How to calculate a PC Power Budget and Energy Costs

How to calculate a PC Power Budget and Energy Costs

Have you ever wondered how big a power supply you need for your computer - perhaps to custom build a new PC, or upgrade or replace a Power Supply in an existing PC. This article will take you through the steps needed in calculating your PC’s power budget, which will help you choose an appropriate PSU (Power Supply Unit) or estimate an electric bill.

What is a Power Budget?

You could say that behind all the Software, lies the Hardware. That is, the software is not possible without some underlying hardware. The hardware requires electrical Power to function correctly. A power budget is a calculation of the electrical power consumed by all your computer’s hardware components while in use.
Electrical power is measured in Watts (W). The actual power used can vary considerably, depending on how demanding a software application becomes, or whether you use Windows Power Plan to changed demand, say, for use in a hibernate state. Regardless, the net electrical power is a useful figure to obtain, say in choosing a Power Supply, or estimating an electrical bill.

There are a variety of PC hardware components that need to be brought together, usually from different manufacturers, in order for the computer to function properly. Depending on how the computer is made, some components will have more bearing on the total net power demanded by your system. Usually, the CPU and Graphics cards demand the most power, but knowing the power of the other components is also critical in a Power Budget calculation.

A. Estimating the Power Budget

OPTION 1 : Measure or calculate the numbers directly.

OPTION 2 : Calculate the Power Budget using an online Power Supply Calculator.

OPTION 1 : Measure or calculate the numbers directly.

If you had all the components in front of you, and you had a multimeter to hand, or, if you had the original PCB (Printed Circuit Board) drawings, then you could obtain the Power Budget by direct measurement, or by using electricians circuit laws to calculate the demand.

For most of us this is not possible, practical, or even desirable. However, there is another alternative.

OPTION 2 : Calculate the Power Budget using an online Power Supply Calculator.

i) For this option, I suggest that you prepare a list of all the computers hardware that you have, or intend to buy: CPU, Graphics Card, Memory sticks, Hard Disk Drives, Solid State Drives, Optical Drives, Mice, Keyboard, LED lights, Card Reader, Cooling devices. You need to know how many of each are plugged into the computer at any one time and, with the CPU and GPU especially, you need to know the make and model.

ii) Next, you need to find an Online Power Supply Calculator. If you conduct a web search you will easily see that there are many to choose from. For example, if you choose Power Supply Calculator - PSU Calculator | OuterVision you will see at that site you can choose either Expert or Basic mode estimate, and with the Expert mode includes parameters for Overclocking as part of the Power Budget Estimate – which not all sites offer.

iii) Enter the site calculator, and click on each component option, one at a time, and select what you have/ or intend to use from the drop-down lists.

iv) Click on the Calculate button to obtain your Power Budget estimate.

B. Estimating Energy Cost

Once you have your power budget estimate (in Watts), you can go ahead and calculate your estimated Energy Consumption (in Kilowatt Hours).

Some people mistakenly assume that the Energy Consumption is based on the Power Supply size. This is wrong – the Energy Consumption is derived from the Power Demand.

i) The basic formula is given by:

Capture 01.JPG


number of hours must be in hours, so half an hour is 0.5 hour.

The factor of 1000 appears in the denominator because you want to convert the Power Budget (Watts) into Kilowatts (KW), otherwise your calculation/estimate would be way out.

Generalising the Load and compensating for Energy Efficiency to form more exact estimates

The above procedure will give a pretty good approximation. However you may want to fine-tune that approximation, and include hours spent playing games as a separate calculation to hours spent using less power-intensive software, or secondly, you may want to include the energy efficiency of your power supply as a factor in the Energy Consumption approximation.

This section will describe ii) how to derive the Energy Consumption when estimating the computer under different loads, and iii) how to derive a Power Consumption formula to account for the Power Efficiency of the PSU.

N.B.: You will need to derive an estimate of how much power your computer draws from the mains when under different loads. So you need to estimate whether the machine is under 10%, 20%, 50%, … 100% load, while running various Software. You could achieve this approximately by running a CPU speed gadget or app, at the same time as the loaded software in question, then multiply the CPU utilisation by the Power Budget. Or using a Portable Electricity Usage Watt Meter to obtain a crude figure for the Power Consumption, that way.​

ii) Load balanced Energy Consumption:

When the computer is in more demand (ie. when using the computer for gaming) it draws more Power, than when the computer software is less demanding. This leads to a situation where estimates for energy consumption, using the above formula, can be artificially high, compared to the actual consumption. The above formula can be revised to factor-in Energy Consumption under different loads:

Capture 02.JPG


Pg is the Power consumed while gaming

Ps is the Power consumed while using less power intensive software.

H1 is number of hours gaming

H2 is number of hours using less power intensive software.

-or more generally-

Capture 03.JPG

where load is in watts and hours is in hours.

iii) Power Efficiency Consumption:

Each power supply has its own energy efficiency. This tells us that, although we have an individual Power Budget for a particular PC, the actual Power demanded by the Power Supply Unit is actually higher due to the electrical energy loss that gets converted into heat energy. What is needed is a formula that compensates for the Efficiency of the individual PSU.

A 600W load with an 80% Efficiency would require 600 x 100 / 80 = 750W. So, a net Power Budget of 600W would translate into 750 / 1000, or 0.75 kwh per hour energy consumption.

A general formula for this estimate would look like this:

Capture 04.JPG

where P is your Power Budget estimate (in Watts), and E is the efficiency (expressed as a percentage) of your chosen Power Supply.

Now you know enough to choose a new Power Supply Unit, or calculate your computer’s Energy cost.

Hope that helps.
Windows Version