Calculations Solar Power/ batteries

[Woutah]

Hi all,

we have some failing old equipment, so time to start thinking of a new off grid system (this time, "designed" by us).
As we need to get this right (as much as we can), please find our calculations below and let us know what you think! (We followed https://www.solarpaneltalk.com/forum...s-why-tutorial)

To make our situation more straightforward, we can split our year in 2 halves
A. "Summer" -6 months of clear blue sky - min 5 sunhours
B. "Winter" - 6 months of often clear blue sky, but only 3 sunhours

1. We use 3500 W (= definitely the maximum!) a day. Times by 1.5 (for efficiency losses) => 5250 W/day
2. 3 days capacity needed .Times 2 (to avoid DoD> 50%) => 6 days
3. 5250 W x 6 days => 31500 Wh
4. A. Summer -> 5250W/ 5 sunhours => 1050 W panels needed
B. Winter -> 1800 W panels needed.
For option B: we're thinking of installing pico hydro. Without technical issues, this should give us a guaranteed power of min 100 W/h (2400W/day) during the 6 months of winter.
Therefore our conclusion is that the 1050 W (5250W - 2400W divided by 3 sunhours = 950W) of solar panels is sufficient. We assume hydro would be very good for the life span of the batteries??

Based on the above, we definitely need more than a 12 V system.

5. Battery amp hours capacity -> 31500 Wh / 24 V => 1312.5 Ah
or 31500 Wh/ 48 V => 656.25 Ah

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Q1: First of all, are the above calculations correct in your opinion. If not, where have we gone wrong.
Q2: Should we go for 24V or 48V batteries
Q3: Included in our daily (3500W) usage is the use of a device which pulls 1200W for 1 hour, continuously. Would the batteries from #5 cope with this without any problem (DoD etc)
Q4: Don't react too emotional people, see the disclaimer below. We want to hear from the pros what's possible and what not!
We have a washing machine which we would like to use without the generator. It's an energy efficient A+++ machine that will use very little wattage per cycle BUT, when washing on 30 degrees C, it will have two surges of about 2000-2200W for about 10 mins, in it's 1h/ 1.5h cycle.
We want to see if we could make this work without (huge) additional costs...

I look very much forward to hearing from you!

Woutah


Disclaimer: yes, we bought the washing machine before we had any idea about off grid systems. We're looking for the most sensible/ economical solution going forward!

[Sunking]

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Q1: First of all, are the above calculations correct in your opinion. If not, where have we gone wrong.

Let's make it simple and answer only 1 question. Batteries require a minimum charge rate of C/10 where C is the battery specified Amp Hour Capacity and the number 10 is charge rate in Hours. So if you had say a 625 AH battery would require a minimum 62.5 amps or just call it 60 amps. Simple enough right. Now here comes the fun and educational part you are going to just love the simplicity once you know that fact. .

To find the panel wattage required is real simple and fun math. All you need to know is the Battery Voltage and Charge Current. So you have 48 volts and 60 amps right? 48 volt battery charges at 50 volts.

Wattage = 50 volts x 60 amps = 3000 watts.

Selection of battery voltage is stupid simple. Run it as high as possible to use the smallest controller possible to save your wallet from extreme pain and suffering. About the largest controller you can buy is 80 Amps, and controllers have maximum Wattage Input vs Battery Voltages of:

1000 watts @ 12 volts
2000 watts @ 24 volts
4000 watts @ 48 volts
8000 watts @ 96 volts.

So your only choices are 48 volts and higher. If you went with 24 volts, you would need a 1200 AH battery with 120 amps of charge current which would require two very expensive 60-amp controllers. At 12 volts would require a 2400 AH battery and 3 extremely expensive 80-Amp controller. So where you went wring is going off grid. Interesting to note regardless of what battery voltage you use still requires 3000 watts ha ha.

Now wasn't that fun and educational as promised? I bet your wallet is thrilled.Solar is super fun to learn about. FWIW you only need 5 days of reserve capacity which is a little less painful, but do not forget either way a Generator and charger are mandatory equipment for off grid. You will need at least a 5 Kva genny. Inverter size no larger than panel wattage.Battery should not cost you one penny more than $4,500 this time around and more in 5 years at replacement time.

[Mike90250]

Also as important as sun hours, is how many cloudy days do you want to go before needing to run the generator ? Start generator at 50% depleted batteries.

Yesterday, i harvested 800 watt hours in clouds, with 5Kw of PV . I ran the generator for about an hour, and pumped 2.8Kwh into the batteries..
3 days before that, I harvested 13.6kwh with the PV on a cold sunny day. You will need to have a generator.

[Woutah]

As usual, thanks for the advice guys This already answers a couple of questions (and raises some more questions)...
So 3000W in panels it is, but have you taken into consideration the pico hydro turbine which will produce 6 months, every day, all day (cloudy or sunny) 2.4KW @ 100W/ hour? Or is it useless and do we need the 3000W panels, end of story? We were hoping that the turbine could help a lot in winter, as it's constantly charging the batteries, adding 2.4KW ever day
During the other 6 months of the year we have a lot of sunny days and a minimum of 5 full sunhours per day. Again, this doesn't matter? We really need 3000W per hour? (It's important, so apologies I asked more than once already )

What about the hoover and washing machine? Can they be used with the suggested setup?

cheers!

ps (edit): we thought our calculations for the panel wattage were the same as on https://www.solarpaneltalk.com/forum...estions-please
In the explanation above you used a different formula to calculate the panel wattage, how come?

[Sunking]

This already answers a couple of questions (and raises some more questions)...
So 3000W in panels it is, but have you taken into consideration the pico hydro turbine which will produce 6 months, every day, all day (cloudy or sunny) 2.4KW @ 100W/ hour? Or is it useless and do we need the 3000W panels, end of story?

You gotta figure that out. You need 60 amps to charge, just how many amps does 100 watts contribute? Answer around 7 amps. Now if you used a battery type like lithium with no minimum requirements is a different story, but explain it to your wallet. Lithium would cost you 4 to 6 times more cash and does not buy you extra cycle life so every 5 years or so you lose 4 to 6 times more money.

On the bright side could keep your batteries topped off after sunset when the batteries are in Absorb. Kicker is Bulk you really need C/10 to C/12 charge current. You could probable get away with 2500 watts @ 48 volt battery and a 50-amp controller.

We really need 3000W per hour?

You will never see 3000 watts. About 90% of that for a few minutes around solar noon. That is why in summer despite having full sun for 14 hours per day only works out to 5 Sun Hours.

What about the hoover and washing machine? Can they be used with the suggested setup?

That is another reason for th eGenny so you can run high wattage loads. Run genny early in the morning, and was at noon.

In the explanation above you used a different formula to calculate the panel wattage, how come?

Because you use worse case. After you do the calculations with sun hours, you then calculate minimum charge current requirement of the batteries.

Edit Note:

Now here is the good news I just caught another mistake you made. To determine Battery AH = Daily wh x 5 / Battery Voltage

3500 wh x 5 days / 48 volts = 364 AH @ 48 volts.

That brings minimum panel wattage down to 36 Amps x 50 volts = 1800 watts

Technically you could go 24 volt battery @ 720 AH, but do not do that because it would require a 80 Amp Controller. At 48 volts would only require a 40 amp controller minimum, but I would recommend a 60 amp controller to allow growth up to 3000 watts down the road.

What you did is apply apply panel wattage to battery capacity by multiplying daily load by 1.5. I just saved you at least $5000.

[Mike90250]

.... have you taken into consideration the pico hydro turbine which will produce 6 months, every day, all day (cloudy or sunny) 2.4KW @ 100W/ hour? Or is it useless and do we need the 3000W panels, end of story?.......

A friend of mine has a pico turbine. it freezes up with ice, clogs with debris, he's about to give up on it, when he needs it, it's non-functional

[J.P.M.]

Also as important as sun hours, is how many cloudy days do you want to go before needing to run the generator ? Start generator at 50% depleted batteries.

Yesterday, i harvested 800 watt hours in clouds, with 5Kw of PV . I ran the generator for about an hour, and pumped 2.8Kwh into the batteries..
3 days before that, I harvested 13.6kwh with the PV on a cold sunny day. You will need to have a generator.

Mike: A bit off topic, but for FWIW, yesterday I harvested 3.55 kWh from a 5 kW system under complete overcast. Because it was all overcast, the POA irradiance was about the same as the GHI at ~ 867 W/m^2 for the day, or 22.62 for the array. System eff. for yesterday was thus about 3.55/22.62 ~ 0.157.

The day before was cool and completely clear. harvest for that day was 22.95 kWh. GHInsolation for that day was 3,417 Wh/m^2, giving a POA insolation for the day of 4,743 Wh/m^2, or 123.75 kWh for the entire array, giving a day long system efficiency for that day of 22.95 kWh/123.75 kWh = 0.1855.

Perhaps an example of low light roll off of panel efficiency.

[Woutah]

Now here is the good news I just caught another mistake you made.

I believe your alter ego made the same incorrect calculation in https://www.solarpaneltalk.com/forum...s-why-tutorial! , so, which Sunking is right?

Anyway, I understand the panel wattage (even when using pic hydro) and we know what type/ capacity of battery we require, so happy days!