When upgrading or adding any device to your computer system, it is essential to know how much power the device requires from your computer's internal power supply. Adding a device to your computer that draws more wattage from your power supply than it is rated for can lead to an unstable system and possible damage to your internal components.
Checking Wattage Without Removing the Power Supply
Unplug your case from any electrical source and press the power button for 10 seconds to ensure any stored electrical charge is dissipated.
Remove the thumbscrews from the rear face of the case that hold the computer's side access panel in place, and pull the side access panel back with light force to remove it from the case.
Locate your power supply within your case. It will be located against the rear wall of the case, either at the very top or the very bottom.
Look at the exposed faces of the power supply unit for a sticker or label that describes the unit's electrical capacities. This label is required to be placed on all ATX form factor computer power supplies. It will designate how many amps are available for each voltage type and give a "watts max" designation. For example, a 300-watt power supply would read "300 watts max." This maximum wattage is your power supply's wattage rating.
Replace you computer's side case panel and securely tighten the thumbscrews back into place.
Plug your computer back into any previously disconnected electrical sources.
Checking Your Power Supply's Wattage if the Label is Obscured
Continue from Step 5 of the previous section if you cannot readily see the power supply's rating label due to its placement on an inward-facing side.
Remove the detachable power cord that runs from the power supply to your electrical outlet. This cord simply pulls out of the back of your computer and is easily reattached.
Locate the four mounting screws on the back side of the power supply. These screws are on the same side the power cord was removing from in the last step. Remove the four screws and set them aside taking care not to lose them.
Use light force to slide your power supply forward toward the front of your case where your hard drives and optical drives are located.
Pull your now semi-free power supply outward away from the case. Take care not to pull too hard or fast, because all your components are still connected to your power supply.
Manipulate the drive and check every face for a specifications label. Once you find it, check the wattage rating as explained in step 5 of the previous section.
Reposition your power supply into its original position, and remount it with the four mounting screws you removed earlier.
Replace the female plug removed earlier from the rear of the power supply back into place.
Reassemble the case as detailed in steps 6 and 7 of the previous section.
Sometimes there are clips that hold power supplies into place. If you find your power supply is not moving easily forward, check to see if there is a small finger clip that needs to be pressed.
Never under any circumstance open a power supply. There are no customer-serviceable parts inside or product information. If there is no label describing the specifications of your power supply, it is highly recommended you immediately purchase a new one. Having unknown electrical devices in any system is not only a device hazard but a safety hazard as well, because you cannot be certain of the product's capabilities and may inadvertently cause electrical shock, discharge or even a fire.