We all know computers need power. Their use of electricity is obvious. They turn it into heat, noise and light – almost like magic.

But how much power does a PC need, exactly? This can be hard to guess. Most people with a computer have at least a few appliances in their home, and they likely came with some sort of power guide or rating slapped on them. Computers usually don’t, and the power they use depends on their components and the tasks asked of them. Let’s take a closer look.

You would expect a desktop computer to use more power than any other PC. You’d generally be correct. Desktop PCs pack more power-hungry hardware than any other type of PC because they don’t have to worry as much about the heat components generate.

With that said, many users over-estimate the amount of power that their desktop computer uses. A typical desktop with a dual-core processor, Intel integrated graphics and a couple of hard drives will be drawing about 50 watts at idle and will struggle to break 100 watts at load. Older processors are less power efficient than new ones, so expect higher power draw from them. AMD processors are also less efficient than Intel ones, so again, they’ll draw more power.

A video card is a significant contributor to system power draw. At idle a PC may use an additional 10 to 30 watts, depending on the card. At load, this can increase to an additional 50 to 250 watts or even more for extreme dual-card configurations.

This may seem like a lot, but it’s as bad as you might expect. A window air conditioning unit will use several times as much when in operation. Electric stoves, dishwashers and dryers can exceed 2,000 watts when in use. This is balanced by the fact your PC is likely to be used more frequently, but computers are on average responsible for no more than 10% of the power use of a modern home. Heating and cooling still take the top spots. Even your home’s lighting may consume more power if you haven’t switched to power-saving fluorescent or LED bulbs.

A laptop computer of course uses less power than a desktop. The processors and video cards inside them are usually rated for half the power draw or less and most come with power adapters that don’t exceed 130 watts. A portable laptop like an ultrabook or netbook may have an adapter rated for half that or less.

Even powerful laptops draw very little power at idle – around 20 watts for a netbook or ultrabook and closer to 40 or 50 watts for a gaming laptop. Idle power draw has reduced significantly as of late due to better processor design and switchable discrete graphics, a feature which turns the discrete GPU off entirely when its added oomph isn’t needed.

The result of all this is an extremely low power draw that has little impact on your yearly power draw and the power drawn from a circuit in your home. Even the most powerful laptop is unlikely to cost more than twenty or thirty bucks in energy per year.

Heavy, bulky, power-hungry CRT monitors are a thing of the past, but that doesn’t mean your monitor’s power draw is negligible. CRTs were notable because they often drew far power than the computer using it. That’s not often the case with new flat-screen displays, but some can consume a lot of juice.

There’s also a difference between displays. CNET’s review of the 27-inch Dell UltraSharp U2711 says it uses about 93 watts of power. A 23” LG Flatron, on the other hand, uses about 26 watts. That big monitor could potentially use more power than a low-end desktop lacking discrete graphics. Want to use more than one? Now you’re up to using as much power as a low-end gaming desktop at load.

Size is a significant factor. Remember, monitors or measured diagonally, so their total surface area increases more than the on-paper specifications would make you think. A 27” monitor is significantly larger than a 24” one and usually draws a lot more power.

Lighting type also matters. Fluorescent tube monitors usually draw more power than their LED brethren, so go for LED if minimum power draw is what you’re looking for.

The external hard drives, printers and speakers you connect to your computer usually don’t budge overall power draw by much. We’re talking a sub-watt power draw at idle. A printer or a pair of speakers can draw a fair bit of power when active, but they’re usually active for only short bursts.

Charging a laptop will increase its power draw. The amount depends on the laptop, but most will add an extra twenty to thirty watts over idle. A laptop that charges quickly is using more power than one that charges slowly – if all other factors (like battery size) are the same.

Even this, however, is not a problem. It won’t move the peg more than a few bucks on your power bill even if you’re charging every day, and the added watts are extremely unlikely to cause a circuit to flip. That would only happen if several other power-hungry electronics devices were already connected.

Let’s finish this up by talking about a few possible situations.

If you use a 15.6” laptop with a modern Intel Core processor you will likely be using 20 to 40 watts at idle and no more than 65 watts at load. Gaming laptops are the exception, which is why they ship with such large power bricks.

Figuring out the maximum drawn when you’re using the laptop and charging the battery isn’t hard. Just look at the maximum load rating on the laptop’s power adapter. It can’t be more than that. A lot of adapters list power draw in volts / amps, so use a volts to watts converter to figure out the power draw in watts.

If you use a an external monitor with your laptop, estimate about 25 additional watts – unless you use a monitor over 24 inches, in which case you could be looking at 50 watts or even over 100 watts.

Desktop computer power draw depends a lot on the hardware, but a basic dual-core with integrated graphics will be using around 50 watts at idle and up to 125 watts at load. Add in a mid-range discrete graphics card like a Radeon HD 7750 and you’re looking at about 75 watts at idle and 200 watts at load. Bump yourself up to a GTX 680 and you’re looking at idle power at about 100 watts and load power up to 350 watts or more.

Once again, you need to add on the monitor or monitors you might use. A simple system with integrated graphics and one 21” monitor will probably never draw more than 100 watts. On the other hand, a gaming desktop with three 27” monitors and two video cards could easily top 1000 watts at load.

The bottom line is that your average computer doesn’t use enough power to cause a problem with either your electric bill or the circuits in your home and surge protectors. Halving the draw of a typical system with a 24” monitor would save you merely tens of dollars every year.

It’s only high-end systems with large monitors and one or more fast graphics cards that can cause a problem. These systems can make a noticeable mark on your power bill, and they can even flip circuits in your home if your electrical isn’t up to par, but very few people run a computer with such extreme hardware.