# wattage chart or calculation advise



## stampee (Jan 31, 2013)

Hi all: I am just getting in to this field. I have two 70W panels I will be building and a 1500W inverter on the way for learning purposes, and hopefully as I get money, I will be adding solar panels and wind in the future.
My question is where is a website or link to help me calculate how much wattage, or amp hours I will need overnight from my system out of the battery?
For example, if I use 6) 13W CFL lamps for 6 hours non stop in the evening, that is 78W, figure 100W needed to be safe.
then that is 100W I need out of the deep cycle battery?
A 100 amp/hour deep cycle battery should provide around 1200W or amp hours power then? yet I only needed 100W so I should have quite a surplus of battery power by this calculation? ( I understand I will have line losses and inverter losses etc)
Also just because it says 100 amp/hour that does not really mean I can draw 100 amps for an hour straight does it? I think it means I can draw 20 amps for 5 hours, correct, or some multiple of that? So in the case of my 1500W inverter all things being perfect no line losses or inverter losses etc the inverter would be drawing 125 amps from the battery at any one time, so I would get maybe 45 minutes from the battery at full amp draw?
Thanks for the advice and pointing me in the correct directions.
-Stampee


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## wy_white_wolf (Oct 14, 2004)

stampee said:


> ....6) 13W CFL lamps for 6 hours non stop in the evening, that is 78W, figure 100W needed to be safe.
> then that is 100W I need out of the deep cycle battery?
> 
> That's only 1/2 of the calculation
> ...


See above


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## stampee (Jan 31, 2013)

Thank you for the response. I think I understand more now. It seems like a person would need half a dozen 100 amp hour batteries then to do anything marginally useful with his power. If I am understanding this correctly.
-Stampee


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## TnAndy (Sep 15, 2005)

That would be about right. I have a 1200amp/hr bank, and I figure it's good for one day at significantly reduced use from our normal.


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## wy_white_wolf (Oct 14, 2004)

stampee said:


> Thank you for the response. I think I understand more now. It seems like a person would need half a dozen 100 amp hour batteries then to do anything marginally useful with his power. If I am understanding this correctly.
> -Stampee


That's right.

No apply what you've learned to your solar panels.

2 - 70 watt panels and a 1500W inverter doesn't make much sence.

As a side note building your own panels is only useful as a learning experience. You can not build them to last as long a factory panels and with how the price has come down, it's even hard to build them for what they cost.

WWW


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## GregYohn (Jan 24, 2013)

Adding wind is not so easy. The pole can cost as much as the wind generator. You might even have a location with low winds too. It also has higher maintenance costs.


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## stampee (Jan 31, 2013)

Thanks to everyone who responded. I do appreciate it.
-Stampee


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## Nimrod (Jun 8, 2010)

I am a newbie too so please excuse my ignorance. The OP wants to run 6 light bulbs. I gotta ask why? does he really need that much light in one place? One at at time would be all I would need.

White wolf wrote;
2 - 70 watt panels and a 1500W inverter doesn't make much sence. 
Why not? Say you wanted to run a big load for a short period of time like an 1100 watt microwave for 3 minutes. You would need the big inverter for this. I would need to do the math to see how long 2 70 watt panels would take to recharge the battery, but isn't this do-able?

Back to the OP's original problem. Isn't the real question how much power can 2 70 watt panels hooked to a 100 amp hour battery provide, per day, over the long haul?

I thought you could repeatedly discharge a deep cycle battery up to 50% of it's rated capacity without shortening it's life. You guys say 20%. Have I been misinformed?


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## TnAndy (Sep 15, 2005)

Nimrod said:


> I thought you could repeatedly discharge a deep cycle battery up to 50% of it's rated capacity without shortening it's life. You guys say 20%. Have I been misinformed?


All batteries have a number of cycles you can charge/discharge them.....you may, or may not, be able to find that info on any particular battery, but from the ones you CAN find it for, you'll see a graph chart. 

In it, 20% DOD ( depth of discharge ) will allow, say, X number of cycles. 50% will give you maybe 1/2-2/3 of X, and 80% will give maybe 20%.

The simple rule of thumb is: The deeper you discharge, the less number of times you can do it.


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## stampee (Jan 31, 2013)

I do not need 6 light bulbs all at once. I was only trying to figure worst case scenario. In my cellar/work shop I have 6 bulbs and even that is not enough light sometimes. You are correct I probably would not have them all on for 6 hours straight. I was just trying to figure the worst case.
I found a good deal on a 1500 Watt inverter so wound up with that. I do want to make coffee or toast, so do need that much wattage for those items. Mismatched solar and inverter is a misnomer. Am I supposed to buy a 200 watt inverter matched to solar panels, then install larger solar panels and have to buy a 300 watt inverter, then a year later install a 600 watt inverter ad-nauseam? And throw away all the other inverters?
Some of us do not have money to burn the way that most of you have.
Using a battery for only 20% of its capacity? Why use batteries at all at that rate?
next I will be told to only use a battery for 1% of its capacity and it will last 100 years.
Once again, I do not have money to burn, installing 15 batteries just so I can use them at 10% of their rating.
-Stampee


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## TnAndy (Sep 15, 2005)

On batteries, it's about matching your loads to your battery capacity so you don't excessively do a depth of discharge TOO often.

You can't match it EXACTLY, nor can you predict with total accuracy your solar input, and so on......that's why you monitor the battery bank with some kind of meter, and either quit using it at some DOD, or fire up a generator as backup on extended cloudy periods.

Can you get by with a real small battery bank ? Sure.....but then you ARE burning money, because you'll have to replace the batteries far more often. The depth of your checkbook and the laws of physics sometimes conflict.


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## GregYohn (Jan 24, 2013)

You better get a pure sine wave inverter or else you risk blowing your microwave. Had 2 go on me using grid electricity. Modified sine wave inverters work, but over heat motors and can spike electronics.


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## wy_white_wolf (Oct 14, 2004)

Nimrod said:


> I am a newbie too so please excuse my ignorance. The OP wants to run 6 light bulbs. I gotta ask why? does he really need that much light in one place? One at at time would be all I would need.
> 
> White wolf wrote;
> 2 - 70 watt panels and a 1500W inverter doesn't make much sence.
> ...


Batteries have minimum and maximum charge/discharge rates.

Maximum discharge rate is usually around C/2. So a 1500W inverter could pull 125A at 12v. That puts the minimum recomended battery bank at 250AH.

Minimum charge rate is usually around C/12 to fully and properly charge them. So a 500AH battery would need at least 21A to properly charge. 2 - 70W panels will only supply about 7 to 9 amps.

Discharging below 50% you start doing irrevesable damage to batteries. Battery lifetimes cycles are less than 1/2 at 50% as compared to 20%. It's one of the trade-offs as a Bank to stay at 20% will cost about double the one for 50%. Plus the 20% will give you 2.5 days atonomy before it hits 50%.

Here's an article on life span/cycles verses DOD. Look at the chart. 50% discharge - 1000 cylces. 20% discharge - almost 2500 cycles.

WWW


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## Nimrod (Jun 8, 2010)

Thanks for the info. I didn't know there was a minimum charge rate.


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