Battery Sizing

[music]

Hello everyone!
I keep on planning on my house's solar setup.

I did a spreadsheet, to add the loads I would like to run.
I don't know if I did the exercise correctly, yet I would like your help with it.

For simplicity, lets say I will have only two loads (for simplicity's sake).
If the math works out, and you guys correct me, then I can work out the spreadsheet with more appliances.

#1
Lets say I have two appliances:
24,000 btu a/c unit - 2,200 Watts - 8 hours per day
refrigerator - 1,411 Watts - 8 hours per day (trying to average the on/off cycles)

#2
So I have
a/c 17.60 kWh/day
fridge 11.29 kWh/day

#3
Which sum up: 28.89 kWh/day.
Multiply that guy by 1.5 for a buffer margin: (28.89 x 1.5) = 43.33 kWh/day

#4
Which is 43,332.00 Total Watts / day

#5
Using a 48v battery bank:
43,332.00W / 48v = 902.75 Amp-Hours


Is this correct?
902.75 Amp-Hours

Wondering about my math... first time I do this!

Thank you very much for your help!

[Bala]

In your introductory thread and other thread you are talking about grid connect systems, no batteries

Here you a talking about an off grid system with batteries.

The KWH a day you are talking about is a MASSIVE amount for an off grid system.

[music]

In your introductory thread and other thread you are talking about grid connect systems, no batteries

Here you a talking about an off grid system with batteries.

The KWH a day you are talking about is a MASSIVE amount for an off grid system.

Thanks for the help!
I started a new thread because I didn't want to mix the Sunny Boy and Sunny Island discussion with my battery question.

And clearly I didn't express / understand my self correctly:
I want to run grid-tied.
Not off-grid.

Yet I though of charging the batteries during the day, and consuming the energy during the night.


Good thing you tell me the battery capacity is massive.
I thought something was off.

Is the math right, then?

[Bala]

Having batteries is not economical

Here is a section from the off grid sizing sticky. http://www.solarpaneltalk.com/showth...Battery-Design

Run your math again,

5 DETERMINE BATTERY SIZE

Determining battery size is very simple. All batteries will last substantially longer if they are shallow cycled. That means discharged only by about 20% of their capacity in a given day. Whereas deep discharge means that a battery is discharged by as much as 80% of its capacity. Second point is no lead acid battery should ever be discharged more than 50%. Below 50% soft lead sulfate crystals harden on the plates which reduce capacity and shorten battery life substantially. So with this said the battery capacity is calculated to be 5 full days. So in real application you have 2.5 days of usable capacity to allow for cloudy days before reaching the 50% discharge.

To figure the daily load, go back to the original load number before the fudge factor—that is, 6000 watt hours. Battery capacity = Daily Watt Hours x 5. So we need 6000 watt hours x 5 days = 30,000 watt hours or 30 Kwh.

Now that we have the battery capacity in Watt Hours we need to convert to Amp Hours. To find the Amp Hours we need to select a battery voltage. Amp Hours = Watt Hours / Voltage. To select battery voltage is based on panel wattages vs controller size. MPPT charge controllers have maximum panel wattage input vs the controller’s current rating in AMPS. MPPT controllers typically come in 20, 40, and 80 amps. So selecting battery voltage is very important. As a general rule you want to run the battery voltage as high as economically possible.

[Sunking]

Is this correct?
902.75 Amp-Hours

No but close for 1 day. You do not multiply by 1.5 for daily load for batteries. That is for panel wattage.

Air Conditioning = 2.2 Kw x 8 hours = 17.6 Kwh
Fridge = 1.4 Kwh x 8 = 11.2 Kwh = 28.8
Total = 17.6 + 11.2 = 28.8 Kwh in a day.

At 48 volt battery requires 28,800 / 48 volts = 600 AH.

However it takes a 5 day reserve capacity to give you 3 days of cloudy weather and maximize battery dollar value or cycle life which brings your to 5 x 600 AH = 3000 AH @ 48 volts. That battery weighs 9000 pounds and cost $35000 every 5 years.

That is going to put a strain on your wallet and spine. . They weight of the money in your wallet or batteries is going to hurt. Not like sex where it hurts so good.

[Bala]

I sort of made an error, Op was talking for grid connect to charge batteries during the day and use that power at night.

Not for an off grid system.

[Sunking]

I sort of made an error, Op was talking for grid connect to charge batteries during the day and use that power at night.

Not for an off grid system.

That would be very foolish.

[Bala]

That would be very foolish.

Yes I mentioned it is not economical.

[SunEagle]

Yes I mentioned it is not economical.

Anytime the word "battery" is used in a scenario to provide power for home appliances it should be considered NOT ECONOMICAL.

[music]

Awesome feedback everyone.
This has been eye opening.

The following question, I ask because I really don't have an idea:
Clearly, such a big battery bank is not "cheap".

Considering my utility will buy a kWh at 0.10$/kWh.
I buy it from them at 0.28$/kWh

What would you do?

I mean, the solar panel system makes sense... yet the batteries, ouch!

What would you do?
Feed the grid during the day, and buy from them at night?

Thanks for the feedback!

[SunEagle]

Awesome feedback everyone.
This has been eye opening.

The following question, I ask because I really don't have an idea:
Clearly, such a big battery bank is not "cheap".

Considering my utility will buy a kWh at 0.10$/kWh.
I buy it from them at 0.28$/kWh

What would you do?

I mean, the solar panel system makes sense... yet the batteries, ouch!

What would you do?
Feed the grid during the day, and buy from them at night?

Thanks for the feedback!

I know it might not make sense to sell your power low and buy it back high but if it was my money and was economically a good decision for me I would install a grid tie system that reduced my electric bill and be happy I received some type of credit for my excess power. There is still a ROI with that action for most people.

Adding batteries just about eliminates any ROI even if you were getting $0.0/kWh for exporting your excess.

[Mike90250]

Awesome feedback everyone.
...
Feed the grid during the day, and buy from them at night?..

Yes, simple grid tie is the cheapest battery you don't have to add water to.

Your solar panels put every harvested watt into the grid, unwinding your meter for 6 hours, and you avoid paying the fee. When your panels don't produce, the meter winds up again. End of the month, you get credited for the difference. The trick is to install the smallest system that will zero your annual bill.