How many 48V 100ah batteries will 3kw worth of solar panels maintain: 1 or 2?

[RonS]

I currently have 12 - 250W Canadian Solar panels that (at optimum which never happens) could produce 3kW / 448.8V / 8A in series.
Is this system big enough to support 2 - LiFePO4 48V 100Ah batteries, or would the 2nd be a waste of money? (I need to know before the Cyber Monday sales go away...=).

Thanks in advance!
Ron

 

[HighTechLab]

Think of it like a credit card. Your batteries are your credit limit and your solar panels are your income.

If you don't spend any money, it doesn't matter what your credit limit is or what your income is.

Start with your load. What are you trying to power?

 

[Delphinus]

It depend of how much power you use, 2 will provide 8.2Kw usable power probably will be short on cloudy days, if you can afford I would buy them.

 

[RonS]

We average 800kW/month, more in summer, roughly 27kw/day?

I guess I'm wondering whether the 3kw system will be able to charge 2 48V 100Ah batteries in a single day?

 

[SignatureSolarJames]

3kwX 4 sun hours is 12kwh per day, if you get 50% during the day for usage you have 6kwh for night, 2 batteries would be within reason

 

[Delphinus]

We average 800kW/month, more in summer, roughly 27kw/day?

I guess I'm wondering whether the 3kw system will be able to charge 2 48V 100Ah batteries in a single day?

I have 8 x 545W= 4.36Kw and an 9Kw battery and consume 32-35Kw winter and 25Kw summer it allow me to be on battery on low rate from 8AM-22PM and I have to fully charge the battery from grid on rain, cloud days and when there is sun I only charge it to 1/3-1/2.

Do your math how much you pay for KW and how much cost the battery KW for 5 years and decide on that.

One important note the more battery you have the less the stress, they will last longer and provide more power.

 

[12VoltInstalls]

important note the more battery you have the less the stress, they will last longer and provide more power.

3kwX 4 sun hours is 12kwh per day, if you get 50% during the day for usage you have 6kwh for night, 2 batteries would be within reason

LiFePo doesn’t care if they don’t 100% charge daily. So for what is described here;

guess I'm wondering whether the 3kw system will be able to charge 2 48V 100Ah batteries in a single

Id be of a mindset to use four batteries if I could afford to, or stretch for three. And just cut consumption a bit and you have some support after a cloudy day or two.

 

[RonS]

Unless I'm figuring wrong, I'd break even at 5 yrs. But if it makes the batteries last longer...?

4.8kw per battery per day (365) x 5 (yrs) x 14c per kw. Of course that's todays price and fuel costs are going to go through the roof, right?

 

[CoreyN3FE]

I am running 2kw worth of solar panels and have two 48 volt 100 Amp batteries, and yes, I can fill them! I just charged them totally Friday. I am glad I did as Saturday/Sunday we had very little sun and today (Monday) there was nothing! All it did was rain! I have to love the weather in the Northern Part of Pennsylvania in the Winter!

 

[RonS]

Nice to hear!

Do you have the batteries in series or parallel?

I know parallel doubles the amps and series doubles the volts. Some say series charges faster?

 

[Bud Martin]

Nice to hear!

Do you have the batteries in series or parallel?

I know parallel doubles the amps and series doubles the volts. Some say series charges faster?

Series or parallel does not matter, it will still be the same power capacity.
In series the Voltage will be higher but the current will be lower allowing you to use smaller wire gauge compared to parallel connection which has higher current so need bigger wire gauge which will cost more, and all the switches/breakers will have to be higher current rating.

 

[robby]

LiFePo doesn’t care if they don’t 100% charge daily. So for what is described here;

Id be of a mindset to use four batteries if I could afford to, or stretch for three. And just cut consumption a bit and you have some support after a cloudy day or two.

Four would be a bad idea if he is looking to use them at night to offset his bill.
He would be lucky to get them to 50% charge on most days, so if he is being prudent on his DOD he would not go below 20% SOC and that means pulling only 30% per pack or 1530W x 4 =6120W.
With two packs reaching 100% and using 80% of the charge he has 4080w per pack or 8160W of power to work with at night.

 

[SamDeleted]

4.8kw per battery per day (365) x 5 (yrs) x 14c per kw. Of course that's todays price and fuel costs are going to go through the roof, right?

We're at £0.34p a KW here in the UK from the grid . Up from £0.15p a year ago.. Running my LPG (propane) generator only costs £0.41p per KW .

Prices like that make solar worth while!

 

[SeaGal]

Four would be a bad idea if he is looking to use them at night to offset his bill.
He would be lucky to get them to 50% charge on most days, so if he is being prudent on his DOD he would not go below 20% SOC and that means pulling only 30% per pack or 1530W x 4 =6120W.
With two packs reaching 100% and using 80% of the charge he has 4080w per pack or 8160W of power to work with at night.

Sorry... don't understand your maths. Whether you have two or four battery packs, you'll only get out what you put in.

So, if you start with suggested min SOC of 20% and add x kWh of power in the day, the batteries will increase to a new SOC that represents the power added. e.g. let's say you had 2 batteries that reached 80% each for given input power (i.e. an increase of 60% each) - but if you had 4 batteries, each one would get an increase of 30%, bringing the overall SOC to 50%. You can then discharge back to 20% and (less conversion efficiencies) get the same back irrespective of whether you had 2 or 4 packs.

The points to consider with only 2 battery (compared to having 4) packs are:-
a) if you hit or get near 100% SOC, you will not be able to store any more power - in fact, many systems will reduce the charge current above 90% SOC so you will start to waste generated power above that SOC.
b) less longevity through greater DoD
c) less longevity through greater charge and discharge rates

 

[onobeka]

To the OP, did you try to run this calculator? For Europe, this is the link: https://re.jrc.ec.europa.eu/pvg_tools/en/tools.html

There is an Offrid tab. You could try to run multiple simulations, based on the consumption for a day in Wh. When you get the results, there is a tab in there called "Performance". For my location, Romania, if I would a consumption of 4kWh a day (which is much less than what you've mentioned), two months will be with empty batteries in average, which looks pretty bad.


With a 4000Wp solar, the situation would be like this, but keep in mind with a 4kWh daily consumption, something like this is maybe manageable.

 

[SeaGal]

Good link - didn't spot the battery performance spec available on that site before - thanks.

IMHO there's also a lot more benefit from batteries - depending on your personal situation. As well as the 'charge-up-in-day-discharge-at-night' scenario, if you live in an area which has odd clouds on sunny days, they can even out the cloud cover - so your washing / cooking etc., won't suddenly go over to grid usage, just because a cloud got in the way.

Also, in the UK and, I assume, other counties, we can charge up at night at a cheap rate. So in the Winter months, when there's not enough sun to charge your batteries, you can charge them up at night and then use that power until midnight, even on a cloudy/rainy day and avoid peak electricity tariffs.

 

[onobeka]

Wouldn't then be better that an grid-tie offgrid inverter is used? Then at night, at cheap rates, the inverter would feed the loads with AC from the grid, instead of charging the batteries and the batteries feeding the loads. At least a conversion is skipped. This is under the assumption that during the day the sun can charge batteries and provide power to the loads - which in this case may not work.

 

[12VoltInstalls]

Four would be a bad idea if he is looking to use them at night to offset his bill.
He would be lucky to get them to 50% charge on most days,

That’s assuming he uses all the full capacity.

I was assuming the extra capacity just being there as backup for more than one day’s use. I wouldn’t use all of it every night.
With flooded lead acid that makes no sense or less sense than with LiFePo cuz LiFePo can sit for weeks at 50% and come to no harm.
That’s the main reason my second system is LiFePo- Nov and Dec kills lead acid batteries ?

 

[RonS]

I decided to pick up the 2nd battery.

Thanks for all the input, guys!! I appreciate it!

And, I'm glad I could inspire a such lively conversation... ?

 

[Delphinus]

I decided to pick up the 2nd battery.

Thanks for all the input, guys!! I appreciate it!

And, I'm glad I could inspire a such lively conversation... ?

Well done, I'm also new here, those guys can't keep it simple