1.Electric motors require 3 times their running power to power on. This is why a lot of servers stagger the start up sequence on drives...

Let's consider just this one parameter: The hard drives need 3 times their running power to start up.

So, here's my system currently running fine. It's a Compaq EVO D310 http://h18002.www1.hp.com/products/quickspecs/productbulletin.html#spectype=worldwide&type=html&docid=11348 that came with a 220 watt power supply unit (psu). If you're interested, the label for the psu can be seen here: http://www.atlantisgadgets.com/index.php?main_page=product_info&cPath=5_10&products_id=100

The power supply estimates below were taken from results here: http://extreme.outervision.com/psucalculatorlite.jsp

Here's only the basic devices from my system powered by the original 220 watt power supply unit:

2.4 GHz Pentium 4 CPU (Socket 478, Northwood, estimated watts below used CPU at 90% TDP)

1 IDE hard drive 7200RPM

1 DVD/CD-R combo optical drive

2 sticks DDR RAM

3-80mm fans

(My modem is externally powered, so I'm not including it.)

If we use the tool to calculate the power supply at this point, we get:

152 watts required / 202 watts recommended. Note that the recommended estimate is adding 50 watts on top of the required amount, which is some degree of headroom.

But consider that the above quote says **the hard drive needs three times its estimated running power** **when it starts up**. If we remove the hard drive from the above estimate and then use the tool to recalculate, the total required watts changes to 134 watts total. That means the hard drive needs about 18 watts. At startup it should need 3 x 18 which is 54 watts, and that's 36 more watts. So now, the required power for the above to run at startup is 188 watts (152 + 36). The recommended headroom applied to this then puts the original 220 watt power unit that came with the system at an underrated amount because the estimator would recommend 238 watts.

But the above devices listed are not all that are running on my system. For example, I've got another IDE 7200 RPM hard drive. So now, we should be adding another 54 watts to the total required watts if it needs 3x the required power for startup. That puts the total required watts at 242 watts. My system should fry at startup because I should need 242 watts and I'm only running a 220 watt power supply unit.

But *additionally*, I have a PCI soundcard installed. The estimator tool uses 13 watts as the average wattage needed for a PCI card. Tack on the watts for my soundcard and I should require 255 watts. The estimator tool would add 50 watts headroom to this. So now, the recommended power supply unit should be 305 watts. Yet, miraculously, I'm running these devices using the 220 watt power supply unit that came with the computer. I didn't even include the power needed for the Intel 82845G integrated graphics controller, which I intend to disable and upgrade with a low watt AGP video card.

So, I have a 220 watt power supply unit that has for years been running all devices fine, BUT (given the hard drives require 3X the estimated required watts for startup) the recommended power supply unit should be 305 watts. How is it possible? Clearly, the estimated power supply requirements must be inflated or my system would fry when powering on.

**Edited by peterk312, 15 January 2014 - 08:44 PM.**