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• Registered Users Posts: 24
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Yes I get basic math like that, I know what watts are, volts, amps, but I don't get the calculations for the batteries, what they mean.
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BB. wrote: »
A battery bank for a 100 watt panel (assuming commercially made)--the battery should be charged around 5-13% rate of charge (based on 20 Hour capacity). So:
• 100 watt panel * 0.77 panel+charge controller derating * 1/14.5 volts charging * 1/0.13 rate of charge = 41 AH minimum battery @ 12 volts
• 100 watt panel * 0.77 panel+charge controller derating * 1/14.5 volts charging * 1/0.05 rate of charge =106 AH maximum battery AH @ 12 volts

A battery's capacity is typically measured in Amp*Hours; Multiply that by the working voltage and you get Watt*Hours.

For a battery bank, the faster you drain the battery, the more internal losses there are. The more internal losses, the less stored energy (AH/WH) the battery appears to have (when discharged at high current levels).

For may reasons, we end up using the 20 Hour rate (how many amps does it take to discharge the battery in 20 hours). It is reasonably close to how a typical off-grid system operates and is relatively efficient at this current level.

The 5% to 13% of the 20 Hour Rate (Amp*Hour capacity) is based on several recommendations from various flooded cell battery manufacturers.

If you go below 5% (C/20), the battery may not mix the electrolyte or equalize well (plus there are self discharge losses from a resting battery, and how long it takes to recharge a battery--those also favor a 5% minimum charge rate).

The maximum charge rate of 13% is recommended because above that rate (~C/8) the battery becomes less efficient when charging and can overheat. Also many batteries will not take much more current than that--So having more panels becomes a waste of money (there are AGM batteries which can take more current, or if you have loads during the day like a washer or A/C unit--you can power those directly from the solar panels and still leave enough energy to continue charging the battery bank).

The 1/14.5 volts is the typical charging voltage for a 12 volt flooded cell battery bank.

The 0.77 derating is based on ~81% losses (PTC rating of panels vs STC) and 5% losses for the charge controller (plus misc. wiring, dirty panels, and other losses).

100 Watts is just an arbitrary value that people sometimes build the solar panel design at.

Also, typically for commercial panels, those at 100 watts or larger tend to be less expensive (\$\$\$/Watt) than the smaller panels. So, in English the following equation:
• 100 watt panel * 0.77 panel+charge controller derating * 1/14.5 volts charging * 1/0.13 rate of charge = 41 AH minimum battery @ 12 volts
• Solar panel Wattage * derating factor * 1/volts to convert watt*hours to Amp*Hours * 1/rate of charge such as 1/0.05 is the same as multiplying by 20 = recommended 20 Hour battery bank capacity for XXX watt solar panel
-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 24
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So I guess I can sum it up now.
I cannot hook up DIY solar panels to the grid, but I can hook it up to anything NOT hooked up to my house.
It would cost a lot more to go off grid than staying on grid.
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Yep,

If you want to save money (and go green), invest in conservation. The Kill-a-Watt meter is great for measuring your current 120 VAC 15 amp plug-in loads. Depending on the age of the equipment--you may find your refrigerator/freezers, computers, home entertainment appliances, etc. take far more power than the newest models.

There are whole house units like the T.E.D. too.

Insulating your home, double pane windows, high efficiency A/C or Heat Pumps, changing out the Compact Florescent Lamps, turning off stuff that is not used (power strips), etc. all can help a lot. Placing outside lights on motion sensors can save a lot of power too.

If you have never addressed energy usage before--It would not be unexpected that you could reduce your energy bill by 1/2... A few of us here have already done it.

Then, if you wish to look at solar--your solar PV system will only cost 1/2 as much.

Solar has its uses--but saving money for people on "cheap" grid power is usually not one of them.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 1,341 ✭✭✭
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If you want a good idea of what you can do with a hobby system, check out the web site of one of the forum members:

http://2manytoyz.com/

He's got a decent little solar system that powers up the outdoor lights on his house, and also a couple of plugs inside the house.

It's not grid-tie, and the stuff he powers with the system is separate from his house wiring, and he didn't use home made panels - but it's still a very cool hobby system.

He's not saving any money with it, but he does have some things that keep working when the grid goes down.
• Solar Expert Posts: 5,436 ✭✭✭✭
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Grid tie runs \$5-8 per watt installed.

Raw panel prices are as low as ~\$1.50-2.00 a watt depending on brand, vendor and quantity, which begs the question, why would anyone wish to build their own PV panels at those prices?

Tony
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I don't know...to me "Bootleg" implies stealing something or doing something illegal and sketchy. Building your own anything CAN be rather elegant and something to be proud of. Sure, there are some horrific home-brews but there are some real craftsmen out there. Some home made equipment is better than manufactured....SOME, not all.

From personal experience, UL or other agency stickers don't always hold water. I'm told fake stickers are showing up on some imports and the European CE sticker has a NEW meaning...ready for this one? "China Export"....and it looks a lot like the EURO CE sticker!

Here's another one...I'm told Euro manufacturers can "self-mark" a legit CE mark, proclaiming that their products meet the requirements of relevant European Directives. Really? Isn't that the fox guarding the hen house?

I am aware of other abuses of agency markings and the only REAL way to be sure of what you're getting is checking dealers and serial numbers and stick with name brands. An few unfortunate oversights of that homeowner who's panels caught fire! Of course those probably didn't have any marks, the mounts certainly weren't UL. As you know UL is not just electrical safety (perhaps some readers don't know they certify mechanical safety too).

So enough `mark-rant...back to DIY...
Somebody needs to perfect a DIY solar panel that is properly sealed on glass and meets UL94V0 flammability, just from a common-sense safety aspect. While I haven't done it yet, I believe it's possible. Is it cost-effective? It would seem that most of you think not, and you may be right. If I had room for ground-mounting 30-40 panels, I think I could cover building a vacuum sealer and several assembly fixtures.

I like a challenge.. Save even \$50/panel and that could be 2 grand. Those really cheap raw panel prices? Are those top-quality? Probably not but they have a mark, probably and can be grid-tied...so the challenge is a tough one.