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Solar Panels Overview

 

Solar panels are widely misunderstood. Many RVers want to know how many items that they can run off of solar panels. But, the truth is that the solar panels don't "run" anything - the batteries do. The solar panels are merely a way to recharge the batteries. Think of them as a trickle charger of sorts. The battery will store up enough energy to run a given size load for a given amount of time. It can run a small load for a long time or a large load for a short time until the batteries run dead. Solar panels are great for utilizing the free energy from the sun and putting it back into your batteries. Ideally, you would use a large load for a short period of time, then allow the solar panels to rebuild the battery charge state before the next load is applied. Unfortunately, the ideal world isn't the real world so we have to adapt accordingly.

Okay, we've established that the batteries actually run our loads. In theory, we could hook up a large pile of batteries and charge them up before we leave on our trip. Then we could use these batteries while RVing without concern for charging them. We could then wait until we get home again to plug in and recharge them. However, this would require pulling a large trailer full of batteries which would be totally unpractical. So, the only solution is to cut down the battery bank to a more manageable size and charge them as we go.

The charge state of our battery bank is affected by two things - how much we take out of them and how often we can recharge them. Today's RVs are pretty complex and come with all the comforts of home. However, when we run our DVD players, coffee makers, surround sound systems, and wide screen plasma TVs we can burn up amp-hours at an amazing rate. As the batteries start to run down, recharging will be needed. Recharging can come in many forms. If we are driving down the road the vehicle's alternator will recharge them while we drive. If we are parked at a campsite with shore power, then the shore power will run the converter or inverter/charger to recharge them. If we are parked where there are no hookups then we have a problem.

When boondocking or dry camping it is important to regulate and manage our energy usage. Eventually, there's only so much you can cut back on and the batteries will need to be recharged. That's when you can fire up your standby generator set. The generator will run any AC loads as well as power your battery charging system. Unfortunately, it's not always convenient to be running your generator. Depending on your present camping situation, it may be intrusive on your fellow campers or there may be quiet times enforced where it's just not possible to run your generator. You may need to recharge the batteries mid-day while you are off sightseeing and there's no one around to start and stop it. Plus, generators do burn fuel, which is an additional expense.

If you are the kind of RVer who moves around often enough, rather than stay in one place, then there may be an easy solution. Just add more batteries. That will allow you to consume power for a greater length of time. Hopefully, by the time they need recharging you'll be headed down the road and you can charge them while you drive. Even if you are spending some time in the same location, the extra batteries may just extend your recharge time to the point where it's more convenient for you to run the generator set to recharge them.

If you still need more battery charging, then solar power may be an option for you.

First of all, solar energy is not "free" energy. While the suns rays are free, the cost to build a solar panel system can add up pretty quick. This is not going to be a great cost recovery investment but it may allow you to enjoy your camping style that much better. Think of it as an investment in lifestyle, not a financial investment. Solar panels are rated in watts. A 120 watt panel is "supposedly" capable of putting out 10 amps at 12 volts. There are a number of factors that change this which we will cover in the Solar Panel System Design topic but for now we'll keep it simple. These 10 amps are only available when the sun is hitting the solar panel at a 90 angle and it is shining at full intensity. Assuming we have this situation, which is not very likely, we can expect 10 amp-hours for 12 hours - which gives us a "free" battery charge of 120 amp-hours per day. 120 amp-hours doesn't go real far but it helps. If we add more solar panels and do a few other tweaks, we can raise the total amps available to recharge the batteries.

By looking at our 120 Volt Appliance Load Chart we can get a good idea of the total wattage needed by our 120 volt appliances. Divide these wattages by 10 to find out how many 12 volt inverter amps will be needed to power them. Then multiply these amps times the number of hours (or fraction of an hour) per day that each item will be run. This will give you the battery amp-hours per day needed to run them. Also figure in your 12 volt loads when calculating your total current demand for each day. Just take the 12 volt amp rating times hours to get those figures. Add them all up to get your daily grand total of battery amp-hours needed. Then look at your battery bank amp-hours. If you have the typical four 6 volt golf cart style batteries you will have 440 amp-hours of battery capacity. You should never draw the batteries down below 50% so that means that you will have 220 amp-hours of usable battery power. If you daily demand is 240 amp-hours, then you will be short by 120 amp-hours. In order to make that up you'll need to provide an extra 120 amp-hours of solar energy. Then, every 24 hours you will have to run your generator set to fully recharge your batteries so that they can run another day's loads because they'll still only be at the 50% charge level. For an example of load and solar panel design charts refer to the Solar Panel System Design topic in this library.

By carefully regulating our power consumption, sizing our battery banks properly, and doing a careful analysis of our power needs we can calculate how much solar power we will need to handle our own unique lifestyle. They are not a cure all, nor are they for everyone, but there are certain applications where they can really add to the enjoyment of boondocking or dry camping.

 

Submitted by Mark Quasius - 2/22/06
 

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