# 6 watt fan and 9 watt leds - WHAT DO I NEED

easytim
Solar Expert Posts:

**57**✭✭✭✭
I'm wanting to power a 12vdc fan that draws 490ma on high = 5.88 watts and three 12vdc led's that is 9 watts total.

For a total of 15 watts at 12vdc

I have two • 6Volt batteries in series• Rated capacity: 12Ah

The three lights and fan will be used only 6 hours each night.

My questions are this

Can I use the batteries I have?

How big of a solar panel do I need (watts) ?

I would like to monitor the charge and voltage, I like this model here -

Morningstar ProStar PS-15 Solar Charge Controller

Will the Morningstar ProStar PS-15 work well in my case and can I monitor the current and voltage or is there a better model available to do this with?

For a total of 15 watts at 12vdc

I have two • 6Volt batteries in series• Rated capacity: 12Ah

The three lights and fan will be used only 6 hours each night.

My questions are this

Can I use the batteries I have?

How big of a solar panel do I need (watts) ?

I would like to monitor the charge and voltage, I like this model here -

Morningstar ProStar PS-15 Solar Charge Controller

Will the Morningstar ProStar PS-15 work well in my case and can I monitor the current and voltage or is there a better model available to do this with?

0

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## Comments

10,311✭✭✭✭that might depend on how long you intend to use those loads each day.

57✭✭✭✭15watts per hour 6 hours per day, 15w x 6 hours = 90 watts used

Does this mean a 90 watt panel?

28,142admin15w x 6 hours = 90 watt

*hoursusedUsing this solar data (pick state of Maine, city closest to you and with similar weather)--we see your "hours of full sun" range from about 2.5 hours to 5 hours per day.

If you want this to work all year round--then use 2 hours (minimum average from November is ~2.4 hours of sun for fixed panel, facing south, at latitude). Use 77% for solar panel * Charge Controller derating, use *80% efficiency for flooded cell battery derating:

90 WH * 1/(0.77*0.80) * 1/2.4 hours of sun per day = 61 watts of solar panel minimum

Battery amp*hour rating (assuming 12 volt battery, 3 days of no-sun, 50% maximum discharge):

90 Watt*Hours * 1/12 volts * 3 days * 1/0.50 = 45 Amp*Hours minimum (at 12 volts)

Your current 6 volt batteries (in series, 12 AH) is just enough to run 1 day's worth of use to a bit deeper than 50% discharge):

90 WH / 12 volts = 7.5 Amp*Hours

(12 AH - 7.5 AH) / 12 AH = 37.5% state of charge after 1 night of use

Your batteries will probably not last very long (months?) -- OK to test your system--but when they die, you will probably want larger batteries (and you will need to recharge them if there is no-sun the next day).

The 15 amp controller should be able to manage:

15 amps * 17 Volt (panel Vmp rating) = 255 watts of solar panel

So, it is large enough to run a 60+ watt panel/string.

What is the end result of your monitoring voltage and current?

A Battery Monitor (which most solar charge controllers do not contain) is the ideal way to monitor the state of charge of your battery.

This small of system may not be worth the cost of a "real" battery monitor... But these cumalitive DC Amp*Hour / Watt*Hour meters used by RC modelers may be a good, lower cost, alternative.

Everything make sense?

-Bill

57✭✭✭✭WOW, Thanks

I'm sorry, I don't know how Maine got there, but I live in Missouri, Not Maine

28,142adminSorry, I guess I got confused somewhere...

Picking St. Louis MO... We get around 3.1 to 5.9 hours of average sun (winter to summer)...

Just take the above equations, and put in your minimum by which months you will be out there working.

Basically, the "winter panel" needs to be 2x larger vs a "summer panel" to run the same loads...

Also, larger panels can be more cost efficient (lower $$$/Watt) pricing.

A 65 watt Kyocera panel may cost $6.33 per watt ($340 per panel)... A 135 watt Kyocera may cost $3.52 per watt ($475)...

So--much of this system design depends on your needs and if you will need any power growth in the future, plus if you have backup power available (bad weather) like a genset, haul the batteries back to home for charging, etc...

-Bill

PS: Make sure you include the costs of shipping, insurance, and taxes... Can dramatically affect overall costs of shipping large pieces of glass (solar panels).

57✭✭✭✭I want to leave a little room for growth.

Please tell me where I might find the best deal on a 135 watt Kyocera .

Thanks,

Tim

28,142adminI am not in the business--so I don't have any suggestions.

Our host, certainly is one place to start (that is where I got the prices I listed)... But you may find something near you with better pricing (once you have included the costs of shipping).

By the way, as the panels get larger--they freqently are at Vmp (voltage rating, maximum power) other than "12 volts" (really Vmp=~17 volts -- what is required to charge a 12 volt battery).

If you get higher voltage panels--then you need to look for a an MPPT (Maximum Power Point Tracking) Solar Charger. These can take higher voltages and downconvert them to lower voltage to charge the battery (and higher currents). Sort of the DC equivalent of the AC transformer.

-Bill