Quote Originally Posted by Irving View Post
Never mind about the battery thing. I just realized how stupid easy it would be to mount inside a box and vent to the outside. This may change my whole plan...

However, I'm pretty shaky on running the numbers to make sure that everything is compatible with each other as far as size of battery vs size of panel.

Can you talk a little bit more about what you expect to be able to charge with your system? I've got a 50W panel and an 18AH battery and I have no idea how long it would take to recharge the battery, or how many devices I could charge at the same time.
Sure. I'll run the calculations for my internet gear as an example.

Currently, (I have new gear on the way that is more power efficient/better technology anyway) I have 2 devices that I want to power to keep my internet service up during a black out:
1: Fiber Modem
2: Asus Wireless Router

Start with looking at the devices to see what they require as power input on the back/bottom:
1: Fiber Modem - 12VDC @ .4A
2: Router - 19VDC @ 1.58A

These numbers are usually, 99% of the time, the "worst case" numbers for those devices. If you want to, you can get a power meter and plug them in and see how much they actually draw on average and build against that. I don't like that method, so I always use the "worst case scenario" and pad that just a little.

Convert them to watts: VDC*A=Watts
1: Fiber Modem - 12*.4= 4.8W - I rounded this up to 5 Watts for my calculations
2: Router - 19*1.58= 30.02W - 31 watts - Power hungry little bastard...

Add them together, 36 watts, and multiply by 24 to get the total number of watts you will use in 1 day. 36*24= 864 watt hours per day.

So, the total consumption that I need to offset with panels and sun is 864 watts per day. If we assume we only get 4 hours of "full" sun in the winter, again the worst case scenario, that means we need 216 watts of panels to provide each day worth of usage. Now, that number has to be larger, because if you go days with less sun, the batteries will get pulled down to make up for the lack of solar. For my own non-critical projects like these, I tend to use 125%, so it would take 4 days of full sun to make up for 1 day of no sun (black blanket over the panel type of no sun). Even on really bad days, the panel will still generate some power, so I build on the smaller size. When I am building for a customer, and it's critical, I usually use 150-200% on the panels to make the system recover as quickly as possible.

So, 216*1.25= 270 watts of panels for this system. Part of the reason I am getting much more power friendly, albeit better network gear!

On to the batteries:

We know we are using 864 watts per day, so we can divide that by the voltage of the battery bank we are using to find out how many amp hours we need in a day. Since I decided to screw around with 48V on this system, we divide by 48.

864/48= 18AH per 24 hour day. Now, we have to account for batteries inability to drain to 100% and recover, so I multiply that by 2 so we only reach a 50% discharge level. Less discharge is better for lead acid, but 50% is safe.

So now we know that I need 36+AH of batteries to run my internet gear for 1 day on batteries alone.

Multiply that by the number of days you want to be able to use the gear without sun (autonomy), I said 3 days in my case, so 36*3= 108AH. I didn't get anywhere near that in the cheap UPS batteries, which also drove my decision to get new/better/less power hungry network gear.

So, that's kind of a rambling explanation of how I calculate solar numbers.

As for your 50W panel and 18AH battery, your 50W panel will provide just over 4 amps with full sun. Now, mounted flat on the roof of your van, it will have full sun for a short period each day during the summer, but significantly less during the winter. It will still likely be plenty to make up for a 50% discharge of your battery on any day.

As for how many devices you can charge, if you figure you have 9AH to use out of the battery, then with no sun you can charge up to 9AH worth of devices, and the battery will still recover when the sun comes up. I know it sounds dumb to say that that way, but you can usually find out how many mAH's your phone/tablet/etc battery actually holds. My LG G3 has a 3000mAH battery, that's only 3AH. You can probably safely plan on a full recharge of your phone being less than the actual battery rating, but for the margin of error, I just use the full capacity. But you could charge 3 phones from dead to full without hurting your 18AH battery. As for how many devices you can charge at one time, that is more a function of the 12VDC-5VDC charger that you choose. Your standard 2.1A USB charger is going to be 2.1A @ 5.1-5.4VDC so worst case, that's 11.34W, which on the 12V side is only .945A. Most batteries have a spec for max discharge current, but its usually in the dozens to hundreds of amps.

If the sun is up, then the solar controller should charge the battery to max capacity, then shunt any solar current to the load output of the controller, which is what you would want to wire your charging ports to, so then you can charge even more devices because you aren't pulling on the battery.

Hopefully my rambling helps.