In order to maximize self-reliance and make the most out of their adventures, many people are adding solar power and energy storage to their RVs, boats and tiny homes. You should, however, first learn about energy basics before buying an energy system you will be relying on, particularly in remote locations. If you plan to be out on your solar-plus-storage boat for several weeks or months at a time, for example, you need to be able to determine how much energy you will need in advance and how to select the appropriate size energy system. Knowing what watt hours and amp hours are and how they differ is the first step in understanding your energy storage requirements.
Watt Hours = Power Over Time
The unit of energy known as watt hours (Wh) is the capacity of power (watts) moving over time. A watt is equal to one joule per second. In practical terms, this is how many watts an appliance, such as a light bulb or toaster oven, uses in one hour. To calculate watt hours, multiply watts by the number of hours an appliance is used. A 50W lightbulb, for example, would use 1200 Watts per day if you calculate its wattage over 24 hours, which equals 50W times 24 hours.
Amp Hours = Current Over Time
The watt hour is a measurement of energy, whereas amp hours (Ah) are a measurement of charge. The first thing you need to know to calculate amp hours is the current, which is defined as the flow of charged particles through an electrical conductor or space, such as electrons or ions. The amp hour is a measurement of how many amps can be discharged by a battery over a one hour period. For example, if the battery is 150 Ah, 150 amps would be discharged in an hour.
You can determine a battery’s charge and discharge rate by looking at its C rating, which measures how fast a battery can be fully charged or discharged. For example, charging at a C rate of 1C means that the battery is charged from 0 to 100 percent in the span of one hour. A 0.5C or C/2 rate represents two hours of discharge time, and a .2C or C/5 rate represents five hours of discharge time. Battery with a 1C (1C current) rating can provide 20 Amps for one hour if it has a capacity of 20Ah.
Converting Watt Hours to Amp Hours and Vice Versa
In order to determine the battery’s energy capacity, the voltage you need, and the thickness of your wires, you will need to know and calculate both watt hours and amp hours. Though amp hours and watt hours are distinct units, it is important to know that they are interconnected and can be used to determine one another. In case you only have one or the other, you can calculate one using these calculations for watt hours and amp hours.
watt hours = amp hours x volts
amp hours = watt hours / volts
Let’s say you have a 12V battery for your van and it’s rated for 200 Ah. You can multiply 200 Ah times 12V to get 2400 watt hours. In order to illustrate how amp hours differ from watt hours, you would also be able to achieve the same 2400 watt hours with a battery rated at 24V (higher voltage), 100 Ah (lower Ah). If you knew the watt hours of your energy storage system, you could calculate amp hours from this. For example, let’s say your battery has a watt hour rating of 1200 Wh. Let’s say the battery has 12 volts, so you would divide the 1200 Wh by that, giving you 100 Ah.
Why Do You Need to Know Amp Hour & Watt Hour Calculations?
In these calculations, you can figure out how much power or amperage your battery is able to discharge and for how long. If you don’t know the amp hours or watt hours that you will need per day, you will be unable to determine with accuracy whether your energy storage system can meet your needs, potentially leaving you unprepared and at risk of energy shortages.
If you are setting up a solar power plus energy storage system, make sure that you add together all the watts that your devices use in one day to determine how much energy you’ll need. By using the equations mentioned above, you can then determine the size of a solar system and an energy storage system.
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