Repairs – AC DC, not about music

One of the changes from moving to an RV is going from one type of power to a system that uses Direct Current (DC) and Alternating Current (AC) power systems.  For most RVs, this is done to add flexibility to an RV to use electrical devices and appliance depending on if you are connected to an AC power source, shore power, or not.  Most RVs are wired to support 110V AC and 12V DC to provide flexibility.

The DC power system is used to provide power to various elements of the RV when not connected to an AC power source like shore power or an electric generator.  Usually, lights, water pump, and carbon monoxide detector, and other items use DC power to permit there use when AC power source is not available.  Some of the systems in the RV like hot water heaters and furnaces use propane for heating while the DC power is used to manage and operate elements like blowers.  Most people are familiar with DC power based on their cars and trucks.  Many RVs include the same 12-volt plug used in a vehicle to all use of items like phone chargers or inverters.

AC power is used to provide power to devices like air conditioners, microwaves, televisions, refrigerators, and water heaters. Many RVs have a couple two-way appliances that can use propane or electric.  Common two-way appliances include refrigerators and water heaters.  When connected to AC power, these devices can use electric power instead of consuming propane.    AC is also used to provide power to other electronic devices you want to use.  AC power to the RV is provided via three main methods.

  1. RV is connected to an AC power source (15, 30, or 50 Amp). The AC devices directly use the power that s connected.
  2. Electric Generator to provide AC power. For many RVs, the generator connects to the same connection used to connect to shore power.
  3. AC power is provided by the batteries using an inverter. The inverter converts DC power into AC power.  This process depends on a method to replenish the batteries such as shore power, generator, or alternate power source like solar and wind.

Watts, Volts, Amps, and Amp Hours oh my

When using DC power when the batteries are not being recharged, you should understand a couple of terms that will clear up some of the mystery.   Wattage (watts), ampere (amps), and voltage (volts) are all used together for electricity.  The following defines the terms.

  • Watt equals one amp at one volt
  • Amp is the amount of electric used
  • Volt is the force of the electricity

This means that the following is true.

Watts = Amps * Volts

Amps = Watts / Volts

Sometimes you will be provided the amps used and other times watts.  The following are examples of the differences of the power consumed by a common light bulb type in RVs.  For this example, a 921 bulb will be used.  The 921 bulb traditionally is an incandescent bulb and now is available in a lower power LED version.   Both bulbs use 12 volts DC.

Traditional 921 incandescent bulb – 1.49 amps

 921 LED bulb – 1.6 watts

The following normalizes the power used between the two bulbs.

1.49 amps * 12 volts = ~18 watts (Incandescent 921)

1.6 watts / 12 volts = 0.133 amps (921 LED)

When the element of time is added it defines the unit of measure based on that time.  Hour is a usual measure of electricity related to watts or amps.  An Amp Hour is the flow of an amp for an hour.  18 watts does not seem like much when.  If you had two lights on for 4 hours each night and considering no other power is used, you would consume 12 amp hours per night for the two lights.

2 bulbs * 1.49 amp hours (per bulb) * 4 hours = ~12AH

Many batteries have between 40 and 50 Amp hours of usable power storage.  Many RVs have the equivalent of one 12 volt battery.  You would need to recharge the battery after 3 nights, just for the two lights.  Keep in mind this does not account for all of the other drains on the batteries like water pump, carbon monoxide detector, and other electronic devices drawing DC power.

If you replaced the incandescent 921 bulbs with LED, you could benefit from the more efficient bulbs.  The two LED bulbs would consume about 1.07AH per night.

2 LED bulbs * 0.133 amp hours * 4 hours = 1.07AH

Keep in mind if you use an inverter with your 12 volt DC system, the power consumption will be a little higher and the amount of power drawn from the batteries might surprise you.  An inverter for using a laptop will need to support between 50 and 130 watts of power.  Inverters are generally 85% efficient at converting DC to AC.

50 watts / 120 volts = 0.42 amps or ~0.5 amps (inverter)

130 watts / 120 volts = 1.08 amps or 1.27 amps (inverter)

If you have one laptop on for 8 hours a day you will use between 4AH and 10AH for on laptop.

With the formulas above, you should be able to calculate amps or watts if you know the other value.

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