# Designing a Solar powered sign. Need help determining how many watt solar panel?

I am trying to run a sign off of solar power. The sign has 40 LED modules. Each module has 3 individual leds. The specs for each module are 12v, consumes .7 watts, and 50 mA. Multiplying watts and amps by the number of modules you get 28 watts and 2 amps. I'm looking at solar panels from walmart and they come in 10,15,20,30 ,60 watt panels. If the sign is ran 12 hours a night(6pm-6am), That means 28 watts x 12 hours = 336 watts used per night. If the solar panel gets 8 hours of sunlight to charge(8am-4pm), that means 336 watts/ 8 hours means I need a 42 watt solar panel? Also, how big of a battery do I need? 2 amps x 12 hours = 24 AmpHour battery?

EDIT:

I'm going to use a 12V timer relay so I can run the LEDs from 6-11 pm and 5-7 am. This decreases the on time to 7 hours and 196 watts. With 8 hours of sunlight, that means 25 watt charge. Also 2 amps x 7 hours = 14 AmpHour battery.

## Discussions

Best Answer 3 years ago

You need to upscale everything, because you have losses and inefficiency to deal with. Your basic maths is in the right order, but you need more than double the panel capacity, to allow for cloud, and panel orientation, and day length changes. You need a MUCH bigger battery, because you will kill it very quickly - like 50 charges, if you flatten it dead. Get a deep cycle, or Cyclon battery, probably 100 Ah+

You'll need a charge controller too.

Answer 3 years ago

What do you think of my answer?

3 years ago

This is a big one with solar.

Where I live we have as little as 50 hours full sun a month. (December) and as much as 300 hours of full sun a month in the summer.

I hope your hours of full sun is more stable.

Take the average number of hours of full sun a month you get. = (A)

Take the number of watts your sign will need in a month. = (B)

Divide the watts by the hours of full sun. = (C)

With solar use the rule of 3.

Multiply the watts divided by sun hours by 3. = (D)

so B / A = C x 3 = D

D is the size of the solar panel you need.

Since your number will be between solar panel sizes go with the larger size.

In generating systems 50 to 60% efficiency is average some expensive systems reach 80% efficiency, and solar is a generating system.

If you look at the back of your microwave oven on the specs label, it will tell you input 1500 watts, output 1000 watts. 1/3 of the power is used just to run the microwave oven.

Take 1/3 of the time for the power used to run your circuit and take 1/3 of the time for when the battery won't run the circuit and LEDs last take 1/3 of the time for the power the LEDs use.

1/3 for the lights

1/3 for the circuitry

1/3 for the point when the battery will no longer run the lights.

45 AmpHour battery.

and that is if you get an average of ten hours of full sun a day otherwise you will need a bigger battery.