Ok, here we go! hehehehe

No, this is not the right direction. The amperages involved are immense.

First a 45 watt kit for $200 is awesome... Please provide website or contact info.

But now, for the bad news.

Those panels would be "12V, 15 Watt panels" (approx. 1 amp at 12V).

That, times 3 is 45Watts at 12V are approx. 3amp (although, it's usually less when really put in service).

Wattage depends on voltage. What I mean is, 45 Watts at 12V is nothing like 45 Watts at 120V. Sadly, I don't remember the formulas off the top of my head, but I do remember amperage that our backup system pulls from the batteries with the kind of load a 1500 Watt, 120V heater would pull.

I've connected a heater to our Sinewave inverter before, which is connected to 5 deepcycle 12V batteries.

If memory serves, it pulled something like 190AMPS @ 12V from the batteries.

For solar panels to return that kind of power, using the above panels (1amp each panel, not 3) would be approx. 190 panels of this type. (or 63 of those kits... at $200 each kit, would be about $12,666 dollars! That sounds about right.

Now, since you are thinking of using the sun to heat, an area, you would be better off using a heated liquid exchange unit or units.

Check out this website:

http://www.windturbine.ca/suncomfort.html

and you get an idea of a much more efficient and less expensive way to heat up something using solar.

With a few of these units and some sort of insulated storage tank, you could heat up a pretty large area.

Hope that helps.

Due to the physical law of conservation of energy, 45 watts at any voltage is equivalent. To heat the average greenhouse, you need 10K - 20K BTU/Hr. of heat, delivered usually at night. The conversion factor to Kilowatts is to divide by 3414. That is 3 - 6 Killowatts per hour. Due to conversion efficiencies, you will need more electrical power than that to get the same amount of "thermal power". Also, converting from sunlight to electricity and then to heat will have conversion losses at each step. If you want to heat with sunlight, it would be better to store the heat in a thermal storage media such as water. Then the water would be circulated into heat exchangers in the greenhouse to heat the air. You could use solar electricity to run the pumps and fans, though.

blondboy the panels are being offered by Harbor Freight (same company as the GH).
Anyway you measure it the cost becomes prohibitive.

MollyD

3 - 6 Killowatts per hour.
Just kilowatts. No per hour (watts come with 'per second's included).

I figured, in his mind, he thought that the heater, at 1500 watts would only require 45 watt panels x33. But 1500 watts at 12V will not power a 1500 watt @120V heater.

Actually, it would be perfectly fine to use 45W * 33 panels to drive a 1500W heater - just wire the panels in a suitable configuration to get 120V. More significant is that the panels will be producing that power during the day when you don't want the heat. Adding batteries will make it completely impractical.

Having said all of this, you can heat your greenhouse with sunlight - that's what greenhouses do! Your best bet is to makes sure that all the sunlight that hits the greenhouse does something useful - use black materials inside to absorb waste light. Then read up about thermal mass and go from there.

If you want to collect more sunlight during the day, just buy some more greenhouse film and put it over say your house roof held up say a foot of the roof, and circulate air through these 'solar panels' using a fan whenever the air temp is higher than your greenhouse. You might put shade cloth under the plastic film to increase the absorption of light. Then you're looking at say $1 per 45W of heat rather than $200.

If your house is 100m^2 (1000ft) in the sun you could probably collect 500MJ a day in winter NY using this approach - that's 20000 btu /hour if spread out over 24 hours!

The person posing this question is in Michigan but I would defy you to find that much sun in NY !!! If we get 3 hours it's an unusually sunny day! I'm not optomistic enough to think I could gather that much energy here :-)

MollyD

5MJ/m^2 day is from a climate averages chart. It's roughly equivalent to 3 hours of direct winter sun.
http://www.wattsun.com/resources/insolation_maps/flat_plate.html
says that michigan might get only 2kWh /day average. That's 720MJ a day over 100m^2. How much of that you can catch depends on how careful you are.

and on getting sunlight everyday! I don't know about Michigan but here in NY we can go long stretches (up to a couple of months in winter) without seeing the sun.Not a dependable heat source in our frigid climate. Wonder where Sail,the original poster got off to?

MollyD

I don't think I have seen this many posts on this board for a while. Thanks for all the feeback.

My original thought is that solar panels are impractical. I guess I was spot on. I just saw those panels and thought I would throw that one out there.

I too have issues with lapses in sun during the winter and the only real solution is to spend $$$ to heat the house. Even tremendous storage of water wouldn't be enough.

Thanks again for the feedback.

nathanhurst

I'm not going to get into an electronics lesson with you, but you (with all due respect) are complete wrong on how amperage, wattage and voltage work.

Real quick.

Using your idea of connecting the panels to get 120V is not correct.

Two problems with your scenario

1) the heaters we are talking about are 120V AC. Your method only provided DC current. While the elements can handle DC, the fan cannot. There may also be other electronics that are running off of a power supply thatÂs designed for AC.
You need an inverter to change DC to AC. While there does exist inverters that will convert, say, 120DC to 120AC, this is not widely used in the Americas.

2) Parallel or SeriesÂ. That is the question.

What you first need to understand is that in order to increase AMPERAGE, you need to connect the panels in PARALLEL. To make things simple, letÂs say that a single panel is 15 watts (thatÂs approx. 1 amp at 12V) Â (which each panel is in the kit aboveÂ. 3 panels make 45 watts, which is approx. 3 amps at 12Volts).

And also, letÂs use small numbers just to get the point across here.

1 panel producing 12V = 15 watts = 1 amp

To product 10 amps at 12V, you connect 10 panels together in a parallel manner (positive to positive, negative to negative).

Now, if you take those 10 panels and connect them in series (positive from 1 panel to negative on the other panel and repeat for each panel you end up with 1 positive and 1 negative when done).

That would make 120V DC, BUT, it would be 120V at 1 amp only.

To be clear:

- Connecting panels in parallel increases AMPERAGE (watts) but the VOLTAGE stays the same.

- Connecting panels in series increases VOLTAGE but the AMPS (watts) stay the same.

So using your idea, in order to product 1,500 watts (which is approx. 12.5AMPS) you would need:

10 panels in series = 120V at 1 AMP (letÂs call this 1 set).

To make the MINIMUM 12.5 amps (we are not including startup amperage or other necessary things to take into account when configuring a system)Â. you need to connect 13 "sets" (rounded off) of the above connected in parallel.

ThatÂs 130 panels (individual panels). AND THEN, that would ONLY product the required amperage ONLY in direct, full sun (we havenÂt calculated for dimmer light levels).

You still need an inverter here, which of course, also costs in amperage loss.

Hopefully, you now understand how this works.

Ok ok, so IÂve been a technician for over 30 years! LOL I just hope that I explained it well enough. I admit, itÂs difficult for me to put my knowledge into short words. LOL

But like IÂve said before, itÂs better to use a water heating system for a greenhouse rather than solar electric panels.

Sail,
I think your climate is similar to mine. How much it would cost you depends on a)what are you growing (how high do you need the temp to be b)how well insulated is your greenhouse and c)how long do you need to heat it. Some folks don't use their greenhouse till March or so to start seeds.
Mine is a cattlepanel hoophouse covered in 6mm film. A solar pool cover lines the inside. I grow daylilies which do best if kept above 50F but not higher than 70F. My greenhouse is 8'w x 17'Lx 8h and it's running me about $200. a month to heat it. I'm running a vent free propane heater and a 1500 watt electric as a backup/supplement. I'm toying with the idea of using kerosene as my backup/supplement instead after a power failure here last week.It would be cheaper too.

Molly

and on getting sunlight everyday! I don't know about Michigan but here in NY we can go long stretches (up to a couple of months in winter) without seeing the sun.Not a dependable heat source in our frigid climate. Wonder where Sail,the original poster got off to?

I imagine climate averages would include that.

Blondboy: I've built inverters, I built a MPPT before people knew what they were. My father's an EE and I've been doing electronics since I was 10. 1500W DC is 1500W AC as far as resistance heaters are concerned. PV panels are essentially constant current, so startup current is irrelevant.

The fan could easily be replaced with a DC one (or driven by a small 100W inverter from the local store). Most of the heaters I've seen have just used a bimetalic strip, a relay and a mercury tilt switch or similar. The point is that it would be a silly thing to do with electricity in the first place. I extend this to a general principle: If you are converting electricity to heat using a resistance heater, you are doing it all wrong. Even lights are better.

But like IÂve said before, itÂs better to use a water heating system for a greenhouse rather than solar electric panels.

Even better to use a super low cost solar air heater as I said in my previous post - no leaks, no freezing, no pumping, no bleeders, no fin tube, no boiling etc. Cost per m^2 is tiny, easy construction, lightweight.