# Solar panels & battery banks (off-grid).

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How does it work in general, what’s the difference between different wattages on a solar panel, what’s the difference between different amps on a battery, why is it important not to discharge your battery over 50%, if I currently use 650kWh in 1 month, how do I properly calculate a solar array and battery bank that would allow me to fuck off the hydro grid?

I desperately want to switch to solar but I feel like I have to get a degree in electrical engineering before I even call an installation company or else I (a) won’t get what I actually need and (b) will get screwed six ways from Sunday.

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Well, it is complicated, but I’ll try to simplify some of the basics.

“Watt” is a rating of power, like horsepower. It is how fast something can perform work — my car has enough POWER to make it to the top of the hill in 5 minutes (Power says nothing about how many times your car can go up the hill). Watt-hours (or kWh) is a rating of energy storage. It’s how much total work you can do – my car has enough gas (energy) to make it to the top of the hill one time (ENERGY says nothing about how fast your car can get up the hill).

The “amp” rating on a battery can be confusing, because “amps” can be shorthand for a few different things such as: cranking amps, cold cranking amps, and amp-hours. For your application, amp-hours is probably the only one the matters. Unfortunately, the confusion isn’t over yet. For the amp-hour number to be useful, you need to know the voltage because voltage times amps gives you watts.

For example, a 24 volt, 100 amp-hour battery holds the same amount of energy as a 12 volt 200 amp-hour battery. That is 24V * 100A = 12 * 200 = 2400 watt hours or 2.4 kWh.

So, now lets look at your 650 kWh number. There are about 730 hours in a month. So,on average, you use 890 watts (650,000 / 730 = 890). Thus, if you solar panels produced full power 24 hrs a day, you would need a minimum of 890 watts of solar panels.

But obviously the solar panels can’t produce full power 24 hours per day. Even on a sunny day, they won’t produce full power when the sun is low in the morning and evening. You need to find out what output you can expect for your local area given your climate. But as a wild guess, I bet you need 4000 watts of panels if you live in a sunny area. More if it’s not always sunny.

Now back to battery capacity. Say you wanted to be able to be able to run you house for 2 days off of just the batteries. That’s 48 hours times 890 watts, about 43,000 Wh (43 kWh). Say you were using 12V batteries. Then, 43,000 divided by 12 equals around 3600 watt-hrs of batteries. Or twenty four 150 amp-hour batteries. Note that when you connect batteries in series, there voltage adds, but the amp-hours does not. And when you connect them in parallel, the amp-hours add, but the voltage doesn’t change. Thus, if you connected the twenty four 12V batteries in 6 parallel groups with 4 batteries in series in each group, then your overall pack would have a voltage of 48 volts (4*12 = 48) and an amp-hour rating of 900 A-h (6*150 = 900). My point is that rearranging the same batteries to give a system with a different operating voltage will have advantages, but the amount of energy you can store (the watt-hours) doesn’t change with the different arrangement. And one last note, the above calculation of 24 batteries assumes you use 100%. To stay above the 50% charge state recommended for lead-acid b\batteries, you would need twice as many batteries.

General solar power and battery system concepts can be ELI5’d.

But asking how to size a system, and how to understand volts, amps, watts, watt-hours, amp-hours, battery chemistry, charge controllers etc. is not an ELI5 topic once you get to that level of detail.

You don’t need an EE degree but you do need a very good understanding of electricity and a general understanding of modern solar and inverter technology. I’d recommend watching Will Prowse on YouTube as another commenter mentioned. He covers the basics.

I suspect at 650 kWh/month you’d need a pretty large and expensive system if you’re trying to go completely off-grid.

You need to find a company who actually does consulting and can explain everything to you and figure out what’s practical and cost effective for you, instead of a company run by salesmen and installers that will just throw you an estimate and a bill. I don’t know where to even start that kind of search.