# Need help calculating small off grid 12v system

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Registered Users Posts: 16
I am looking to setup a small greenhouse lighting system that is using 12v lighting.

We use solar completely out here off grid and want this to be independent of anything ese.

Basically, I am going to be using about 100 watts a day for 5 hours a day.

I was looking at maybe something like this http://www.amazon.com/Solar-Panel-Starter-100W-Monocrystalline/dp/B00BFCNFRM/ref=cm_cd_ql_qh_dp_t

but I am having trouble sizing the battery bank.. no inverter is needed, so my calculation ended up coming to like 333ah or something? Any help?

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Re: Need help calculating small off grid 12v system

Are you looking for "grow lights" or just lighting to work when the sun is down?
• 100 Watts * 5 hours per day = 500 Watt*Hours
• 500 Watts*Hours * 1/12 volt battery * 2 days of storage * 1/0.50 maximum discharge = 167 Amp*Hour @ 12 volt battery bank

At least how I understand your needs... With 1-3 days of storage (2 days recommend) and 50% maximum discharge (for longer battery life).

If you are just doing this for lighting... There are some neat LED Flood lamps that work on a wide range of DC voltages (some are 10 watt and claim 10-35 VDC) and around \$10 each + shipping (lowest price I have seen).

I would suggest a 24 volt system--Easier to send the power longer distances...

If you can use a 300 Watts TSW 12 volt inverter for 120 VAC--That would be a interesting option too if you have a large area to wire up:
• 100 Watts / 12 volts = 8.3 amps
• 100 Watts / 120 volts = 0.83 amps -- Much smaller awg wire required @ 120 VAC

Something else to watch for. If you are using 100 Watts for 5 hours per day, you would probably want an array minimum of:
• 100 Watts * 5 hours * 1/0.52 AC system off grid eff * 1/4 hours minimum sun (9 months a year) = 240 Watt array minimum for 120 VAC system
• 100 Watts * 5 hours * 1/0.62 DC system off grid eff * 1/4 hours minimum sun (9 months a year) = 205 Watt array minimum for 12 VDC system

If your green house will be used into the winter, you may get down towards ~2 hours of sun per day or less in some regions (depending on local weather and latitude).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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Re: Need help calculating small off grid 12v system

100 Watts for 5 hours is 500 Watt hours. On 12 Volts that's about 42 Amp hours. If you limit depth of discharge to 25% that would be 168 Amp hours. You'll need to size that to meet available batteries. Upwards is preferable, as having more power on hand is better than having less.

As for recharging it, if the lights were used during the day as well as at night that would need to be taken into account. If they are only for nighttime use then you could use minimal recharging current. With a PWM controller that would be 147 Watts for the theoretical battery size, or 294 Watts at the standard 10% current rate.

The 100 Watt panel you link to would produce about 5.7 Amps and would not meet minimum requirements for the theoretical battery size.
• Registered Users Posts: 16
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Re: Need help calculating small off grid 12v system
BB. wrote: »
Are you looking for "grow lights" or just lighting to work when the sun is down?
• 100 Watts * 5 hours per day = 500 Watt*Hours
• 500 Watts*Hours * 1/12 volt battery * 2 days of storage * 1/0.50 maximum discharge = 167 Amp*Hour @ 12 volt battery bank

At least how I understand your needs... With 1-3 days of storage (2 days recommend) and 50% maximum discharge (for longer battery life).

If you are just doing this for lighting... There are some neat LED Flood lamps that work on a wide range of DC voltages (some are 10 watt and claim 10-35 VDC) and around \$10 each + shipping (lowest price I have seen).

I would suggest a 24 volt system--Easier to send the power longer distances...

If you can use a 300 Watts TSW 12 volt inverter for 120 VAC--That would be a interesting option too if you have a large area to wire up:
• 100 Watts / 12 volts = 8.3 amps
• 100 Watts / 120 volts = 0.83 amps -- Much smaller awg wire required @ 120 VAC

Something else to watch for. If you are using 100 Watts for 5 hours per day, you would probably want an array minimum of:
• 100 Watts * 5 hours * 1/0.52 AC system off grid eff * 1/4 hours minimum sun (9 months a year) = 240 Watt array minimum for 120 VAC system
• 100 Watts * 5 hours * 1/0.62 DC system off grid eff * 1/4 hours minimum sun (9 months a year) = 205 Watt array minimum for 12 VDC system

If your green house will be used into the winter, you may get down towards ~2 hours of sun per day or less in some regions (depending on local weather and latitude).

-Bill

BB, those LED flood lights are exactly what I'm using. We are stringing a line of 10 of them down the greenhouse, the reason is to add a bit off supplemental light in the evening and mornings to keep plants into vegetative cycle outdoors.

We are in central CA, past hollister aways. TONS of sun, and it will not be needed in the winter.

So basically, no usage during the days.. lights come on around 8 for a few hours and then again early AM for a few hours. Also you think these 12v floods can take 24v fine? Then I am fine with it!
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Re: Need help calculating small off grid 12v system

You will have to be very careful about the LED floods... They are probably built overseas and how they are configured appears to be an open question with feed back I have seen for some vendors (i.e., 120 VAC versions, 12 VDC parts ordered but have a 120 VAC transformer inside, etc.).

I would suggest working with a reliable vendor and only getting a few of them to test out first. I have not used any of these--So I cannot talk about their acceptable input voltage range (some are listed as 12 volts, other as 10-35 volts--Sometimes the same web page will list 120 VAC in a different section).

You may also have to experiment with what LED fixtures work correctly for your needs... White LED's may not be the best for your needs????

Have you researched the "Grow Lights" that have Blue and Read or other combinations (sometimes different combinations between growing vs flowering?).

Also, how much wattage per square foot? One site said about 3 watts of LED lighting per square foot????

I have a brown thumb--But I am interested to hear more about what you will be doing.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 16
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Re: Need help calculating small off grid 12v system
BB. wrote: »
You will have to be very careful about the LED floods... They are probably built overseas and how they are configured appears to be an open question with feed back I have seen for some vendors (i.e., 120 VAC versions, 12 VDC parts ordered but have a 120 VAC transformer inside, etc.).

I would suggest working with a reliable vendor and only getting a few of them to test out first. I have not used any of these--So I cannot talk about their acceptable input voltage range (some are listed as 12 volts, other as 10-35 volts--Sometimes the same web page will list 120 VAC in a different section).

You may also have to experiment with what LED fixtures work correctly for your needs... White LED's may not be the best for your needs????

Have you researched the "Grow Lights" that have Blue and Read or other combinations (sometimes different combinations between growing vs flowering?).

Also, how much wattage per square foot? One site said about 3 watts of LED lighting per square foot????

I have a brown thumb--But I am interested to hear more about what you will be doing.

-Bill

Yup, I've properly researched and have experience with them. Solar math is my weak points. The flood lights are a cool white, light blue spectrum that mimics the sun's color temperature in the spring time, thus tricking the plants into thinking they are still in the early months of the year while keeping them at around 18 hours of light per day

Basically, you can use any lights. Most people just do CFL light bulbs strung across.

Wattage isn't a huge issue. Just need light covering all of the plants, penetration/lumens isn't an issue because we aren't necessarily trying to grow the plants via these lights, just trick them to further their cycle until we can finally transition them into the 90 footer house

They are def 12 volt, I have tested them off my trucks batteries. I am down with trying either 12 or 24.. what 12v batteries do you think would be proper for my needs? 2 day battery life sounds fine