Magic Ratio? Watts to Ah

[Sr WiNdTeCh]

Is there a "magic ratio" between watts of solar to AH of Battery? I've been reading a lot and all I find is people guessing on what hey need. I'd like to know if anyone has come up with anything better than a guess.

I have 4 125AH 12v batteries wired series parallel for 24v and 120 watts of Solar, I may use 500WH a day, and assume 5-6 Peak hours of rated sunlight. I should be producing around 600WH average a day...

So would I need more solar or more battery and why.... and how did you come up with that?

[dnix71]

If your batteries hit float during the day the mix is correct.

If your batteries go too low at night, then you need more of both, or load shift to daylight hour when your are hitting float.

If your batteries don't hit float on a sunny day in the spring or fall, then you need more solar.

[wpowokal]

Your 24v system has 250A/h rating, you use 0.5 Kw/day that's about 20amp hours into an inverter.

Your solar panel is 120 watts, the name plate rating is rarely achieved, but.... at a battery voltage of 25V that 120 watts is 4.8 amps x average 5.5 hours sun = 26.4 A/hs.

Your daily draw is within 10% depth of discharge, for flooded lead acid batteries the charging efficiencies are average 91% overall from 0-84% state of charge (SOC) but 79-84% SOC is only 55% efficient so your charging is somewhat less than 50% efficient. This does not take into account usage while there is incoming charge.

Read http://photovoltaics.sandia.gov/docs/BattIntro.htm

In summary you need more solar panels. And there is a ration between battery size and inverter size, if you are using an inverter. Inverter rating in watts / battery volts X 3 minimum!

Hope I made a mistake somewhere so others will chime in.

allan

[independent]

I read an article on Homepower that used to be on the front page as a free sample. In it it had an elderly couple with several kW of solar (I think 5). Their battery bank was 600ah at 24v. So not too big. They rarely drew down their battery bank below 25% and their batteries lasted a very long time. Whenever they had spare money they'd buy some more solar. Sounded like a good way to do it to me

[Sr WiNdTeCh]

I'm using a 1800watt Xantrex Prosine inverter... So 1800/24*3= 225ah my bank is 250ah

You say I use around 20ah a day, and make around 26ah a day... seems to me as long as I don't use anymore power then I have the ratio just about perfect. Or did I miss something?

Thank you, very informative!

[ghurd]

I has to be a guess.

If the system is to power a calculator in a 9 to 5 office, the batteries can be small because the sun is sort of semi-directly powering the system.

If the system is a security light with a daylight sensor, then the batteries have to hold all the power used.

Irrigation pump for a small garden can be a small battery, because most people don't water the garden in the rain (cloudy), or low solar hours (winter).

Something to consider is how the battery is regulated.

Just because a 100AH battery is half full, and the PV supply 10A for 5 hours that day, does not mean the battery will accept all the amps.

The battery voltage gets up to regulation, then the charging amps are reduced by the controller.  Too much panel quickly gets to regulation voltage, then a lot of potential power is not harvested for use.

Generally, I believe 20:1, or 5% is good.  5A of solar for 100AH of battery.

Anyone who ever put a 85W PV on a 100AH 12V battery will tell you it gets up to regulation voltage in a pretty reasonable time frame.

And even 3-4% is still not too bad, (50-60W PV on a 100AH 12V battery) but it does depend on the load's timing.

G-

[wpowokal]

I suspect you are still missing the point about charge efficiency, however what dnix71 said will tell you if you have enough.

allan

[Sr WiNdTeCh]

I understand the charge efficiency, but I'm asking if there is some magic ratio, like If you have 500ah of batteries then you should have 500 watts of solar or 200ah then 200 watts or something like that, just so that way no matter if you use 80% of your battery bank at night you will still hit float by the end of the day... just some ratio to help new people (me) to solar, instead of well just buy some stuff and see what you need to do from there.

[Bruce S]

Nope no magic ratio, even schools teaching Solar installs don't have one.

Most schools now are teaching grid-tie so they don't have to figure this out.

One way to figure this out would be take the total load you would have, make sure your battery bank can handle this without taking it below the 20% drain-max for Acid-based batteries, then with the total average solar input make sure the worst day will get your batteries backup to full/float. This minus battery self-discharge is a very good place to be.

My batteries are NiCds so my discharge is a bit higher than Acid-based, but then again I can drain them down to 80% empty before worrying.

Hope this helps

Bruce S

[Rob Beckers]

As Bruce mentioned, there is no magic ratio (unfortunately, as it would make life much easier). It all depends on the size of the batteries, and your daily loads. The usual is to size batteries such that you can run on them for 3 days without any RE input for recharging (a winter storm), at 50% discharge (if you want any lifetime out of those batteries, beyond 50% DOD the number of cycles you get out of batteries starts to nose-dive). Under 'normal' conditions you want to be able to daily produce as much energy from solar/wind/hydro as it takes to fill up what is used that day. However, it is often prohibitive to scale this to the worst mid-winter day (that is where a small generator can be a life saver), and those 'normal' days rarely happen. That is more or less the standard. Every situation is different, as are budgets, so there are at least as many variations on this theme as there are following these rules-of-thumb.

The last issue of Home Power Magazine had a very good article on off-grid system sizing, that went quite in-depth; "Off Grid Design" page 78, Apr-May 2010 issue.

-RoB-

[XeonPony]

My personal sizing strategy is: every load added up size the solar to run every thing by its self at 75% cell loading then multiply by 2.

So if every thing that will be run at once draws 6amps, then make sure you are generating 15A of solar. that way there is plenty to charge the batteries while running the load during the day! As a plus you aren't stressing the cells much.

[edit]

Keep in mind I live 100% off grid and cooking and heating is don through either gas or wood, and every thing is native 12v, so I don't have things like stoves or microwaves to run.

My system used to be 120w panel mated to a 240AH battery bank and my average continuous load was around 3amps a day and 1.2 amps all night. and the battery stayed pretty full all the time

[jerrywc]

Hello friends,
   Once upon a time I was talking with someone knowledgeable, I don't remember who, about sizing the system for our bus. They said I should have 1 amp-hour of batteries for each 1 watt of solar panels. Thats what I did. I have 1580 amp-hours of batteries and 1580 watts of solar panels. Giving me 790 usable amp-hours of batteries. I have been using this system for about 6 years. On a perfect day the batteries are full at about 1:30. On a cloudy day they are full by about 4:00. On dark rainy days I can last a week or two by cutting back on usage. Then I have to run my 120 amp charger. Thats a 10cf electric refrigerator, doing a load of laundry, baking bread (bread machine), Computers, tv, killer music, ham radio, satellite internet, electric coffee maker, toaster, etc, etc. In the afternoon when the batteries are full I use my electrical tools, skill saw, drills, grinders, etc. and other heavy loads without touching the batteries. I spent many years with too few panels so My goal here was not to have to think about electricity if the sun was shinning. So far so good. We also have a 400 watt Air-X.
I'm not sure if there is a correct formula. BUT, the weak link is always the ability to recharge. So get as many panels as you can afford. I wouldn't worry about ending up with too many panels. They cost way too much for that to happen.
Good Luck
Jerry
'75 Crown Coach


Edited DamonHD amp --> amp-hour

[DamonHD]

I suspect you mean amp-hours rather than amps?

Rgds

Damon

[jerrywc]

You are correct. That is exactly what I meant.
If I could edit the post I would add the word hours.
Perhaps you could do that.
Jerry

[DamonHD]

Done!

Can you see the 'modify' link above your posts, at least when you have just made them?

Rgds

Damon

[wdyasq]

Something I DON'T see mentioned is the loss of Watts due to over-voltage of the panels.

Example:

120W of 12V panels deliver 6A @ 20V.

Your battery accepts the 6A @ ~12V (Varies and we could actually throw a bit of Calculus in here and get closer, but it won't really matter.) so of that 120 Watts, the battery gets 72 Watts. AND, with a battery being ~85% efficient we will get about 60 Watts out.

An MPPT can raise that but one will NEVER get the 120W produced out of the Battery bank.

Ron

[ghurd]

They said I should have 1 amp-hour of batteries for each 1 watt of solar panels. Thats what I did. I have 1580 amp-hours of batteries and 1580 watts of solar panels. Giving me 790 usable amp-hours of batteries. I have been using this system for about 6 years. On a perfect day the batteries are full at about 1:30. On a cloudy day they are full by about 4:00. On dark rainy days I can last a week or two by cutting back on usage. Then I have to run my 120 amp charger. Thats a 10cf electric refrigerator, doing a load of laundry, baking bread (bread machine), Computers, tv, killer music, ham radio, satellite internet, electric coffee maker, toaster, etc, etc. In the afternoon when the batteries are full I use my electrical tools, skill saw, drills, grinders, etc. and other heavy loads without touching the batteries. I spent many years with too few panels so My goal here was not to have to think about electricity if the sun was shinning. So far so good. We also have a 400 watt Air-X.
I'm not sure if there is a correct formula. BUT, the weak link is always the ability to recharge. So get as many panels as you can afford. I wouldn't worry about ending up with too many panels. They cost way too much for that to happen.
Good Luck
Jerry

Edited DamonHD amp --> amp-hour

Meaning a 100W panel on a 100AH battery?
A 100W panel makes about 5.7A?  Which 5.7% of 100AH, which is pretty close to my 5% rule of thumb.

Like Ron said, With solar, it is easier to think in Amps than watts.
G-

[hydrosun]

100 watts solar into 100 amps battery would only work with a 12 volt battery.  Otherwise you need to compare watts to watts or amps to amps.
But like others said, each system needs to look at loads and time of use to determine the storage needed.  I've got way more panels than battery storage (2500 watts solar, 400 amps @24 volts battery so about 3 times the ratio above)  but I have automatic switches to turn on optional loads when the batteries are full. When house heating is needed a heat pump is the primary optional load. A water heater element is pwm for fine tuning of the voltage.  In summer a valve is opened to turn on a sprinkler which is supplied by a well pump. By making large loads optional the solar can run them directly without the need for battery storage. When there isn't extra power the sun can still  quickly fill the batteries ensuring they spend more time in float.
 So for me a smaller battery bank is likely to last longer.  To me a battery is a consumable part of the system that has to be periodically replaced. I'd rather put my money into solar panels that actually produce energy not consume it without benefit like an over sized battery bank.
Chris

[tecker]

 This is a bit esoteric buuuut  I know there's a way to manage the banks as an impedance .The impedance changes as the ions exchange and heat rises and falls with the load demands and again when charging . The charger has to be large enough to take over the projected load and include the batteries as load . For solar this requires some boosting controller over line voltage devices and or low voltage replacements ( that's from the stand point of a standard household ) In reality It's probably a mixed bag of transfers with some active monitoring .
 Which brings me to my current focus . Typing all the loads in use and separating the resistive loads from more complexed impedances  .So the over all equation could be for total impedance and usage  rather than a crapshoot based on AH       

[B529]

I shoot for 20-30:1 ratio. 

Example 1000 amp hour bank, 35 to 50 amps PV charging.

My 5yr old very healthy bank is 1140 amp hrs @ nominal 48 volts, PV charging current is about 40 amps. I have a Proven 2500 watt turbine, did not take it's current into consideration when sizing the battery bank, the turbines current is a bonus.

Then obviously the charging current/battery capacity should be determined by power used.

[Ungrounded Lightning Rod]

Can you see the 'modify' link above your posts, at least when you have just made them?

Doesn't that go away as soon as somebody else has posted?

Like, for instance, to tell them that there's something they ought to edit?  B-)