Note: As in the last two installments, this information is designed to help newbie RVers better understand the electrical needs of their RV. As with anything electrical, there are exceptions to every rule of thumb, but the basic information shared below is applicable in most RVing situations. Results will vary depending on temperature, battery condition, large loads, and loads over long periods of time.
In our previous article, we looked at how to calculate your RV’s 120-volt shore power needs by calculating watts and watt-hours. In this post, we will look at how to determine the electrical demands of your 12-volt battery system by learning about amps and amp-hours.
As we shared in the last couple of installments, the electrical needs of 120-volt appliances are typically expressed in “watts” on the back of the appliance. When they aren’t expressed as watts, we looked at how to multiply voltage times amperage to determine watts, then by including run times to determine watt-hours.
Norcold 12-volt needs – Photo from iRV2
While watts and watt-hours can also be applied to the 12-volt system in RVs, it seldom is. Rarely are the electrical requirements of the 12-volt appliances (water pump, furnace motor, lights, etc) expressed in any form other than amps.
This is definitely more convenient than watts as deep-cycle RV batteries (aka “House Battery”) are rated in amp-hours rather than watt-hours. Note that amp-hours are commonly abbreviated as “A.H.”, “Ah” or similar.
Just as we added up the 120-volt shore power needs in watt-hours, we will add up the needs of 12-volt (battery-powered) appliances in amp-hours.Examples:
- An incandescent 12-volt ceiling light bulb has a draw of 1.5 amps. If you leave the light on (burning) for one hour it will consume 1.5 amp hours of battery power. If you burn it for 2 hours it will consume 3 amp hours of battery power, etc.
- The blower motor and related 12-volt components of your propane furnace draw 7 amps while running. If the furnace runs a half-hour, it will consume 3.5 amp hours from your battery bank. 7 amps X .5 hours = 3.5 amp hours
- An LPG leak detector that draws .2 amps and operates 24 hours will consume 4.8 amp hours from your batteries.
Water pump requirements – Photo from iRV2
Now, if you are connected to 120-volt shore power, none of the above matters much, as your converter charger will replenish the RV batteries as loads (amps) are drawn. Where it becomes important is when you are dry camping (no shore power) and the reserve power (amp-hours) in your RV batteries is all you have to supply your electrical needs.
A typical deep-cycle RV battery will be rated around 80 amp-hours, which in theory would supply one amp for 80 hours. However, in reality, if you discharge a lead-acid battery (what you likely have in your RV) more than 50% of the rated capacity you will greatly shorten the life span of the battery. Therefore, when calculating the reserve amp-hours of your RV’s batteries, the usable power will be 50% of the rated capacity.
Now, it is just a matter of adding up your amp hours for every 12-volt item in your RV (like we did for watt-hours in the last installment) and divide that number into the amp-hours available in your RV’s battery bank to determine approximately how long the battery will last.
Now that you know how long your batteries are likely to last, you can plan accordingly for your next dry camping adventure in RVing!
See our previous installments here: