Solar Panel Watts = Volts x Amps

When determining the overall wattage of a solar panel we use this simple equation:

Watts = Volts x Amps.

When building a solar panel or looking for a solar panel, you should always know the final wattage, volts, and amps that you want the solar panel to produce. Your solar panel should always use the same size and same type of solar cells as well to keep the power consistant. Larger cells produce more current, or AMPS, and smaller cells produce less current. Larger cells will produce more power, but will make your solar panel heavier, and smaller cells will keep your solar panel lighter, but it will not produce as much power as larger cells. You also do not want to mix different sized cells together because the current or AMPS of your solar panel will be limited by the smallest cell in the group - wasting the power of the larger cells. SO ONLY USE SOLAR CELLS OF THE SAME SIZE AND SIMILAR AMPS.

What is a solar bypass diode?

Most solar panel systems consist of individual solar panels being connected in a series to produce higher watts at a steady amperage. For example, a 180 watt, 3.6 amp solar panel system may consist of three 60 watt, 3.6 amp solar panels connected together in a series. The overall productivity - 180 watts - of the solar panel system depends on all three of the 60 watt panels working efficiently. However, if one solar panel connected in a series string fails, due to shade, dirt, destruction or other factors, it will produce electrical resistance (called a hot spot) and now the weakest panel in the solar panel system determines the total current. The covered or damaged solar panel now acts like a resistor and it can actually damage the entire solar panel. A bypass diode creates a path or least resistance around the disabled solar panel and can help to protect your solar panels from damage.

Many panels are set up as actually two strings in one frame. So one 60 watt panel is actually made up of two 30 watt panels. Each half of the panel has a bypass diode, usually inside the junction box, or built into the frame to allow for electricity flow should one of the panels begin acting like a resistor.

It is recommended that a bypass diode be placed in between each solar panel in a multi-panel system to help keep the system working effeciently. I have read that bypass diodes are not needed in all systems, and usually not at all in 12 volt systems, but I feel that it is best to use them to prevent any long-term damage to your system.

What is a solar blocking diode?

If you think of the flow of electricity like the flow of water, then a diode can most closely be related to a one-way valve in a sewage pipe. It lets electricity flow one-way only and blocks it from coming back in - just like a one-way valve operates in a sewage pipe - letting sewage out, but not back into your house.

A solar blocking diode (or low voltage drop diode) is a diode that is used in the electrical schematic of a solar panel to restrict or block the electrical current coming out of the solar panels to reverse or flow backward through the solar panels. If the current were allowed to flow back into the solar panel, the solar panel could get damaged by the current and suffer from thermal destruction. So you do not want the power coming out of a solar panel to ever flow back into it.

It is claimed that if you have a charge controller and a single panel or single string of panels, that you do not need a blocking diode - but I still recommend that you put a blocking diode on every solar panel you create or that you purchase solar panels with blocking diodes to prevent any possibly damage to your solar panels.


It is important to note that the rating on a blocking diode - for example; 8 amps, 45volts - is rated on a free-air system. The effeciency of a blocking diode decreases as the temparature around the diode increases (as is the case when blocking diodes are installed inside solar panels and solar panel junction boxes, so make sure that you purchase blocking diodes from manufacturers that allow a safety factor when selling diodes to you. A diode I like to use is a Schottky blocking diode. The amps and volts depend on the application. Some sellers advertise an 8amp 45 volt blocking diode for sale, but the diode is actually rated at 16 amps. The company is good about selling this diode with a safety factor so that you can use it correctly in your application. Always check for the full rating of a diode and how temparature affects its performance if that information is available.

Understanding a solar cell - The solar panel building block

I am build a few homemade solar panels, and I am working on writing up an instructable and a video to show how easy these are to make. Before I post that information however, I wanted to introduce the basic building block of a solar panel and describe how to look for them, how they work, and their energy output. Once this is understood, building a solar panel becomes an easier task.

So what is a solar panel and what are solar cells? A solar panel is basically just a box that holds a bunch of individual solar cells that are connected together. A solar cell is the component that actually converts the sunlight into electricty. It takes a lot of these individual solar cells working together (in a solar panel) to produce enough electricty to be useful. When you look up solar panels, you will see that they come in all types of various wattages - ranging from 15watts, 30 watts, 60 watts, 240 watts, and higher. The number of solar cells in the panel, and the way that they are wired is what determines the solar panels overall power production.

To explain what I mean about how solar cells ultimately determine the overall power of a solar panel, I will discuss the power rating of an individual solar cell, solar cells and overall wattage, wiring solar cells together, and how to connect solar cells together in your wiring schema.


POWER RATING OF AN INDIVIDUAL SOLAR CELL:
Solar panels are rated on three measures of power: Watts, volts, and amps. The first thing you need to know about solar cells is that typically all of them creates a little more than .5volts of DC electricty - no matter what physcial dimension they are. The only thing that changes with the size of a solar cell is their current, or AMPS. So if we take our 3"x6" solar cell example and look at its electricity output - it is more than likely rated at .5volts with 3 amps. If we were to break that solar cell into half, we would get two solar cells that rate at .5 volts and 1.5 amps each.

When determining the overall wattage of a solar panel made up of combined solar cells, we use this simple equation:

Watts = Volts x Amps.

When building a solar cell, you should always know the final wattage, volts, and amps that you want the solar panel to produce, and you should always use the same size and same type of solar cells. Larger cells produce more current, or AMPS, and smaller cells produce less current. Larger cells will produce more power, but will make your solar panel heavier, and smaller cells will keep your solar panel lighter, but it will not produce as much power as larger cells. You also do not want to mix different sized cells together because the current or AMPS of your solar panel will be limited by the smallest cell in the group - wasting the power of the larger cells. SO ONLY USE SOLAR CELLS OF THE SAME SIZE AND SIMILAR AMPS.

HOW MANY SOLAR CELLS DO I NEED:
So now that we have an idea how individual solar cells are rated and how their power production is rated - how do we know how many solar cells to use to make a final solar panel with a pre-determined amount of wattage? We do this by using the equation for wattage:

Watts = Volts x Amps

So lets say that we want to create a solar panel that puts out 60 Watts. We know that our 3"x6" solar cells put out .5 volts and have 3.3 amps. So we take 60 watts divided by 3.3 to get our overall solar panel voltage. 60watts/3.3amps = 18volts.

Now that we have our overall solar panel voltage, we can find out just how many solar cells we need to get 18volts out of the solar panel. To do this we take:

Solar Panel Voltage / single solar cell voltage = Number of cells needed per panel

So in our example, we found out that a 60 watt solar panel will have 18 volts. So now we take 18volts/.5volts = 32 solar cells needed. So we know now that it will take us 32 .5volt solar cells to create a single 60 watt, 18 volt solar panel. This means that we will need to wire these solar cells together in a series to achieve the voltage increase we need.


WIRING SOLAR CELLS TOGETHER IN SERIES OR PARALLEL
There are two ways that you can wire your solar cells together in your solar panels

1) Series wiring (positive to negative and negative to positive)
2) Parallel wiring (positive to positive and negative to negative)

Wiring solar cells in series will increase the voltage of the solar cell, but will not increase the amps. Wiring solar cells in parallel will increase the amperage, but will not increase the voltage.

Using our 3"x6" solar cell with .5volts and 3.3amps I will explain how these two different methods of wiring solar cells together drastically changes the power output of your solar cell.

Series Wiring
If we take two 3"x6" solar cells by connecting the positive terminal of one cell to the negative terminal of another cell, and the negative terminal of the same cell to the postive terminal of the other cell, then we will have series wired the two together creating an increase in voltage to 1 volt with the rated 3.3 amps not changin. If we were to series wire six of the solar cells together, we would get 3volts (.5x6) at 3.3 amps, and so on.

Parallel Wiring
Parallel wiring refers to connecting solar cells to increase amps, but not volts. If you have two .5 volt solar cells rated at 3.3 amp hours, for example, by connecting the positive terminal of one solar cell to the positive terminal of the other, and the same with the negative terminal, then we will have parallel wired the two together. In this case, we now have a .5 volt solar cell and the rated 3.3 amps increases to 6.6 amp hours.


CONNECTING SOLAR CELLS TOGETHER:
A single solar cell will not yield a lot of useful electricity, so you need to connect them together into a solar array - inside the solar panel. Connecting solar cells together is relatively easy, but it is a slow, gentle process that should be done carefully and with a lot of attention. Each solar cell has postive leads on the bottom of the cell, and negative leads at the top of the cell. There are two ways to connect solar cells together. The first way is to solder the tabs (or wires) that are already connected to the solar cells to the tabs or another solar cell, and the second way is to buy a metal solar cell ribbon and solder a strip of it to the back of each cell you want to connect (soldering to each of the six rectanglar points on the cell)

It is important to note that not all solar cells are sold with the tabs already connected, but that all solar cells have connection points on their back-side that look like a metal rectangle (6 total) which are used for soldering the ribbon into place. I recommend that if you can find solar cells with the tabbing already connected - that you purhcase these. They are easier to solder, the will cost you less in material, and they are faster to install with less steps and less soldering. Using the solder ribbon to connect solar cells will more than double the time it takes you to create a solar panel, and leaves more room for errors and connectivity issues.

When soldering solar cells together, I recommend the use of a flat-tip 30 watt soldering iro and silver bearing solder. You can find both of these at Radio Shack.

Be sure to solder the connections in series or parallel, based on your application.