Solar panel questions from a noob :)

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

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

10 Watts times 8 hours per day of sunlight = 80 watt-hours.... plus or minus depending on season, location, and weather.

Perhaps subtract about 15% for inefficiencies in charging the battery, so we end up with around 70 watt-hours/day to work with.

So on average, that would allow running one 10 watt appliance for 7 hours per day.
My blender requires 230 watts, so it could be operated for 18 minutes per day.
My cell phone charger uses 4 watts, so it could be used 17 hours per day.

Two panels would charge one battery twice as fast.

Thank you for your replies - I really appreciate you taking the time to get back to me

So if I have two batteries could I connect these together to provide double to amount of power and would this be a better option as opposed to having one in use while the other charges?

'My blender requires 230 watts, so it could be operated for 18 minutes per day.' Can I ask how you figured this out?

If I was to search for products to buy (appliances) what should I look for with regards to determine how long I could use it for (or even if I could use it)?

Also (apologies for the questions) how does voltage tie into this? Is voltage simply something which can be used to avoid too much electricity going to an appliance and overloading it?

Once again thank you for your replies,

Samuel

Think of a battery as a swimming pool. You can take water out and put water in at the same time. If you put multiple hoses into the pool it fills faster. If you put bigger hoses into the pool it fills faster. You can take water out through a small hose, or lots of small hoses, or through a huge hose, but when it's dry you gotta wait while it fills up enough to use again.

I measured the power consumption of the blender by using a 'Watt Meter'. It plugs into the wall, then I plug the appliance into the meter. Alternatively the blender has a tag on it that says 120V @ 3 amp. Watts equals voltage times amps, so that equals 360 watts. To use that number in an example... Assuming that 70 Watts/day get stored in the battery divided by 360 watt hours = an average of 0.19 hours per day that a blender could be used, or 12 minutes per day. Or if the blender was only used once per week it could run for 12 X 7= 84 minutes per week. (Presuming the battery is big enough.)

I intentionally avoided introducing voltage into my math... But in simplest terms there are 12 Volt appliances that are most often used in RV and automotive applications, and for solar systems, and remote cabins. Then there are 120V systems that are typically used in grid-tied conditions. There are pricey converters to switch between voltages.

Gerard

Could you post a link to the calculator you use for A/H.
Thank you