Solar Panel System Design

Once you have decided to go with solar power you have to properly design a system that will meet your needs. Solar is not a magic word for free power. If you just hang some stuff on your RV that isn't designed to satisfy your particular energy needs you won't be very happy and the money that you have invested into the system is wasted. In order to properly design a system you need to first calculate your energy needs, then design a system that will meet these needs.

Calculating Your Loads:

The first thing you need to do is define the load requirements of whatever items you will be running off your batteries. Remember that we learned in Solar Panel Systems that it's the batteries that really run our loads and the solar panels are merely a source to help recharge them. When we run any 12 volt loads, such as lighting or water pump we are drawing amp-hours from the batteries. Whenever we run 120 volt AC items we will be running these items through an inverter, which converts battery power to AC power. We'll need to find the wattage of each item that is being used. Remember that volts times amps = watts so if we know the amperage but not the wattage we can easily calculate the wattage. Because inverters run at right around 90% efficiency there will also be some loss in the inverter. When you take your AC loads divide by 10, rather than 12, to derive any amps.

In the sample table below I'll use some of the calculations I used when sizing my system:

The best method of measurement to use when calculating battery consumption is to go with amps. Therefore I've converted the 120 volt wattage ratings to DC amps. Because we mentioned earlier that inverters have an efficiency loss it's real easy if we just divide the 120 volt wattages by 10 to get to DC amps. I purposely eliminated heavy AC loads such as air conditioning, electric hot water heaters, and washer-dryers. These large amperage loads will draw down your batteries in a hurry so they are scheduled for operation when the generator set is running. The same holds true for a convection/microwave oven. If using in the microwave mode the runtime will be less so it's fine for inverter use. But when running in convection mode the runtime gets fairly long and the draw on the batteries will be too severe so we'll reserve that for generator use as well.

Calculating Your Energy Needs:

The next step is to calculate how long we will run these items. The longer we run them, the more amps they will take out of the batteries. The intention of this system is to run the generator set once a day to recharge the batteries and run various high AC loads. This means that we'll need to calculate battery loads per day. The following table shows the expected daily runtime in hours of each item as well as the total amps consumed per day. In the case of the portable freezer we will factor the daily amps by 3 because we're assuming that, while plugged in for 24 hours, the freezer will only run 20 minutes out of each hour.

The above conditions show that we will consume 232 amps from our batteries each day. Now let's determine how many amps we have to spend.

Calculating What We have:

A bank of 4 golf cart style 6 volt batteries (220 amp-hr rating) will have a total 12 volt capacity of 440 amp-hours. You can't draw the batteries down below 50% so we have the ability to draw up to 220 amp-hours before they will need recharging. Because we plan on running the generator set, or moving to a new location, each day we won't really need any solar power. Technically, we're drawing an extra 12 amps below the 50% threshold but that's not a large amount so it's not critical. However, if we consume more power or decide to dry camp for 2 days without generator set usage we'll be in trouble. Realize also that these are optimum conditions. What if we didn't have enough time to run the generator set to bring the batteries to a full charge? What if we encountered cooler temperatures and the propane furnace ran quite a bit in the evening? What if our particular RV didn't have that many batteries or the capacity of each battery was lower? These situations would change the above consumption figures.

If we decide to boondock for 2 days without running the generator set our load will increase to 464 amps over those two days. We only have a maximum of 220 amp-hours in the batteries so we are short by 244 total amp-hours, which is an average of 122 amp-hrs per day. Assuming that we don't want to increase the battery bank size, or run the generator more frequently, we can look to solar to fulfill that need. The next step is to calculate how much solar we will need.

Calculating Our Solar Needs:

Solar panels only work when the sun hits them. If you're camping in Arizona in the desert in summer you'll get great output from your solar array. But, if you are camping in the north woods during the rainy season you won't get that much output. They also need to be at a right angle to the sun's rays in order to put out maximum performance. Most RV installations are flat mounted. Even at noon the sun is never directly overhead (unless you are RVing on the equator) so there is some loss here as well. Therefore early morning and late afternoon may only give you 25% of the performance that you are expecting. In the following example we'll assume that the sun is out for 8 hours per day. To allow for the varying angles we'll assume an average efficiency of 50% when making these calculations.

120 watt panel = maximum output of 12 amps (average output of 6 amps)

6 amps x 10 hours = 60 amp-hours per day

The above scenario shows that we can realize an average of 60 amp-hours from this configuration per day. The problem is that we need at least 62 so it's not enough. In order to continue to RV in this manner we will have to change some things. We have a number of options available to us. First, we can add a MPPT Charge Controller to boost our output by 30%. Actually if we don't add one of these we'll get even less output from our solar panel at peak sunshine when the volts are high and the amps are low. Secondly, we can add a second solar panel to double our output. Thirdly, we could add more batteries to increase the runtime between recharge cycles. Finally, we could run the generator set more often.

Keep in mind when doing these calculations for your own situation that there are a ton of variables that come into play. Sunny Arizona versus the rainy Pacific Northwest is one of them. Also, the brand of solar panels makes a big difference. Some panels, such as the Shell-Siemens, put out fairly decent output in low light conditions while some brands rapidly fall off when the light isn't perfect. While it's not a hard and fast rule, generally the less expensive ones tend to fall off more. Also, some panels put out very high voltage. You trade volts for amps so a good MTTP charge controller is needed to bring those amps back. You aren't going to get maximum efficiency unless you buy tilt-up mounts for your panels. But, do you want to have to go up on the roof to set them up every time? And, do you want to crawl up on the roof when it's time to break camp and it's raining so that you can drive down the road? What about weather? Cold weather really puts a load on an RV's electrical system and your batteries will need more capacity. Yet, when the batteries get cold their capacity decreases. It's best to upsize your system enough to allow for these and any other variables. By doing so you'll have a solar energy system that will comfortably accommodate all your needs.

Submitted by Mark Quasius - 2/23/06
 

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