100W Solar System

[shimabuku]

Hello guys and gals! New here and to solar so please be gentle. Now that I finally have the time I want to dive into off grid solar. Been doing some homework these past few days and have several questions. I have an access point in the house which can use direct DC power and would like to power that AP by an off grid solar solution. These are the calculations I have come up with;

Access Point 15W (Peak) x 24 hours = 360Wh
100W Solar Panel = Average 300Wh/day
15Ah Battery Bank 15Ah x 12v = 180Wh
Tilt angle set to latitude (panel will sit on roof with no roof pitch)

Is it safe to say with a 100W panel and 15Ah battery, my access point can be powered 24/7? I assume that going with a MPPT charge controller rather than a PWM is the better option. These past few days I have been browsing around ebay and other sites but am unsure of which panel, battery, MPPT charge controller to get. I would really like to hear your suggestions.

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[Sunking]

Hello guys and gals! New here and to solar so please be gentle. Now that I finally have the time I want to dive into off grid solar. Been doing some homework these past few days and have several questions. I have an access point in the house which can use direct DC power and would like to power that AP by an off grid solar solution. These are the calculations I have come up with;

Access Point 15W (Peak) x 24 hours = 360Wh
100W Solar Panel = Average 300Wh/day
15Ah Battery Bank 15Ah x 12v = 180Wh
Tilt angle set to latitude (panel will sit on roof with no roof pitch)

Is it safe to say with a 100W panel and 15Ah battery, my access point can be powered 24/7?

No you answered your own question already. Lets start with battery because that is where you are way off the mark. If you use 360 wh per day and want to use 12 volts you are going to need 150 AH, not 15. A 150 AH battery if you use a PWM controller will take a minimum of a 225 watt solar panel to maintain it properly. That does not mean it is enough to generate you daily power requirement as that is determined by where you live.

If you use a MPPT controller the minimum panel wattage is 150 watts. Again does not mean it is enough panel wattage. Either PWM or MPPT would require a minimum 15 amp charge controller.

You are in Hawaii and that is good news as it means you can get by with minimum panel wattage. On paper with minimum 4.9 Sun Hour day in Hawaii a 100 watt panel using a PWM controller will generate 245 watt hours of usable energy which falls short of your 360 wh per day usage. If you used a 100 watt panel with MPPT would get you up to 330 watt hours which is still not enough to cover 360.

So here is what you need, two options:

Option 1
Panel = 225 watts, 12 volt Solar panel. It will actualy be two 110 to 120 watt panels as there is no such thing as a 225 watt battery panel $450 to $500
15 amp PWM Charge Controller = $25 to $40
12 volt 150 AH battery = $200 to $300
Total estimate = $675 to $840. Does not include any hardware, wiring, or inverters.

Option 2
150 Watt Grid Tied Panel. $150 to $200
15 amp MPPT Controller = $130 to $200
12 volt 150 AH battery = $200 to $300
Total estimate = $480 to $700. Does not include hardware, wiring, or inverter.

Take your pick and happy shopping. For your first 5 years you will be paying roughly $1.10 per Kwh. After your first five years after you replace the battery (if it last that long) around $0.60 per Kwh for the next 5 years if the battery last that long, and if battery prices do not increase. How much does your electric company charge you for a Kwh in Hawaii?

[tarobrand]

I'm new too, here are my thoughts



Just to contribute my thoughts, though they might be off. So if I am incorrect, please let me know - I enjoy learning.

Amount of solar hours per day is needed to determine what type and how many solar panels you will want to spec out. Recommend location to get solar hours can be referenced from the National Renewable Energy Laboratory (NREL). There are many other resources out there that have done studies for specific geographical regions. Might be a good idea to research several outcomes and take an average.

http://www.nrel.gov/gis/solar.html

http://www.nrel.gov/gis/images/eere_pv/national_photovoltaic_2012-01.jpg

Your question is in regards to running the access point for 24 hours, might be a good idea to plan for 48 or 72 hour battery source in the even of heavy overcast days. Although the reference above does provide the average solar hours per day, it's an average and not a minimum. So there might be days where the solar hours could be significantly less than stated.

[shimabuku]

No you answered your own question already. Lets start with battery because that is where you are way off the mark. If you use 360 wh per day and want to use 12 volts you are going to need 150 AH, not 15. A 150 AH battery if you use a PWM controller will take a minimum of a 225 watt solar panel to maintain it properly. That does not mean it is enough to generate you daily power requirement as that is determined by where you live.

If you use a MPPT controller the minimum panel wattage is 150 watts. Again does not mean it is enough panel wattage. Either PWM or MPPT would require a minimum 15 amp charge controller.

You are in Hawaii and that is good news as it means you can get by with minimum panel wattage. On paper with minimum 4.9 Sun Hour day in Hawaii a 100 watt panel using a PWM controller will generate 245 watt hours of usable energy which falls short of your 360 wh per day usage. If you used a 100 watt panel with MPPT would get you up to 330 watt hours which is still not enough to cover 360.

So here is what you need, two options:

Option 1
Panel = 225 watts, 12 volt Solar panel. It will actualy be two 110 to 120 watt panels as there is no such thing as a 225 watt battery panel $450 to $500
15 amp PWM Charge Controller = $25 to $40
12 volt 150 AH battery = $200 to $300
Total estimate = $675 to $840. Does not include any hardware, wiring, or inverters.

Option 2
150 Watt Grid Tied Panel. $150 to $200
15 amp MPPT Controller = $130 to $200
12 volt 150 AH battery = $200 to $300
Total estimate = $480 to $700. Does not include hardware, wiring, or inverter.

Take your pick and happy shopping.

Thanks for the reply. Ok, from what I understand if going with the 150Ah battery and a 225 panel I will get roughly a 1.5 -2 day cushion? No inverters are required as this access point can be powered by DC.

[Sunking]

Amount of solar hours per day is needed to determine what type and how many solar panels you will want to spec out. Recommend location to get solar hours can be referenced from the National Renewable Energy Laboratory (NREL). There are many other resources out there that have done studies for specific geographical regions. Might be a good idea to research several outcomes and take an average.

You are on the right track, but your track is missing some rail and will derail your train.

Off-grid is a different animal than grid tied. Off grid is very inefficient and must be designed for worse case. In this case his minimum sun hour month is 4.9 Sun Hours, and if you were to use average yearly Sun Hours will leave you dark in months with less than average.

Secondly off grid is very inefficient. With MPPT controllers around 66 to 70%, and with PWM very best is 50%. So if you need 1 Kwh per day with MPPT you need to generate 1.5 Kwh and with PWM 2 Kwh.

[Sunking]

Thanks for the reply. Ok, from what I understand if going with the 150Ah battery and a 225 panel I will get roughly a 1.5 -2 day cushion? No inverters are required as this access point can be powered by DC.

On paper you get 5 day battery reserve, but in practice only 2.5 days because you never want to take your battery lower than 50%. You will have 150 AH available but only 75 AH usable. That is done for 2 reasons. One is to CYA for a couple of cloudy days in a row. Second reason is to get the most bang for your buck. The deeper you discharge your battery, the fewer cycles you will get.

Batteries are not created equal, you get what you pay for. Get a cheap battery even if you limit to 20% daily discharge will only last a year or two. A good quality battery at twice the cost limited to 20% daily discharge should get you 5 to maybe 7 years if you give them TLC. For a beginner you will destroy your battery in a short time. So look at a mid range battery like a golf cart battery and a 12 volt 150 AH battery on the mainland will cost around $200. I am certain in Hawaii will be more. After you destroy that battery and gain experience you can then move up once you know what to do and care for your battery. In Hawaii batteries will not last as long as in a cooler dryer climate. You are blessed with abundant sun, but cursed with tropical climate.

[Mike90250]

Thanks for the reply. Ok, from what I understand if going with the 150Ah battery and a 225 panel I will get roughly a 1.5 -2 day cushion? No inverters are required as this access point can be powered by DC.

Then you need to include a DC-DC converter to provide stable DC. The battery will be charging sometimes at nearly 16V
which may make the AP unhappy.

[jony101]

I had a alfa r36 12 volt AP and I'm not sure its actual wattage but I would have no problem keeping it on 24/7 on a 75 ah battery and 120 watt panel. Its amp usage was insignificant and if my battery was reading 12.6 when I plugged it in, in the morning it would be reading maybe 12.5 and that includes using my netbook and lights during the night. It was one of those items that I only unplugged if it was within easy reach.

I think you need to find out how much power your AP will actually use over 24 hours. A DC wattmeter connected to it will be very accurate. Thats how I measured how much amps my 12 volt fridge used in a 24 hour period. 15 watts doesnt sound like you need a very large battery. But a large battery will allow for a cushion during cloudy days.

For 100 watt panel using an expensive MPPT controller wont make a difference. From my experience MPPT requires high voltage panels to work. You'll get about 5 amps whether you use PWM or MPPT on low voltage 100 watt panel.