Samuel,
Let's start with the 10 watt panel. I'm going to make a few assumptions as we go along but I think you'll get the idea of the general principles involved.
A 10 watt panel tends to mean it will provide 10 watts at a given voltage when being exposed to unobstructed sunlight. So, let's say the given voltage is 12 volts. Most off-grid solar systems utilize batteries to store the power until needed, and deliver it in larger quantities for shorter periods of use.
A battery is a bank where watts can be stored until needed. You're assumptions regarding a battery which can deliver 10 watts for 100 hrs would also deliver 20 watts for 50 hrs is correct.
With most solar designs, we start with the needs (or loads) as the base requirement.
So, let use your message and start some calculations:
LED lighting:
Very efficient, especially if using 12v
lights which do not require an inverter. I use 3-10 watt/12v LED floods inside my barn, and 2-20w floods outside the barn on a motion sensor. These lights are used only during tending the horse at night, or late or early feeding, and the outdoor motion controlled lights seldom run for more that 10-20 minutes a night. Since most devices uses that watts number to indicate watt/hours, here's my power requirements for my lights assuming all lights will be used for 20 minutes a day:
70 watts/20 minutes= 23.33 watt/hrs per day.
So now I need to convert that to numbers that match battery rating which are typically AH (amp/hours). I use a web based ohms law calculator for this and get 1.94 AH per day. Let's say 2 to make the calculations easy.
We also have to consider how many days we want the system to work without sunshine. I use 3 days. So I need 3 times my needs to be available in the battery when I need it. That's 6AH. When using lead-acid batteries it is wise to only drain them down to 50% of their rating to prolong life. That means for the above example, I
should purchase a battery with 12AH of capacity.
This has told be how much battery I need to power the stuff I want to power, when I want to power it.
Now I can calculate how much solar panel I need to make this all work. I want the system to be able to recharge from a 50% drain in 1 day. Using the same ohms law calculator, I need panels that can put back 6ah at 12 volts in 1 day.
It gets tricky here because each location is unique and has different sun exposure issues. In solar terms, we call that insolation, and you can find the insolation for your area using google. Mine is an average of 4.5 hrs of sun a day.
I need to put 6ah back into the battery in 4.5 hours. Using the ohms law calculator I find out I need 72 watts in 4.5 hours to equal 6ah. That's 16 watts/hr. I need at least 1 16 watt panel to recharge my system.
This approach applies to all off-grid systems using batteries. Start with the work to be done, determine the battery capacity and the amount of backup power (how many days without sun), and then calculate how much solar panel is needed to recharge the system.
You can use the same calculations for the 12v pump. The blender is going to be a little more complicated unless it's a 12v bender, and I'm thinking it's going to be a power pig.
Also, for any moderately small system you'll find you'll need a solar charge controller to prevent battery over-charging and manage the battery discharge.
Samuel Morton wrote:Good evening everyone,
This is my first post and forgive any ignorance regarding my electrical knowledge.
I am thinking of buying some solar panels for my shed roof which I hope will power (not full time but as and when said appliances are needed) a LED lighting strip and a small water pump to help me wash my produce (and maybe a blender and phone charger).
The solar panel system I am looking to buy is 10watt 12 volts and I was just wondering about the following things:
- what appliances could this power?
- if this solar set-up can power a 10watt appliance for 100hours (for example) would a 20 watt appliance be able to be used and would last 50 hours with this set-up?
- if I buy two systems and have two batteries and connect these together will I then have 20watts and 24volts?
- Will 1 panel take twice as long to charge two batteries and would two panels charge a single battery twice as fast?
Thanks for getting back to me,
Samuel