Do I understand this correctly?

[Thunder Dan]

Say I have a machine, and it's the only electric thing I use. It uses 60 watts at 12 volts, so that's 5 amps. If I want to run that machine for 8 hours, that's 40 amp hours. And I wouldn't want my battery going below ~ 60% of its amp hours, so I would want at least a 100 amp hour battery (assuming it is under ideal conditions, getting optimal sunlight to charge it back to full every day).

Is that all correct? Thanks for helping me understand.

 

[highdesertranger]

you got it. you are also doing it the right way. figure out your usage, size your battery according to your usage, then size your solar according to your battery. highdesertranger

 

[LeeRevell]

50% is the commonly stated standard.  I like having enough battery to get by three days at least, to account for those less-than-ideal days when solar isn't charging at it's best.

 

[Canine]

50% is the commonly stated standard.  I like having enough battery to get by three days at least, to account for those less-than-ideal days when solar isn't charging at it's best.

Great point. For three days of zero sunshine you would want 200 a/h to keep the battery from going below 60%. By doubling the battery bank, you get triple the usage.

 

[Thunder Dan]

you got it.  you are also doing it the right way.  figure out your usage,  size your battery according to your usage,  then size your solar according to your battery.  highdesertranger

Now here is where I might get confused. Say I have another machine that uses 65 watts, and it only has an AC 120 volt plug. I want to run it 3 hours per day. It will need to go through an inverter. For this scenario, let's pretend I'm using the Tripp Lite 375W Powerverter:

https://www.tripplite.com/power-inverter-for-car-375w-2-outlets~PV375/

I cut the cigarette plug off of it and connect the wires to the battery. I will only turn on the inverter when I'm running the machine. If I read their website correctly, the inverter will use 0.65 amps by itself when it's turned on. And the machine I'm using will be 5.42 amps, for a total of ~ 6 amps, * 3 hours = 18 amp hours of additional battery needed.

Is all of that correct, or have I missed something?

 

[ZoNiE]

Correct: You did the conversion for 12V and added the loss on the inverter.

 

[SternWake]

I am not sure if that no load draw of 0.65 amps is included in the general 80 to 85% efficiency of the inverter.


However you should not be planning on 'cutting it that close' because then you will only be able to run when you want, for as long as you want, in ideal conditions.  

I am rather surprised how much more juice my 800 watt PSM coleman inverter uses powering a 55 watt heating pad, compared to my 400 watt PSW Wagan elite inverter.  A full extra amp!.

As for no load consumption the wagan is 0.24a, the coleman 0.68a.

I rarely use the Coleman anymore.

Please keep in mind getting the battery back upto full takes more than math, it takes time, and after many successive days of not getting to 100%, then the time required at absorption voltage increases, making it even harder to get to 100%.

So, Go for as much solar as you can fit and afford, rather than just enough.  If all you can fit is not affordable right now, use thicker wiring and make sure the controller can handle the extra wattage when you can afford it.

Couple this with only expecting 80% of the panels rated output and it producing that 80% only a few hours either side of solar noon when aimed at the sun.

Running numbers is great and logical, but there are too many efficiencies to put a stamp of approval on 'theoretically enough'

Youse gotsta plan for inefficiency and much less than ideal solar conditions

One other consideration is the Peukert effect.

a healthy fully charged 100 AH battery can support a 5 amp load for 20 hours before voltage hits 10.5v which is considered 100% discharged

Loads over 5 amps on this battery, reduce that 100AH figure to something less. the bigger the load the less the battery has to give.

Combine/account for the inefficiencies, or the math, can't work