by Bill Moeller
Dividing the watts by 12 volts gives the load size; multiplying by 4 gives the 25% capacity of the battery bank in amp-hours. Here’s an example:
1,000 watts ÷ 12 × 4 = 333. 33 amp-hours
This would be the safest minimal size battery bank to use with this inverter, although we believe it would be prudent to make the bank even larger than the calculated figure. One inverter manufacturer states in its literature that 50% of capacity can be used. The reason is that most inverters operate for only a few minutes at a time, and the batteries will not be damaged with that amount of short-term use. To recalculate our example:
1,500 watts ÷ 12 × 2 = 250 amp-hours
The manufacturer is probably right because you rarely run an inverter at full rating, and so the load would be smaller than required by the full rating. However, we would still be cautious in operating a 1,500-watt inverter with such a small battery bank capacity, and would be more comfortable with a battery bank capacity of 440 amp-hours.
Although there are large inverters capable of operating high-wattage items, such as an air conditioner, these items are very poor loads because of their high-current demand.
SYSTEM SIZE
The capacity of your battery bank and your charging equipment should be something that suits your individual needs. If you don’t use an air conditioner while boondocking, you may not need a large generator. If you don’t want to run a microwave, a small-wattage inverter may be satisfactory.
Remember that with any equipment, the higher the wattage output, the more it will cost. You can start out with a very modest charging system and then upgrade it gradually, if necessary, according to your needs. Our original setup included the following:
160 amp-hour battery bank
600-watt inverter/charger
two 40-watt solar panels and one 48-watt solar panel, totaling 128 watts
This system was adequate for our limited needs in our old trailer as long as we were conservative in our appliance use. However, we later added a 1,850-watt portable generator to take care of battery charging on cloudy days when the solar panels were not working at peak efficiency.
Using the tables we’ve provided, you can calculate your own typical amp-hour and wattage consumption, and then design your system accordingly. If you do a lot of boondock-ing, it’s a good idea to have at least two means of battery charging.
A FEW WORDS OF CAUTION
Finally, we offer a couple of thoughts about generator and inverter safety. Many of these units are wired with the ground (safety) conductor and the neutral conductor bonded together, which can be a very unsafe condition in an RV. This configuration grounds the metal of the RV’s skin to a current-carrying conductor, thus creating the possibility of an electrical shock if you touch the side of the RV. To be on the safe side, have a knowledgeable electrician check your equipment for you to see if yours has this type of wiring. Our 1,850-watt portable Coleman generator had a sticker on it stating that the generator had a "floating neutral" (not bonded to the RV chassis), which meant it was safe to use.
Fuses
Installing a fuse in the positive wire from the inverter to the battery is a necessary safety precaution. But you must use a fuse designated for DC use. Using an AC fuse is dangerous since if the fuse blows, the current can fuse or arc across the gap in the fuse. Use a Class-T DC fuse in a proper insulated fuse holder designed to prevent accidental contact with the fuse or the studs. Select a fuse size that is at least 25% larger than the maximum load the fuse is designed to protect. We have a 100 amp fuse on our 600-watt inverter positive lead, for instance. It is also a good idea to install a 300 amp fuse on the main ground wire of the batteries where it connects to the chassis ground and the ground side of the circuit breaker panel bus. This will act as a catastrophe fuse in the event of a major electrical short or failure, preventing your RV from burning to the ground.
Ground Fault Circuit Interrupters
Most small to medium-sized inverters have a ground fault circuit interrupter (GFCI) installed in the receptable. If you have a transfer switch (relay) in your AC wiring between the land-power cable and either an inverter or generator, you may find that the GFCI will constantly trip whenever you try to use the inverter. It may be necessary to change the receptacle to one without a GFCI to get the equipment to work.
Chapter 12
Sun and Wind Power
We think all serious boondock campers should consider using solar power and wind power. When you’re "off the grid," it’s great to be able to keep your batteries charged with alternative energy sources. We have had a lot of experience with solar panels over the years. In 1989 we bought two 40-watt panels producing 2. 55 amps each when we acquired our first 29-foot fifth-wheel trailer. They did a good job for us in a limited way. A few years later we added a used 48-watt, 3. 02 amp panel, and this increased our maximum charging capability to a total of 8.12 amps. When we traded the old trailer for a new 34-foot fifth-wheel just four years ago, we replaced all our old panels with two new 85-watt panels producing 4.72 amps each, plus a 100-watt, 4.51 amp panel. The 100-watt panel has a lower operating amperage than the 85-watt panels do, but it has a much higher operating voltage of 21.5 volts instead of the 18 volts of the 85-watt panels. We now have more than an ample charging capability of 13. 9 amps for all of our boondocking needs, and at times the amperage has been even higher, which we will explain later.
(RVIA)
Solar panels have become more and more popular with RVers, and many RVs are manufactured with installed solar panels. Wind generators haven’t gotten as much publicity, but we’re familiar with them because we lived aboard a sailboat for twelve years.
SOLAR POWER
Solar power is probably the most popular and best way to charge your batteries and offers many benefits to RVers:
Quiet: there are no moving parts to create noise.
Clean: there is no fuel or oil to clean up.
Safe: since there is no combustible fuel, there are no risks of fires or carbon monoxide poisoning; the low levels of electricity generated means there is a reduced risk of shocks
Easy: there is no maintenance except keeping dust off the panels; there are no complicated start-up steps—you just turn on the controller.
Efficient: it takes the free energy from the sun and turns it into pure DC electricity.
Good for your batteries: since it generates pure DC current, it charges your batteries slowly and steadily.
Solar panels give RVers more independence when boondocking. (AM Solar)
SELECTING A SOLAR POWER SYSTEM
A solar power system consists of the panels, a controller, the batteries, and an inverter. We’ve covered batteries and inverters elsewhere; here we’ll focus on the first two components.
What’s important when you go looking for a solar power system? Here are some things to keep in mind:
First and foremost, make sure the panels are rated for RV use. Many bargain systems are not.
Have a good idea of what your daily amp-hour consumption will be so you can talk knowledgeably with the dealer.
Look at the specifications: watt rating, amps delivered, tolerance, peak power voltage (see the Solar Panels section).
Know your budget. The cost of a basic charging system with one panel rated at 50 watts should be around $395. This should include the panel, the controller, the mounting brackets, and the necessary wiring, but not an inverter. More elaborate four-panel systems can run to as much as $4,000 to $5,000 for everything you’ll need. Individual panel prices can run from $295 to $700 depending on wattage size without a controller or the wiring. Controllers and regulators range in price from $45 to $350.
Make sure you have the right components for your proposed system. Is everything you need there and functioning?
SOLAR PANELS
Solar panels are made up of photovoltaic (PV) cells, which basically means they create electricity from light. The basic unit of a panel is a cell. Cells are wired together to form a m
odule, or panel, and panels are wired together to form an array. There are 30 to 36 cells per panel, each generating about 0.5 volt. The voltage of a cell remains constant, but the number of cells and the total area they cover determines amperage.
* * *
Buyer Beware: How Much Will It Cost?
Be on the lookout for misleading information. If you look through the magazine ads or go on the Internet, you’ll find many companies offering solar packages that promise you can have so many hours of power for your electrical needs. But often they are offering panels and equipment not suited for RV use.
Some ads express these figures in watts per day, which are not useful ratings, as they usually are not achieved. Other ads state that a package will give you AC power for your TV, computer, and other appliances, when all a solar system will give you is DC power for battery charging. If you want to run AC appliances off your batteries, make sure—before you buy—that the solar power system package includes a suitable inverter to convert 12 VDC battery power to 120 VAC.
So as you shop for a solar power system, be an informed consumer. The information in this chapter will give you a good start.
* * *
Cells are made from silicon, which is derived from quartz. The high-grade quartz used for the cells is mined and then refined to produce nearly pure silicon. Crystalline silicon is grown in the form of large crystals, which are cut into thin wafers to form cells. These cells can take one of three forms:
1. Monocrystalline cells are single-crystal cells that produce 0. 5 volt and are the most efficient.
2. Polycrystalline cells have many crystal pieces per cell. They are cheaper to produce than single-crystal cells, but also less efficient.
3. Amorphous cells are thin-film cells with no crystalline structure. They are made by depositing vaporized silicon onto a metal substrate. They are the least expensive to produce, and also the least efficient.
Choosing Solar Panels
Solar panels are rated in watts, which are derived by multiplying the panel’s peak power voltage by its peak power amperage (remember, watts = volts × amps). These ratings are based on standard test conditions of 1,000 watts per square meter of light input, a cell temperature of 77°F, and an air mass of 1. 5 (slightly above sea level). These conditions are rarely found in real-world operation, but manufacturers had to settle on standard parameters so that panels could be rated after being subjected to the same test conditions. This gives buyers a basis for comparing panels from various manufacturers.
As a boondocker, it is important to know the peak power voltage of your panels because efficiency decreases as cell temperature increases. We learned this the hard way one year at Quartzsite on a moderately warm January day. The panels were in full sunlight and they got so hot because of their dark color that the voltage dropped to the point that we were getting only couple of amps of charge into the batteries. These older panels had a maximum peak power voltage of only 15. 7 volts, so a 2-volt drop greatly affected the charging rate. We decided that if this happened again, we would try to restore full power by pouring cold water over the panels to cool them. However, before we got the chance, we purchased new panels. These had a peak power voltage of over 17 volts—a much better rating.
Table 12-1 lists specific brands of solar panels along with their specifications.
Another important specification to check is the manufacturing tolerance used in producing the panels. Most manufacturers use a 10% tolerance, which means that your 100-watt panel may, in actuality, only produce 90 watts. Look for companies that use a higher tolerance, perhaps 5%, since this means the panel’s output should be closer to its rated wattage.
How many solar panels do you need to boondock? It depends on the type of panels and their intended use. We’ll look at a few ways to answer this question:
Use 1 watt of solar panel output for every 2 amp-hours (1:2) of battery capacity—for example, a 50-watt panel with a 100 amp-hour battery. This is only a rough guide, but in our experience we have found that one 48-watt panel will, in bright sunshine, easily charge a 105 amp-hour battery with a modest DOD of 10% to 20% of capacity. If you have two 6-volt golf-cart batteries of 220 amp-hours, you would probably need about 110 watts of panel power, or two 53-watt panels (although two 48-watt panels would suffice with diminished electrical use).
Table 12-1. Solar Panel Specifications
If you spend a lot of time in the Northwest, as we do, use a more conservative guide: 1. 5 watts of panel output per 2 amp-hours (1. 5:2). For example, use a 75-watt panel with a 100 amp-hour battery.
Use your daily consumption estimate from Chapter 8. For a daily use of 30 amp-hours, you’d need two panels capable of delivering 15 amp-hours per day.
Identify when you’ll do most of your boondocking—summer or winter. If in summer, figure on getting about 40 amp-hours of charging time per day, and in the winter, about 30 amp-hours (with full sunshine). Then factor in your need to return 1. 2 amps to the batteries per 1. 0 amp used. As an example, if you have one 100-watt panel, your daily use could be 33 amp-hours per day in the summer (33 Ah × 1. 2 = 39. 6 Ah) and 25 amp-hours in the winter (25 Ah × 1. 2 = 30 Ah).
Keep in mind that these estimates are for minimal charging capabilities. You honestly can never have too many solar panels. It would be nice to have an array delivering up to 30 amps of charging power, and it is now possible. Only your budget and your RV’s roof size define the limits. Our current solar system provides 1. 28 watts of panel per 1 amp-hour of battery capacity which gives us a large amount of backup charging capability. The main thing is that a minimal number of panels can do the job, if you control and limit the amount of battery discharge.
Also, other factors may affect the actual charge rates of your panels, so they may occasionally deliver much more than the estimates suggest, such as a boost when cloud conditions permit it.
Remember Rule 7 from Chapter 9: "Never discharge your batteries to more than your charging equipment’s capability to recharge them within the next daily charge period. " We can see here that this rule definitely applies to the use of solar panels.
Insolation
Insolation is a measure (in kilowatt-hours per square meter—kWh/m2) of equivalent full sun-hours. A sun-hour is the maximum, or 100%, of sunlight shining on a module for 1 full hour. Even though the sun may shine for 14 hours from sunrise to sunset, the same light intensity is not falling on your panels constantly. The amount of insolation received varies according to the angle of the sun (which varies daily and seasonally), the state of the atmosphere, the altitude, and geographic location.
When the sun rises in the morning, the flat angle of the sun’s rays causes a lower output because of the reflection of light off the surface of the panels and the density of the atmosphere. Only the sunlight at noon penetrates the thinnest portion of the atmosphere and therefore gives maximum amperage output. As the sun sets, the rays again have to penetrate more of the atmosphere so the amperage starts to drop. In general, the hours from 9 a.m. to 3 p.m. are the best hours of sunshine during both the winter and summer. So it is important to calculate or monitor your amp-hour battery consumption every night. (For more on solar panel output, see the Solar Panel Tests section later in the chapter, which includes hourly solar output tables.)
Maps and tables showing insolation values are available for people building or equipping homes for solar operation. They are less useful for RVers, since we are usually traveling from one part of the country to another. But they do show how effective solar panels are in different areas, so we’ve included an example here. We have developed our own method of estimating panel size based on insolation values as seen in the scenarios below.
Scenario 1: Let’s say you only go RVing in the summer, and/or you have minimal battery requirements; you can use 48 amp-hours as your average daily amp-hour usage. Because it’s summer, figure 8 hours of usable sunlight (although it won’t always be at full intensity). Divide your daily amp-hour use by the number of hours
of sunlight to estimate the solar panel amperage output you’ll need. (The extra time the panels will be delivering amperage will make up for the fact that they will probably not be at their rated amperage for the full 8 hours.)
48 amp-hours ÷ 8 hours = 6 amps
This map shows variations in solar panel efficiency across the United States. (Renewable Resource Data Center, National Renewable Energy Laboratory)
In this example, an array of two 48-watt, 3 amp panels might be sufficient for most of your needs.
Scenario 2: You are a snowbird or a fulltimer and go south every winter; let’s use 65 amp-hours as your average daily amp-hour usage and 6 hours of usable sunlight:
65 amp-hours ÷ 6 hours = 10. 3 amps
In this case, you should have a minimum of two 90- or 100-watt panels of 5 amps each.
Scenario 3: Let’s work this problem backward using 6 hours of usable sunlight and the amperage output of the panels we currently use, 13. 9, to calculate how much we can discharge our batteries:
6 hours × 13. 9 amps = 83. 4 amp-hours
This tells us we could discharge our batteries a total of 83. 4 amp-hours, or approximately 40% of our batteries’ capacity, and still recharge the batteries during the next day’s charging period. But we would need a good amount of full sunlight during this period with no shadows on the panels.
Scenario 4: You plan to travel to Alaska or the Pacific Northwest, where insolation decreases. Your average daily amp-hour usage is 60 amp-hours, and let’s use 4 hours of usable sunlight: