small solar system: check my math plz

[stack]

Smart people: how's this look for a small solar system I'm building? I'll use it mainly for lights and a small swamp cooler/exhaust system that I made for the hexayurt.

Power: solar setup from Costco( http://tinyurl.com/d9kw92 ) makes 60w (3panels 18w ea.) 3.9 amps. 150w inverter, 4a charge controller.

Battery: 110ah deep cycle from wally world.

Draw: I'm calculating around 32a draw per day + 10% for loss in the system for a total of 35amps from the battery per day. Solar panels should produce 36amps with 9 hours of sunlight. The battery shouldn't drop below 65%.

Tell me what I'm missing and/or any advice will be appreciated! Thanks!

[Token]

Since you don't list specs on your lights, fan and swamp cooler it is hard to tell.

Overall it looks really thin and weak. You basically have 60W of solar panels which will provide power for both day and night use. If you are running the swamp cooler I doubt there will be much charging going on.

This setup would work for lights and charging batteries n stuff. The swamp cooler seems like too much.

[Oldguy]

From the Costco site you posted about the Coleman battery charger:

12V DC Power Consumption Chart: Typical WATTAGE/HR requirements for everyday electronics:

Bilge / Sump Pump 100
CB Radio 5
Clock Radio 5
Computer (Laptop) 50
Cooler, 12V (3A) Electric 36
Depth Finder, 12V 5
Lights (Compact 40W Equiv.) 10
Satellite Dish 30
Stereo 50
TV 12" 20
_____________________________
How to convert Watts to Amps

Basics

You cannot convert watts to amps, since watts are power and amps are coulombs per second (like converting gallons to miles). HOWEVER, if you have at least least two of the following three: amps, volts and watts then the missing one can be calculated. Since watts are amps multiplied by volts, there is a simple relationship between them.

However, In some engineering disciplines the volts are more or less fixed, for example in house wiring, automotive wiring, or telephone wiring. In these limited fields they often have charts that relate amps to watts and this has confused people. What these charts should be titled is "conversion of amps to watts at a fixed voltage of 110 volts" or "conversion of watts to amps at 13.8 volts," etc.


Some tidbits of information that you might need a refresher on:
To convert mA to A (milliamps to amps) 1000mA = 1A
to convert µA to A (microamps to amps) 1000,000 µA = 1A
To converter µA to mA (microamps to amps) 1000µA = 1mA
To convert mW to W (milliwatts to watts) 1000mW = 1A
To converter µW to W (microwatts to watts) 1,000,000 µA = 1A



The Following Equations can be used to convert beween amps, volts, and watts.

Convert Watts to Amps (at a fixed voltage)
Convert Amps to Watts (at a fixed voltage)
Convert Watts to Volts (at a fixed current)
Convert Volts to Watts (at a fixed current)
Convert Volts to Amps (at a fixed wattage)
Convert Amps to Volts (at a fixed wattage)

Converting Watts to Amps

The conversion of Watts to Amps is governed by the equation Amps = Watts/Volts

For example 12 watts/12 volts = 1 amp
Converting Amps to Watts

The conversion of Amps to Watts is governed by the equation Watts = Amps x Volts

For example 1 amp * 110 volts = 110 watts

Converting Watts to Volts

The conversion of Watts to Volts is governed by the equation Volts = Watts/Amps

For example 100 watts/10 amps = 10 volts

Converting Volts to Watts

The conversion of Volts to Watts is governed by the equation Watts = Amps x Volts

For example 1.5 amps * 12 volts = 18 watts
Converting Volts to Amps at fixed wattage

The conversion of Volts to Amps is governed by the equations Amps = Watts/Volts

For example 120 watts/110 volts = 1.09 amps

Converting Amps to Volts at fixed wattage

The conversion of Amps to Volts is governed by the equation Volts = Watts/Amps

For Example, 48 watts / 12 Amps = 4 Volts

Explanation

Amps are how many electrons flow past a certain point per second. Volts is a measure of how much force that each electron is under. Think of water in a hose. A gallon a minute (think amps) just dribbles out if it is under low pressure (think low voltage). But if you restrict the end of the hose, letting the pressure build up, the water can have more power (like watts), even though it is still only one gallon a minute. In fact the power can grow enormous as the pressure builds, to the point that a water knife can cut a sheet of glass. In the same manner as the voltage is increased a small amount of current can turn into a lot of watts.

[Oldguy]

I agree with Token.

110 amp hour battery will provide enough juice for lighting alone.

A swamp cooler has both a water pump and fan for blower, you can run it for a short time but recharging will take all day.

W=AxV, V=W/A, A= V/W

[mdmf007]

110 amp hour battery will provide enough juice for lighting alone.

W=AxV, V=W/A, A= V/W

46.8W=3.9Ax12V amount of charging from solar panel for lighting, if charging 9 hours /day you can get 9 hours/night of 46 W of lighting on 12 volt system.

9 hours of charging at a specific level, will not equate to the same level of power out.

[Oldguy]

Thank you, I stand corrected. I have edited my post

[stargeezer]

In addition to the concerns stated above, I will add another that has not been mentioned. That 150W inverter that you are using is probably not that efficient, and I would expect it to be a modified sine output. Transformers and motors work great on a clean sine wave, but generate a lot of waste heat with the modified sine. If run for a long time, I have seen motors destroyed by this heat. Also, startup currents for motors are very large, and can require an inverter with 10 times the continous power consumption just to start (your 150W inverter may not start anything larger than a 15W motor). Since you are already running with little to no margin, I would suggest that you eliminate the inverter and modify everything to run directly off the battery.

[rodiponer]

I agree, ditch the inverter. It probably has a parasitic load that would wreck this whole system-- 0.5 amps really adds up over 24 hours on a small system.

I think the main problem with your math is to assume the solar panels will generate their rated output for nine hours a day. In my experience they only do that for two or three hours around noon. By 2pm my systems have always been down to a third of their rated output. I suspect that with this system you'll generate about 20 amps/day, provided that you keep the panels clean, don't let even the smallest sliver of a shadow on them, and point them towards the sun a couple times a day.

But I think 20 amps/day is plenty of power if you are very efficient. On my sailboat we used LED lights that drew next to nothing (0.2 amp "Sensibulbs", though there are lots of options), and had very efficient fans (0.7 amps). If your swamp cooler is made with one or two low power fans and something like a computer water cooling pump (0.1 or 0.2 amps) to wet the media, I think you could run your yurt off of 20 amps/day.

But I wouldn't even try to charge a laptop (usually 5 to 10 amps) or phone (2 or 3) off of this system. Or if you do, bring a 12v adapter, instead of running it through the inverter. This is way more efficient, but still, it runs the risk of blowing your very tight energy budget.

[stack]

Looks like its back to the drawing board for me.... I had some doubt about using the inverter, and I really wasn't sure what to expect from the panels on a daily basis. Sounds like my original plan to stay 12v was the smarter way to go.

I'm pretty comfortable with the watts=amp*volts math, and can make reasonable guesses as to how much power I'll be using (I'll list the specifics of consumption below).

Originally I was going to build a 12v only system using a 12v water pump, computer fans, and string LED lights that I would cut and put resistors on (this site has been really helpful for that http://led.linear1.org/led.wiz).

I shyed away from this for 2 reasons: I was planning to use 14ga speaker wire to minimize loss, and and it turned out to be around $50 for 300 feet, and when I went to Harbor Freight to get the 12v water pump for the swamp cooler all they had was a 120v pump. So I figured why buy all that speaker wire and spend the time to make lights when I can just plug everything to the inverter.

So now looks like not using the inverter will be the way to go. I'll be making some lights and re-thinking the swamp cooler (which I'm not sure will really be that effective anyways, its pretty small). Here's some specs on what I'm planning to run:

LED lights: 60 count string = .09a@120v (will cut into loops of 3 lights w/ 120ohm resister using 20ma ea). I have 138 lights, *if* I ran all of them for 8 hours load would be 8a for the night.

computer fans: 5 @ .35a 12v. I was planning on three for exhaust of yurt, 2 for swamp cooler. if on for 6 hours = 10.5a

So just the lights and fans equal 18.5a for the day, not counting a pump for the cooler. Unfortunately, I'm having a problem finding anything less that a 2a draw on a 12v water pump. So looks like I can't afford the power....oh well, we'll just have to dip the wicking material by hand every now and then.

oh, yeah, laptop charging or any other heavy draw was not something I was planning for BTW.

Rodiponer: how do you calculate how much amperage you're getting out of your panels?

Thanks again for all the help.

[rodiponer]

You can measure the output of small panels with an inexpensive multimeter. Mine has a 5 amp limit, but look for the writing on the back to make sure you don't overload yours. Note that you won't see any amps going into a charged battery, so discharge the battery some or run the lights or fan to measure the solar panel output. You'll need a bigger amp meter for larger solar panels. Some of these have a little computer inside that add up the amps generated during the day. I had one of these computer amp meters when I lived off grid with solar panels, and this is how I made up my unscientific rule of thumb:

I think you can estimate the amps/day a system will generate by dividing the rated output wattage by 3.*

*12 volts only.

Please tell me if there is a more scientificaly accurate rule of thumb for daily solar panel output.

I think thick wires are important between your solar panels and batteries. The batteries will not charge if there is a large voltage drop.

But I do not think you need to worry much about voltage drop here. LEDs and fans work well enough at a low voltage, and your yurt is small. You have to be more careful about voltage drop with electronic devices and big motors, which will freak out or overheat at a low voltage. So consider using the '10% voltage drop' wire sizing table instead of the 4% table. But please use fuses sized to the size of the wire.

For your swamp cooler, have you looked at computer cooling pumps? Many of them draw less than 1 amp and pump about 100 GPH, which should be more than enough to keep the media wet. For example:
http://www.thermaltake.com/product/Ligu ... -w0132.asp

You may not need separate exhaust fans with your swamp cooler. The air pressure created by the intake fans will be enough to blow exhaust air out of a cracked vent (this is how we lived in Tucson, with a cracked window)...

If you want to be really nerdy and futuristic, there are plans for a small heat exchanger made with straws. This lets the inside exhaust air cool the outside intake air, so that the swamp cooler is working on a lower intake temperature. These are surprisingly efficient in commercial systems. This and very thick swamp media seem to be the best ways to have a large temperature drop.

[motskyroonmatick]

Do a search for 12 volt bilge pump and you will find a bunch of 12 volt water pumps in varying capacities. I use a 600gph pump on my grey water evaporator. I use the brand that starts with an "A" it seems to use less juice than the ones that cost just a couple bucks less.

[gyre]

Resistors turn power into heat.
How about using D cell packs sized for the LEDs?
Or if you need more light, consider 12 volt fluorescents.

Though fluorescents totally overmatch LED efficiency, I'm finding an advantage in LED (luxeon) tolerance for falling voltage, especially with a pwm controller.
I'm using batteries in a tv first- a model intolerant of voltage drop.
Then in the portable fluorescents (tolerance can vary a LOT) but power demand is fairly high.
Then I switch to some brinkmann D cell flashlights and they seem to run forever, though getting dimmer.
They are supposed to have digital controllers and this seems to be true.

I'm planning on making some tent light kits using wide angle luxeon LEDs in 1 and 3 watts, running off D cell packs.

LEDs have the advantage of no power surge on startup and no warmup.
Fluorescents are still better for a substantial amount of light.
They generally need at least 12 volts though.


How about not venting the swamp cooler?