Monday, February 19, 2018

Basics of Solar Panels: Sizing Your Array

So you have decided that you want to install solar panels. Now you need to determine how many panels to get, and which kind.

To figure out the sizing of the components of your power system, you will need to answer a few questions first.

What Will You Be Running?

What appliances will you be running off your panels? 
The first thing you will want to do is make a list of each of your appliances that you want to use when using solar/battery power, which will form the foundation of what you will need.

As you make this list, list the power use (in watts) of each item next to it. Every appliance has a wattage rating, which tells you the amount it should consume as you use it.

For example, I have a light fixture that uses fluorescent tubes. Each tube in this fixture consumes 17 watts of power.

How many of them?

In the case of my light fixture, I can run it with 1-4 bulbs in it. Make sure to mark this on your list.

How many of them at once?
Look at your list of appliances, find which ones you plan on running at the same time, and mark them. You might (for example) mark each appliance on the list with a M for Morning, D for Day, E for Evening, and N for Night.

Add up each the wattage of each appliance used during each of these times separately, and find the highest total out of them. Take that number and multiply it by 1.1 (this gives you a little extra room so that if there is a problem, you are not stuck) and this will give you your minimum peak wattage. Go ahead and round to the nearest ten watts.

In my example, I have a light that uses 34 watts (2x 17 watt bulbs) and a laptop that uses 12 watts. You will probably want more, but this is just to show you an example.
  • 34 + 12 = 46 watts. 
  • 46 * 1.1 = 50.6 watts
I need at least 50 watts for this example. This lets me know how much wattage I will need for the inverter, to get AC (household) power from the battery bank, or directly from the panels. This will also let me know how many watts of charge controller to use, if I decide to charge batteries and use those for power.

Remember that many items will pull slightly more power as they start up than when they are in use!

Most real world systems will use much more power. I use a 2000 watt inverter at home and a 400 watt inverter when I am out doing extended camping trips. I keep a 100 watt inverter (with two USB charge ports built in) in my car most of the time, just to use for charging electronics on the go. It is better to have more than you need than not enough!

When Will You Be Using It?

During the day?
If it's only the middle of the day, you shouldn't need a lot of extra solar panels, and only a minimum amount of battery backup. 

If you are running something directly off of solar power, you will want to multiply the minimum peak wattage by 1.2, in order to make sure that you have enough continual power output. Having your equipment shut off and on because a cloud passed overhead, or a butterfly landed on the panels, and caused just enough power drop is both maddening and often damaging to the equipment. The extra power also makes up for inefficiency in the inverter itself.

In the example with the 50 watt inverter, you want at least 60 watts of solar power (50 watts *1.2= 60 watts) if you want to run the inverter directly off of the panels.

At night?
If you are using your power setup at night, you will have to have a large battery bank and a sufficiently large solar panel array to charge them during the day.

There will be a later article in this series on sizing your battery bank

In the summer? 
Days are longer in the summer, giving you more time that the solar panels can be used to power devices or to charge batteries. You will not need as large a solar panel array if you are using it to charge batteries at night.

In the winter?

Days are shorter in the winter, and panels can become obstructed by things like snow or fallen leaves. Bad weather, such as clouds, will reduce effectiveness of the panels, meaning that you must have a larger panel array in order to get the same amount of power.

If you plan on using the solar panels on a cloudy day, assume that you need between 30% and 50% more panels to get the same amount of power. Less sunlight means less power; I multiply my normal solar power needs by 1.5 in order to get a reliable cloudy day power amount.

In my example, if I need 50 watts of inverter power, and I am running the inverter directly off of the panels, I want to calculate the normal operating amount of solar power (in this case 60 watts) and multiply it by 1.5 (60 watts * 1.5 = 90 watts), giving me a minimum of 90 watts of solar power in order to generate 50 watts of power on a day with light cloud cover.

What Other Power Sources Do You Want to Use?
Will you have access to grid power on a bad day, or is this totally off grid? Will you have a gas or propane generator on hand for hard days, or perhaps a wind power generator?

Remember that if your plan is to occasionally “plug in”, either into a generator or grid power, you don’t have to worry about the worst weather and other situations. There are a number of RV users that use this method, as well as people who have off grid cabins.

In addition, if you are planning on using a generator to supplement your solar panels, you can use it to charge batteries on bad days.

Having supplemental power allows you to size your array for the majority of your use time, without having to size it large enough to deal with the extreme situations.

A Note on Batteries
I strongly prefer charging my battery bank from the solar array and using that to power my appliances and electronics. Having a battery bank means that if I have slight cloud cover it is not an issue, and if I want to watch a movie on my tablet after dark it is not an issue.

While it is possible to run things off of the solar panels directly, I strongly recommend that you also have at least a small battery backup from which to charge.

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