Hacker News Reader

Comment by JSadowski | Hacker News Reader

They claim that their 50 Whr battery can power a MacBook Pro for 6 hours, a MacBook Air for 12 hrs. By my math, that would mean that the MacBook Pro would only be consuming an average of around 8.3 watts an hour, the air only around 4.2 watts an hour.

That seems absurd to me, considering the power supplies that ship with the MacBook Pro and the MacBook Air are 85-watt and 45-watt respectively. They are assuming that each only draws about 10% of the power that the official power supplies are rated at. Anyone have a Kill-A-Watt to see the actual draw of their device?

View on HN

Think of the power supplies as battery chargers.

The 13" MBA has a 54Wh battery, and Apple claims it get 12 hours of "wireless web" use. The 13" MBP has a 72Wh battery and claims 9 hours of "wireless web" or "iTunes movie playback". In practice, people find the Apple claims fairly reasonable.

While that does not prove how precise or accurate their projections are, I don't think they are absurd. Software-based monitors of power consumption on MacBooks show variation between low single digits and low teens under light use, and shoot up dramatically if you are doing something intensive.

It's entirely true, and means I could run my MacBook Air off of USB power at 5 volts and one amp (5 watts), but zero battery charging. Unfortunately Apple doesn't allow this scenario, but the math works out...

The Air has I think a 50 WHr battery and will last 11-12 hours for Wi-Fi surfing. I think the idle load is just a couple of watts. Peak load is of course higher.

Good point, I didn't think to check the capacity of the Air's internal battery.

The difference in power draw between idle and 100% utilization on a modern computer is enormous.

The power adapter needs to be able to handle 100%, but the sort of usage where you get 6-12 hours of use while on battery is very much near idle almost all the time.

I have a 15" MacBook Pro, which has a 95Wh battery. Apple claims 8 hours of battery life on it, and in my experience that's an understatement. It also ships with an 85W power adapter.

> The power adapter needs to be able to handle 100%

Actually it can't; there used to be a support article on Apple's site named HT2332 that details how Macbooks will underclock with the battery removed, because the adapter cannot supply enough power with the system at full load. Other manufacturer's laptops have also been doing this, so Apple is not alone.

That's true, I'm able to drain my MacBook's battery while plugged in if I stress it enough. It's pretty close, though!

I don't suppose you have any idea exactly how they arrive at the power adapter size for these things? It seems to be almost but not quite 100%, but they still try to optimize it where they ship 45W, 65W, or 85W depending on what computer you have.

At least for the Macs I think it's more to do with how fast it can charge the battery while the system is (mostly) idle or off. A larger adapter is inconvenient, but so is a slow-charging battery.

"watts an hour"

FYI, the unit you're looking for is "watts", not "watts per hour".

Reply on news.ycombinator.com