What is the consequence of using a lower wattage power adapter with a laptop?
Similarly with your laptop.. the supplied power brick is specified based on the expected worst case power demand of your PC. When you go to a lower spec supply, there will be use cases in which that supply won't fully power your computer. Knowing that Apple switches the power in software, I assume that as the power supply starts to brown out, the laptop's internal battery would take over. Once that's depleted, you would find that higher power conditions on the laptop, more GPU and CPU use, would lead to a system wide brownout... a crash.
Even at lower power levels, you might find that the power supply can't both operate the PC and charge the battery. That alone can be annoying... most folks expect the computer to charge while plugged in.
Long term use of the power supply in this way means that you're regularly trying to push it past its limits. For a well designed supply, it'll be current limited, so it won't try to operate past safe current and heat limits, the main issue will be lower power efficiency at those loads than for a properly sized supply. However, a not so properly designed supply will suffer thermal stress with repeated overloading. It will not last as long as the full spec supply.
And all of this assumes your Mac Book will even agree to try to work with the lesser supply. I don't know the details of Apple's power interface, but if there is an actual communications channel there, your MacBook may simply refuse to operate with unauthorized supplied. That seems like a very Apple thing to do.
If your laptop needs 40 W then it will work but you battery will take forever to charge while the laptop is running.
If the laptop takes 50W to run it will not run properly if the battery is flat and if the battery is not flat you won't be charging your battery either, you'll be discharging it to make up the shortfall.
If they give you an 85W adapter with the unit that is what you should use and nothing less.
Let’s start by saying that I have done this. And… I’ve done it on more than one occasion, and I’ve done it on purpose to know what will happen.
I ran a Toshiba Satellite A75 on a 90 watt adapter for a couple of months, while it required a 120 Watt adapter. Here is what happened. Under normal usage conditions… checking email, surfing the web… light tasks, the battery would get hot, but otherwise the laptop would perform normally. If I played any games, the laptop would turn off within 60 seconds of starting the game. If I did any CPU intensive tasks while not gaming, whether or not the laptop would turn off, depended on how long I ran the tasks for. If I removed the battery, the unit would not turn itself off when doing CPU intensive tasks outside of gaming, but gaming would still bring the laptop to shutdown… albeit just more like 20 minutes into it, instead of 45 seconds.
I’ve tested laptops that required 90 watt adapters with 65 watt adapters.
Is it dangerous? No. The laptop isn’t going to explode. The power adapter isn’t going to explode. The battery isn’t going to explode. What *will* happen then? All depends. Nothing might happen. Or… the laptop might turn off because it is trying to draw more power than is available.
But it won’t explode. It won’t rise up and attack you either because you are starving it of power.
A “ lower wattage power adapter”
I think you mean a lower rated power adapter.
If an adapter can not deliver the power necessary to operate the load, a laptop in this instance, the load will not operate correctly.
If the load tries to draw more current than the adapter can supply, the adapter’s output voltage will drop.
If your laptop come with an 85W adapter it is reasonable to assume that it needs this much power, or some figure very close to that, in order to work correctly. Try to operate it from an adapter with a lower power rating and it won’t work .