Figuring out electrical usage to determine how many 6 volt batteries to get...

[magentawave]

I'm trying to figure out my electrical usage so I'll know how many 6 volt batteries to get.

I have an old 2008 MacBook Pro with a 17" LCD monitor and I have a DC adapter for it.

The DC adapter (see image below) says:
Input: 11V-16 DC 5.6 amps
Output: 18.5V 4.6 amps

NEWB QUESTION, PLEASE...

If I ran the computer with LCD screen at it's highest level and with hard drive spinning and internal fan on for 1 hour, then how much energy would it consume using that 12 volt adapter?

Thanks

 

[proper4wd]

Safe to assume 5.6A constant draw, that should help you figure it all out.

 

[dwh]

5.6a x 1hr = 5.6ah (amp hours)
5.6a x 8hr = 44.8ah

 

[dlh62c]

A good starting point is a 200ah battery bank and 200 watts of solar.

 

[Bbasso]

The basic rule of thumb I have learned from reading about solar system is for every 100 amp hour battery you need 100 watt of solar. Hopefully that helps you too build a better or proper system.

 

[Shadow503]

Why not test it to see what kind of power draw it actually has? They may have significantly overbuilt their power adapter. I use one of these in my system to measure power usage: https://www.amazon.com/gp/product/B017BDQHE6/ref=oh_aui_search_detailpage?ie=UTF8&psc=1

 

[Shadow503]

A sound idea, you probably find on average it draws less than its rating.
But its rated power is a good basis which to estimate battery needs.

True, but based on the estimates we've been giving, people are assuming that it is going to be doing a 5.6A constant draw for 8 hours. That power adapter is probably designed to provide enough power to charge the laptop battery from dead while the laptop is maxing out the CPU/GPU/Display Brightness + a little extra for accessories. OP shouldn't need a 200AH bank to power a MacBook.

A good test would be to meter it (he can use an A/C Kill-O-Watt and inflate the value to leave room for inefficiency of the offbrand adapter) for an hour simulating worst case power draw (whether that is rendering video, watching a movie, or playing a game). If the MacBook's onboard battery is healthy enough, you could also try draining it and then just measure how much energy it takes to charge back up to full - that's probably a more realistic use case. If you think you'll only need to charge once a day, then you have a pretty good figure for your daily energy needs.

 

[IdaSHO]

The basic rule of thumb I have learned from reading about solar system is for every 100 amp hour battery you need 100 watt of solar. Hopefully that helps you too build a better or proper system.

I'm sure some places you could get away with that, but most are not that ideal.

Especially if you are trying to maintain things full-time, not just summer.

Living in the PNW and traveling regardless of season and weather, I like to see 2x the solar than battery AH

 

[e60ral]

I bet your actual usage is about 15 watts at idle and probably about 50 watts max when you are using some programs

 

[Shadow503]

I bet your actual usage is about 15 watts at idle and probably about 50 watts max when you are using some programs

Exactly. I've run a laptop + two 22" external LCD monitors on full brightness while running a GPU & CPU intensive program on my laptop and only hit 90 - and that was going through an inverter. His DC to DC converter should be much more efficient.

 

[dwh]

True, but based on the estimates we've been giving, people are assuming that it is going to be doing a 5.6A constant draw for 8 hours.

The OP asked a simple question - how much would an old MacBook draw running for an hour.

Not knowing exactly how many watts it would actually draw, I simply used the max draw of the power adapter when I showed him how to do the math for himself.

That is the worst case assumption, and in the absence of more detailed information, the worst case assumption is the correct assumption.

And since I was already providing an example of how to do the math, I provided two examples - one of one hour and one of eight hours - so that the OP could see how the amp*hours pile up.

After all, estimating battery bank size and energy budget are not just about the power draw - it's about power draw over time - and he did say this whole exercise was to help him figure out his battery bank size.


So now, let's take some of you guys' statements, and turn them into something that, combined with my previous example might actually help the OP to refine his estimated power budget and help him figure out his battery bank size:

90w ÷ 12.8v = 7a (battery full)
90w ÷ 12.0v = 7.5a (battery 50%)

50w ÷ 12.8v = 3.9a
50w ÷ 12.0v = 4.2a

5.6a x 12.8v = 72w
5.6a x 12.0v = 67w