# Solar organic garden

Posts: 2,511 admin
Hello and thanks in advance.

I need some assistance.

I want to do some urban gardening and I live in a very cloudy area (Vancouver, BC, Canada) so I thought about getting a solar panel, use a (home-made) green-house and place full-spectrum lights powered by a solar panel on top of the plants.

The questions is:

Other than the solar panel and the 5 bulbs I will use, what do I need to generate 110V to light up the bulbs?

Thanks/Regards

## Comments

• Solar Expert Posts: 5,436 ✭✭✭✭
Re: Solar organic garden

The cost and area required for any such plan would far outweigh the benefit. The answer is if you don't have sun to grow things, you don't have sun for PV.
The answer for your plants may lie in making sure they get as much light as possible, reduce shading, increase reflection. My guess is that plants are much more efficient in converting solar energy net/net than any panel arrangement lighting the plants.

Icarus

PS You would need a battery bank of some sort to feed an inverter, and the needed wiring. You need the battery because I don't know any way to feed an inverter and thus the loads directly with any efficiency. Assuming 40 watt full spectrums, you would need panel capacity somewhere well in excess of 200 watts, perhaps 30 square feet?
• Posts: 6,290 admin
Re: Solar organic garden

Really?

Well, Germany doesn't have more sun than we have (actually less) and they are the top world producer of solar energy.

I know I can do this and it's not about cost. It's about the principle.

200W? Are you sure? To light up a few bulbs for a few hours a day? It doesn't seem right...
• Solar Expert Posts: 5,436 ✭✭✭✭
Re: Solar organic garden

Do the math.
How many bulbs? How many hours do you expect to run them? How many hours of full sun. (Solar panel efficiency drops off very considerably if partially shaded)

Lets use a down and dirty example.

5 40 watt bulbs= 200 watts. Assume for this argument 4 hours/day=800 watt hours.

Now assume input. Assume 300 watts of panel @ 75% efficiency for 4 hours per day= 300X4/.75=900 Now, assuming 80% (Optimistic) charger/inverter efficiency=720 watt/hours. You can see that you are already in the hole, with no accounting for grey days, or partial cloud cover etc.

Yes indeed germany does have vast solar installations. The difference is both the scale, AND, I don't suspect they are using the electricity to power grow lights per sey.

As I said, I think the plants are net/net way better users of solar energy than the arrangement your proposing. Remember, nothing comes for free. When you convert one form of energy to another there are considerable losses.

I applaud your desire to act on principal, but you would be way better off putting your energy into some other "green" project. IMHO.

Good luck,

Icarus
• Super Moderators, Administrators Posts: 33,402 admin
Re: Solar organic garden

Icarus/Tony is correct... For any practical use of solar panels for green house "grow" lighting of plants--it will be pretty much cost prohibitive...

But, we can always give it a shot.

First, we need to make some assumptions.

1. Lights--the most efficient lamps for "grow-lights" are not full spectrum but HP Sodium in conjunction with natural lighting... So, lets assume 100 watt HID lamp for supplemental lighting a 3'x3' area for 12 hours per day.

100 watts * 12 hours = 1.2 kWhr per day.

Now--lets also assume that you are doing this to get an early start on the season rather than trying to raise plants in the dead of winter.

From this link, we will figure out how much in solar panels you will need in February to start your seedlings (as you head into summer, you will get more sun).

Assume that you will be using batteries and an 120 VAC inverter, plus charge controller... Use a derating factor of 0.52 (based on losses of listed components):

A 1kW fixed mount solar array in Vancouver BC, will produced an average of 36 kWhrs in a month, or, on average 1.2 kWhr per day. (lucky guess).

So, for this system, you will need a 1,000 watt solar array, and for a battery bank:

(1,200 watts / 12 volts) * 3 days of no sun * 1/50% max discharge = 600 amp*hours at 12 volt.

Of course, we can play with the solar panels (tilt them up in winter and down in summer to get more power). But--we have the bare bones of a system here...

5x 205 watt panels for \$4,600

1X 80 AMP Outback charge controller \$580
Morningstar SureSine, 300 Watt Sine Wave Inverter 115VAC \$259
12x Crown 225 Amp-Hour 6 Volt deep cycle battery 6 Volt Golf Car Battery \$1,329
Wiring, timer, frame, concrete, fuses, panel ~\$500 (place holder)

=====================
Total (excluding shipping, taxes and duties)= \$7,268

Note: Batteries will last approximately 3-7 years before replacement is needed.

At least this gets you into the ball park (soccer pitch?) for approximate cost of a system to power a 3'x3' seedling bed.

And just for fun... What would a system that ran for three months using a gas genset to power 100 watts of lights look like (and pump the CO2 and waste heat from the generator into the greenhouse for better growing conditions):

1x Honda eu1000i (900 watt) \$700
Roughly 5kWhrs per gallon of gas. 4x 12 hour days of runtime per gallon (should power 200 watts of lights or 3'x6' bed).

90 days / 4 days per gallon * \$4.50 per gallon = \$101 worth of fuel...

For less than \$1,000 USD, you can build and run a twice as large "fossil fueled" bedding green house with a brand new generator every year (even at \$9.00 per gallon gas prices--sorry for the US gallons and pricing--I don't know your fuel/parts costs up there in BC).

Of course, the two solutions are not equivalent... The solar one would work 9 months out of the year--the fossil fueled one I sized for 3-6 months of the year. And I know that you would not want to go with a gasoline genset to run your green house--but many parts of the world do exactly this.

Just trying to scope the problem out here... The devil is in the details.

But, truly, if you only want the grow lamps for a limited time of the year (early spring), and you have utility power available (i.e., home garden)--the "greenest" way is utility power... And look at Solar Grid tie for your home. Solar PV is best used when you have a 9-12 month of the year load--which grid tie with net metering is the best solution... And use solar thermal design for your green house to warm up the soil (if needed in your area).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 5,436 ✭✭✭✭
Re: Solar organic garden

Bill,,,
You just love doing this don't ya!

T
• Solar Expert Posts: 9,583 ✭✭✭✭✭
Re: Solar organic garden
BB. wrote: »

First, we need to make some assumptions.

1. Lights--the most efficient lamps for "grow-lights" are not full spectrum but HP Sodium in conjunction with natural lighting... So, lets assume 100 watt HID lamp for supplemental lighting a 3'x3' area for 12 hours per day.

Gee, the LED Red & Blue grow lites are pretty efficient too..
http://ledgrowlights.com/u_minn.htm
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