All solar modules create electricity when sunlight photons hit the PV cells and dislodge electrons. These electrons move from the cell through the internal PV panel wiring to exit the panel through the junction box. Simple enough right? Sure. However, solar cells are arranged to provide an optimal output voltage for a given application. Historically, that application was charging batteries. Similar to batteries, the cells of a solar panel are wired in series to increase voltage and in parallel to increase current. Also (again, similar to batteries), solar panels themselves can be wired in series or parallel to achieve the desired voltage and current output of the array.
The above information holds true for every solar panel. It is important to understand this relationship as it has important relevance to how we use solar panels today. Therefore, I’ll explain further.
Note: I will simplify some voltage numbers along the way, just in case you’re wondering…
Let me introduce you to two friends of mine on the left, the poly-crystalline and mono-crystalline solar cells. Simply put, poly-crystalline solar cells are made from a number of different ingots (solar raw material), and mono-crystalline solar cells are made from a single ingot. They both do the same job, but mono-crystalline cells tend to slightly outperform their poly-crystalline cousins in a few applications. For our purposes, we’re going to treat them the same (no prejudice here!).
Let’s say each of my cellular friends (above) produces 0.5 VDC. Solar panel manufacturers put a bunch of these cells together in series to create a “string” of cells.
As we discussed earlier, manufacturers made solar panels for specific applications. Therefore, solar panels were made with series “strings” of 36 cells per string, like the image on the left to create an output of around 18V to charge 12V batteries. Why 18V? Remember, voltage is pressure. You need that pressure to overcome resistance in wiring, circuits and battery banks themselves in order to charge them. As solar cells became more efficient, they got smaller with the same voltage output. Solar panel manufacturers began building more powerful solar panels. They still needed the same voltage output, so they made 72 cell panels, which were internally wired with two parallel 36 cell strings in series. Confused? Don’t be. Remember, we do that all the time when we build battery banks. Sometimes, these panels were built specifically to charge 24V battery banks, so they simply wired all 72 cells in series.
Solar Panel Evolution
Years go by. Solar charge controllers are built to properly charge battery banks within the well defined voltages of the panels being created. The solar universe was at peace, until the day solar became popular for doing more than just charging batteries. This new market didn’t care about battery voltage. they wanted inexpensive solar. The manufacturers obliged with a number of different cell configurations until they settled on what is now the most common cell configuration of 60 cells.
These new, 60 cell solar panels are less expensive and mass produced to fill the much bigger grid-tie market. In most cases, grid-tie solar arrays don’t care about battery voltage. They are wired to match the (much higher) voltages used by grid tie inverters.
Eager to utilize these readily available and less expensive solar panels, the battery based application industries wanted to use these new panels in their installations as well. But, as I mentioned, these new panels don’t provide the right voltage for charging batteries with the existing solar controllers. They can (in some cases) work, but will do so very inefficiently. Sometimes, they don’t work at all. To understand why, you have to do some research into how Pulse Width Modulation (or PWM) works to use solar energy to charge batteries, or have a look at the Charge Controller Toolbox for how to select the right controller for your application. This page will just assume you’ll select the right controller.
How Much Solar Do I Need?
First off, let me say that a “Watt” is a “Watt”, is a “Watt”. What do I mean by that? I simply mean that a Watt is a unit of power, and power is the rate at which energy is produced or consumed. So, the rating (in Watts) on a light bulb, appliance, or solar panel tells us how much power can be either consumed (load) or produced (solar panel).
A Watt/Hour is simply a measurement of power over time. A Watt rating all by itself tells us how much power can be consumed or produced. A Watt/Hour rating tells us how much power was (or will be) consumed or produced over a given amount of time..
There are many ways to discover how much solar you need, and it all depends on what you’re trying to do with it.
- If you have a single load such as an industrial application, then you use the daily use of that load.
- If you’re trying to power a cottage or parallel generation system, then you use a load calculator (like the one on our Calculators page) and discover how many watt/hours of energy you use, and therefore need to produce.
- If you’re trying to reduce your electrical bill through net metering, you’ll want to know how many kWh of energy you consume in a year, broken down into months, then divide into days.
- If you’re trying to make money through a “feed-in tarrif” then you look at the tarrif rate paid per kWh and look at your budget and available installation area to see how much solar you can install and what that solar will do for you.
In all of the above cases, you need to know how much your solar array will produce on a daily/weekly/monthly/yearly basis, depending on application and location of your solar array. To determine that, you need to use solar charts or maps.
A fabulous resource for Canadians is the Natural Resources Canada Photovoltaic and Solar Resource Charts. This website is categorized by municipality or city and lists the PV potential at a number of installation angles. For those that don’t know yet, solar panels need to face the sun, and the angle of installation will impact the performance of the array differently depending on the season. The average number of sun hours per day varies greatly from month to month, season to season and tilt angle of solar panel installation. The graphic below explains tilt angle.
Use the above graphic to pick the number (1 through 5) that best describes your solar installation tilt angle. Use that number with the NRCAN Solar Insulation chart values to determine your base number of sun hours per day, which you will use to size your solar array. Remember that the base number you choose will also depend on your application.
Off-Grid seasonal cottage: Select the lowest number corresponding to a month that you will still be using the cottage.
Off-Grid year-round cottage: Select the lowest number of the year, unless you indend on supplementing your energy production with another source (wind, gas generator, etc…)
Grid Tie net meter: Select the average number for the year.
In all cases, you can use our solar calculator excel workbook to help you. The output graph sheet is editable so you will be able to input your actual sun hours per day in the chart and see how your solar output may cover your needs from month to month.