Your RV batteries produce power in Direct Current (DC) that run at low voltages. Power companies and AC generators produce sine wave Alternating Current (AC), which is used to operate 120-volt appliances and electronic equipment. An inverter takes 12-volt DC power from your RV batteries and electronically changes it to 120-volt AC.
If you’ve been RVing for a while you have probably been in a situation where you needed an inverter at one time or another. Maybe it was when you were dry camping and didn’t have access to electricity, or maybe you have a generator but it’s after quiet hours and you still want to watch a TV program or use the microwave.
You don’t want to buy a bigger inverter than you need, but you also don’t want one that’s too small for how you plan to use it. Inverters come available in small portable units that plug directly into 12-volt outlet; to larger high power, hardwired units that can provide electricity for the entire RV electrical system and are permanently installed in the RV. Many of the inverters found in RVs today are inverter/chargers. What this basically means is they are inverters, battery chargers and a transfer switch all in one. They act as a battery charger when you are plugged into an electrical source, or using the generator, and they invert stored DC battery power when no electrical source or generator power is available. Many inverters are capable of transferring from inverter to battery charger automatically.
Inverters are rated in watts and come in a variety of sizes ranging from 75 watts to 3000 watts. The size of the inverter you will need depends on several factors. If most of your camping is done at campgrounds where electricity is plentiful you may not need an inverter. On the other hand, you might enjoy boondocking where all your 120-volt requirements depend on batteries and an inverter.
When you purchase an inverter the output capacity must be capable of operating the loads that will be placed on it. Inverters have two different capacity ratings, continuous output rating and surge capacity rating. Continuous output is the maximum wattage the inverter can output for a long time. Surge capacity is the maximum wattage the inverter can output during initial start-up. All appliances require more power when they initially start, compared to what they use when they are running. Some can use as much as two or three times the amount to start that it uses to run, so the starting power required for any appliance that you plan to use with the inverter must be within the surge capacity rating.
Let’s say you only plan to use an inverter to run one or two small appliances. You might want to use a 20-inch TV, a small fan, and an overhead light all at one time. You total all the wattages, about 96 watts for the TV, 40 watts for the fan, and 20 watts for a fluorescent light. This is a total of 156 watts. In this case you can probably get by with a 300-watt inverter. Other RVers use inverters to operate microwaves, coffee pots and other larger appliances which require a larger more sophisticated inverter.
Another consideration is the type of AC power being produced. There are modified sine wave inverters and true or pure sine wave inverters. Modified sine wave inverters are less expensive and will power most types of appliances. The down side to modified sine wave inverters is that some electronic equipment will not run on this waveform, and because it’s not true or pure sine wave you may get some electrical noise or a snowy picture on your TV screen.
True or pure sine wave inverters are more expensive, but they can produce power as good as a power company, and all appliances and electronic equipment will run the way they are intended to. Microwaves, motors, and other inductive loads will run quieter and will not overheat and electrical noise will be reduced. If you plan to use a computer or other sensitive electronic equipment you may want to consider a true sine wave inverter.
Another important consideration when using an inverter is your RV battery(s). The more electricity you plan to use not only requires a larger inverter, but a larger battery bank too. Batteries are rated in amp hours. The amp hour rating is basically how many amps the battery can deliver for how many hours before the battery is discharged and needs to be recharged. Amps times hours. In other words, a battery that can deliver 5 amps for 20 hours before it is discharged would have a 100-amp hour rating.
You need to compute the amperages you plan to use and the amount of time you plan to use it to determine if your battery(s) can provide enough stored power. Keep in mind when you make your calculations that a battery discharged to 50% of its capacity is basically dead and needs to be recharged. The power, in watts, drawn from the batteries by your inverter is the same as the power, in watts, drawn by the 120-volt AC items, plus about 15% for losses in the inverter.
Since Watts = Volts X Amps, and the 120-volts is 10 times the 12-volts of the batteries, about 10 times the amperage is required from the batteries than the amperage drawn by the 120-volt loads (ignoring for the moment the inverter losses). For instance, a TV might draw 480 watts from the 120-volt AC supply, which is 4 amps of current. To supply these 4 amps at 120-volts, the inverter must draw 40 amps at 12-volts from the batteries, plus roughly 6 amps for the inefficiency of the inverter.
Finally, you need to consider that any battery power used has to be put back in through some type of effective charging system. Batteries need to be charged in three stages. The first stage is a bulk charge that replaces about 80% of the battery capacity very quickly. The second stage is the absorption stage that replaces the remaining 20% and the last stage is the float stage which is a lower voltage designed to keep the battery(s) topped off, but not overcharge them.
Older RV converter/chargers charge battery(s) at a fixed voltage in the range of 13.5 volts. This will not recharge batteries that are discharged to 50% of its capacity, and it can be too much for a float charge for fully charged batteries. When you determine how much battery power you will be using you can decide on an effective charging system. It may be that you only need to keep the batteries topped off with the converter charger, or you might need a complete set of solar panels to put back into your batteries what you are taking out.
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