A laptop typically uses around 60 watts of power, which is about 0.5 amps. A typical brand new laptop battery has a capacity of around 40 watt-hours, meaning that the battery can provide 0.5 amps for 40 hours before needing a recharge.
You can find out how many watts your laptop uses by looking at the power brick that you plug into a wall or a power strip to charge your computer or to run it from an AC power source instead of using its batteries. The wattage rating is normally printed on the outside of the power brick, but some manufacturers may put it on the bottom of the laptop itself as well.
You can also determine how long a fully charged battery will last if you know your laptop’s “load” and “runtime.” Load is simply referring to how much energy is being added to all electrical components of the machine at once. A load of 100 watts means that all components are fully operational and pulling as much power as they need from the battery or AC adapter. In this fashion, a laptop’s load varies as you use different applications that require more or less processing power.
You also have to consider average battery life expectancy in your calculation. Because batteries have a finite number of charges in them before they lose capacity, it is generally advised not to use more than 50 percent of the total capacity of a new battery until it has been fully discharged and recharged three times so as not to reduce its overall lifespan before it needs replacing.
In this calculation, lets assume that the laptop only draws an average load of 60 watts from its AC adapter, but that the battery is only charged to half its capacity before being discharged.
Using this information, you can determine how long your laptop will run using AC power by dividing the wattage of the adapter by the watts per hour rating of a new battery — or in this case, 60 divided by 0.5 = 120 hours of runtime. Using half of your batteries total watt-hour capacity means that it will last twice as long on AC power compared to when running off of batteries alone.
Most laptops are designed with an “auto-switching” power supply meaning that they contain circuitry designed to detect whether or not they are plugged into an AC outlet and deliver alternating current at appropriate voltages depending on what’s available rather than simply provide direct current delivered at a single voltage. Therefore, you can use the AC power adapter for your laptop wherever international AC power is available as long as it’s rated input voltage and frequency parameters match with local standards.
Since most laptops don’t draw more than half of their batteries capacity when running off of an AC power supply, you won’t need to worry about overloading the adapter as long as your battery is fully charged before switching off of battery-power alone or vice versa. However, if your laptop must operate on both sources simultaneously and one source provides significantly less wattage than the other, then it’s advised not to exceed 50 percent load from either source combined in order to prevent damage to either yourself or your equipment.
The typical current used by laptop manufacturers is around 0.5 amps for up to 60 watts of power with auto-switching circuitry that detects input voltage and frequency. Therefore, most laptops can be used off of any standard AC outlet without worry as long as you pay attention to the load that your laptop draws from its power source.
To answer simply, a typical new laptop uses about 0.5 amps when running on it’s own battery according to manufacturer specifications. However, if you’re plugged into an AC power supply then it will draw whatever wattage is required by the circuitry in order to deliver alternating current at appropriate voltages while being monitored by sensors inside the computer so not to overload either your adapter or your batteries while remaining compatible with international standards for AC current delivery.