 24/7 on a home PC. Part 1 There has been much discussion about this very topic and there are many who give their opinion, however there is nowhere that gives the facts for potential users to read this information which they can then show to those who may complain about the use of Electricity. Lets start by explaining the simple principles of Electricity and the power consumed. As we all know, electrical circuits consume power (va), one Watt of Electricity is also 1va. Calculating the voltage in a circuit and multiply it by the current used arrives at this figure. Example 1: Circuit A has a supply rated at 12V (Vn) and measurement shows it draws some 4.5A (In) from the supply. To determine the power rating of the circuit you simply multiply the Voltage by the Current thus.. P = Vn * In = 12 * 4.5 = 54 watts or 54va Now we understand that, there is one important point to make at this stage. If you alter the Voltage, you will also alter the current drawn from the circuit, however the power will remain the same at 54va. Example 2: P = Vn * In = 200 * 0.27 = 54 watts or 54 va. If you are wondering about the 0.27, then that is the current drawn from the circuit. As I said, the power must remain constant for the circuit, so by increasing the voltage you are reducing the load taken from the circuit. If you have any doubts about this then do the calculation in reverse. Example 3: In = P / V = 0.27A Part 2: Now we have the basics out of the way, lets go on to the important part of cost. Lets assume you turn your monitor off manually when not in use, so we can just concern ourselves with the actual computer itself. Please bear in mind these are rough figures with regards power used and only as an example. The maths are correct, but the power used by each users machine will depend on the hardware and the setup of the machine. Your computer has a 300W (300Va) PSU and runs at 120V, therefore it will draw some 2.5A from the supply Remember Examples 2 and 3 above. 300 / 120 = 2.5 where 300 is the maximum power of the PSU 120 is your supply Voltage. There are things inherent in all electrical circuits that have a bearing on the actual power used and these are called losses. They occur due to electrical circuits being far from perfect and take the many forms. The average PSU in a computer will operate at about 80% efficiency due to the nature of the circuits employed. This means that for the PSU to deliver 300W (va) as rated then it will actually draw about 375W (375va). Now we have that information, we can begin to look at a more accurate cost running calculation. Whilst we know that your Computer will not run at full PSU load, if ever, we will use the full load to give the MAXIMUM cost that should be seen to any user. Your PSU, as we have shown above, uses 375W (va) of electrical energy from the supply. Therefore to do the cost calculation is rather simple. Example 4: Power used at maximum (Pmax) = 375va So now calculate the total power used in 24 hours, P(max) * 24hrs = 375 * 24 = 9000va or 9000W (9Kva or 9Kw) If you pay, for example, 10 cents for each unit of electricity, then your cost of running the machine is : 9 * 10 = 90c per day If you wish to know the monthly cost, then calculate as follows: 90 * 7 * 4.3 = 2709c or $27.09 per Calendar month. The 4.3 in the above equation is the multiplier required to calculate any figure on a calendar month basis. Now we all know that electricity costs vary across the nation and from supplier to supplier. If you have a look at your last bill, you should see the unit cost of electricity clearly indicated on there. This is the figure you need to use to calculate the approximate cost of running a PC 24/7 for a month. Some of the information at the top of this in Part 1 may seem a little irrelevant to the cost calculation, however that is information that you can use to calculate all sorts of costings if you know the rating of various pieces of equipment. Part 3. Estimates: The following are based on the following figures. That all of the PSU's run at 80% efficiency, about average, and that each Kw/h of electricity will cost $0.06c per Kw/h. Please note that you will need to adjust the calculation for your given tarrif of costs from your electricity supplier. This information will be available on your electricity bill. 235w PSU Assumed maximum demand due to losses. 293w In one calendar month this would consume 211.6632 Kw/h at a cost of 0.06c per Kw/h, the maximum cost is $12.70 300w PSU Assumed maximum demand due to losses. 375w In one calendar month this would consume 270.90 Kw/h at a cost of 0.06c per Kw/h, the maximum cost is $16.25 350w PSU Assumed maximum demand due to losses. 437.5w In one calendar month this would consume 316.050Kw/h at a cost of 0.06c per Kw/h, the maximum cost is $18.96 400w PSU Assumed maximum demand due to losses. 500w In one calendar month this would consume 361.20Kw/h at a cost of 0.06c per Kw/h, the maximum cost is $21.67 450w PSU Assumed maximum demand due to losses. 562.5w In one calendar month this would consume 406.35Kw/h at a cost of 0.06c per Kw/h, the maximum cost is $24.38 500w PSU Assumed maximum demand due to losses. 625w In one calendar month this would consume 451.50Kw/h at a cost of 0.06c per Kw/h, the maximum cost is $27.09 As you can see the costs escalate incrementally as you would expect. These costs do not include the cost of running a monitor, however as this is not on 24 hours a day these costs are minimal, a few extra dollars a month. The figures quoted above are only if the machine is running at maximum potential load at all times. In most cases the machine will probably consume only about 50% of the estimated maximum and thus cost, however this will vary due to hardware differences and usage. The maximum you can use is listed above and is accurate for consumed power, actual cost will vary with supplier costs per Kw/h. Hope this is of help to those whose spouses, partners or parents are concerned about the cost of allowing a machine to run 24/7.

You can't really use the PSU Watt rating as that is a max use. You need a wattmeter to measure. Also a PC will use more watts if the CPU is running 100 precent. Stuff I've measured (idle-full load).. Pentium M 1.5Ghz Laptop 40-60watts Core 2 Duo 2.2Ghz + 8600GTS 120-150watts Pentium 4 2.5Gz + 7600GS 200-250watts If you want to save the planet get a laptop! (most though not ideal for gaming)