Solar setup

[Harp]

Can anyone suggest a simple solar setup to run 4 LED lights and a vent fan. We run the fan a few hours a day for weekend trips and the lights usually one or two at a time a few hours in the evening. Clear sun (no shade) 8+ hours a day. Should be permanent roof mounted.

[camper357]

It really depends on how much power you are using. How many watts are your lights and fan pulling? Are you currently using a second battery or just running everything off of your vehicle? More info is needed.

[dwh]

I'll take a stab at this using rough guesstimates in order to show how it's done. You'll need to replace my numbers with your actual numbers.

Assuming this fan:
http://www.amazon.com/Fan-Tastic-Ven...5118091&sr=8-1

Which according to reviews on Amazon draws about 35 watts on high.

I'll also assume this light fixture:
http://www.superbrightleds.com/cgi-b...pecs%2Ftdl.htm

Which draws .24 amps at 13.5 volts so .24a x 13.5v = 3.24 watts.


Assuming 4 hours a day of running the fan would be:
35w x 4hr = 140wh (watt*hours) per day

Assuming two lights, 4 hours a day each would be:
2 x 3.24w x 4hr = 30wh per day

Okay - now we know we need to replace 170 watt*hours per day.

Batteries are not 100% perfectly efficient. In general, we use 80% as the rule of thumb. So if we draw out 170wh/day, we'll need to supply 120% of that to top the battery up again:

170wh x 1.2 = 204wh

PV modules (solar panels) are likewise not 100% efficient - the hotter they get, the less they put out, and they are usually hot from being in the sun - and here again we use 80% as the rule of thumb:

204wh x 1.2 = 255wh


Okay, so we need the capacity to generate a total of 255 watt*hours per day to replace what we've used. The next question is how many hours are available to do that? PV modules don't put out much if the sun hits them at too much of an angle, so most solar estimators use either 4 or 5 hours as the amount of "peak good sun" (full direct sun almost straight down onto the PV module) that can be expected per day.

Mounts that angle the PV toward the sun help quite a bit. Permanent flat mounts are just about "worst case".

So just for giggles, I'll use 4 hours/day "good sun" in this guesstimate:

255wh / 4h = 61w


Tada! We'd need at least a 60w solar panel.

[Dirtytires]

^^^ You sure are on wise fellow. Kind of remind me of a owl.

[Harp]

The figuring I had done had me choosing an 80W panel. I guess the question now is which panel and controller can you suggest. Thanks for the great response.
Marty

[dwh]

^^^ You sure are on wise fellow. Kind of remind me of a owl.

"One...two...three...CRUNCH! Three."


Haha! I appreciate that. Though in person, I'd probably remind you more of a gorilla.

I'll put that down in the "win" column to help offset all those times I've been called an arrogant conceited bastard.

[dwh]

In general, it seems like 100w is sort of a breakpoint on price per watt. You might find a 100w or 120w PV module for the same or less than an 80w.

Solarblvd.com usually has good prices, and if you can get your hands on one of the Solar Cynergy modules they sell, that'll probably be about the best deal. BUT, Solar Cynergy modules are made with solar cells from Q-cells and Q-cells just went bankrupt, so it's a crapshoot if you can get those modules anymore.

I like to recommend Northern Arizona Wind & Sun because they've been around a long time, have a great rep, fair prices AND they spend their own time and money to run just about the best solar forum on the net.

You can't go wrong with a Morningstar PWM controller like a SunGuard or a SunSaver.

[Captm]

So why watts instead of amps in figuring the power requirements? Basic question, but I monitor my consumption and charge in amp hr (link pro monitor).

[dwh]

So why watts instead of amps in figuring the power requirements? Basic question, but I monitor my consumption and charge in amp hr (link pro monitor).

Good question.

Hrmm...

In visualizing electrical systems, it's sometimes handy to use a compressed air analogy.

Watts would be the basic unit - like cubic feet.
Amperes would be a flow rate - like cubic feet per minute.
Volts would be pressure - like 12 psi.

So what I did was figure out the number of cubic feet we'd use, and thus how many we'd need to put back. And then how many CFM the compressor would need to supply to get it done in 4 hours.




The problem with specifying amperes, is that it's meaningless without ALSO specifying voltage.


Let's say we pull 1200 watts out, over a period of 10 hours. That's 120w/hour.
If the system were at 12v pressure, that would be 10 amps per hour flowrate.
If the system were at 24v pressure then the same 120w/hr would be a 5a per hour flowrate.


So it's not exactly "technically" correct to say something like, "We used 100 amp*hours".
Technically, it should be stated, "We used 100 amp*hours at 12 volts".
Or, we could say, "We used 50 amp*hours at 24 volts".

Either way, we'd be saying the same thing, "We used 1200 watts". [EDIT: Woops. Should say, "We used 1200 watt*hours".]


For estimating solar, there are many different ways to do the rigging.

You could have two "12v nominal" solar panels in parallel feeding into a pair of "12v nominal" batteries rigged in parallel into a 12v nominal bank.
Or you could rig the panels in series, and also rig the batteries in series, and have "24v nominal" on both sides.
Or you could rig the panels in series to have 24 nominal on the PV side, and feed through an MPPT controller to a 12v nominal rigging on the battery side.

Each different sort of rigging is going to end up with different voltages and thus different amp*hours numbers.

But the watts won't change no matter how you rig it, so it's just a lot easier to do the ballparking in watts.

[Captm]

Thanks, that made sense.