Did I Do This Right?

[firebuild]

Originally Posted by john61ct

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I calculate my 24 hour usage to be about 1100Wh or 74Ah. That includes:

1) MacBook Pro, which charges at 75 watts. I run it off battery down to about 20 percent, then charge again, so even though I run the MacBook probably 10+ hours a day it's only drawing external power for about 2 and a half hours.

So to be conservative call that 100W * 5hrs =
500Wh

Why would 100W *5 hours be conservative? It seems like total overkill. After the 75W charging time it drops to about 50W, so I would never be using 100 watts even once, let alone for 5 hours. Even if I used 5 hours - which I doubt - and overestimated to 80w, it would be 400, not 1000. I used 75*2.5, which is realistic based on my experience with this laptop.

Originally Posted by john61ct

2) Refrigerator - averages 30 watts or 2.5 amps, running 24/7

Too low, 1000Wh accounting for high ambients, food-safe temp setpoints, frequent opening.

I'm already using this fridge, have been in 90 degree temps, with a 34 degree temperature set on it. This is the actual, not a projection. So why is it too low? Truthfully it actually averages only 26 watts, so 30 was already stepping it up.

Originally Posted by john61ct

3) A double headed 12 volt fan - 13 watts/1.1 amps

need hours, but call it 300Wh

300wh is 23 hours a day! I calculated it at 16, and that was very conservative.

Originally Posted by john61ct

4) Misc USB Items - mobile hotspot, phone, etc., charge only

best to actually measure, but 200Wh

I have powered all of these off of my starting battery for a week without starting the bus and had no issues when it was time to drive. I really, really doubt they come to 200wh. But I will add them up and see.

Originally Posted by john61ct

5) 2 LED lamps - 3.5W each

another 200Wh

Unless I am grossly misunderstanding the concept I don't even see how that's possible. For 7 watts to run for 200 wh, the item would have to run 28 hours! Right? I run lights for at most 5 hours a night.

While I understand the need to be conservative, I thought the point was to come up with a realistic budget and then figure out how much reserve you would like to have on top of that, not to make an unrealistically high budget. Calculating this by your numbers just about doubles mine, which might be good to aim for, but certainly not with a reserve on top of that!

[john61ct]

If you have actually been measuring over 24-hr periods at 12V not mains power,

maybe a 25-50% cushion is enough.

I also assume other load devices will get added to the list.

Just a different approach, obviously you do you.

To the extent shore power is conveniently available **and** you can recharge your pack in an hour or two, then of course little reserve is needed.

However depending on your solar panelage ratio and weather conditions, once off grid I like at least a 3-4 day buffer.

Even if you only do that a few times per month, need to plan for worst case rather than best.

Unless you carry a genset?

All depends on context, other factors unknown here. . .

[john61ct]

Not directly relevant, but IMO pretty funny

https://youtu.be/r8xfzeD0ZK4