How many watts does an RV refrigerator use? Which uses more watts – a conventional halogen light bulb or an LED bulb? How many watts of solar power do I need to run a fan all night on my boat?
Watts matter to us all day and all night, even as we sleep. But what are these abstract things called “watts,” and why do we need to know about them? Well, stick with us! In this article, we’re going to show you why understanding watts is more important than you think.
What Are Watts In Simple Terms?
Electrical power is measured in watts. One watt is a single unit of power. When we use the term “watt”, we are putting a number on the rate of transfer of energy. Meaning, a watt is a unit of power, and power is the rate at which energy is produced or consumed by an object.
If we think of watts as measuring electrical flow, then we can think of any device or appliance as needing either a large electrical flow or a small electrical flow. For example, if you have a 100-watt light bulb and a 60-watt light bulb, you can think of the 100-watt bulb as needing a larger flow of energy to work.
If you want to run a 750-watt microwave oven, you’re going to need a much bigger flow of energy – 750-watts, to be exact!
In the same way, if you have solar panels on the roof of your RV, the solar energy that flows into your RV is measured in – you guessed it – watts!
Common Watt Multiples and What They Mean
Watts are measured in multiples of 1,000. You’ve probably heard of these multiples before, but let’s look at what they mean in terms of power.
The smallest measurement of watts commonly used is a milliwatt or 1/1000th of a watt. This measurement is commonly used in small circuitry like your computer or phone.
A kilowatt is a measure of 1,000 watts of electrical power. Abbreviated as kW, a kilowatt is a globally understood standard for measuring electricity. You may notice that your home’s energy usage or power consumption is rated in kilowatts on your electric bill.
A megawatt (MW) is globally understood to be equal to 1,000 kilowatts of power. Depending on their size, generators can be rated in megawatts, kilowatts, or watts.
Any equipment that will run on electricity in your home will have its usage or consumption rated in watts or kilowatts. Again, a light bulb might be rated at 60 watts, while a microwave is rated at 750 watts.
Interestingly, some appliances have two different ratings. Your household refrigerator, for example, is an appliance that would have a start (or surge) rating and a running rating. This means the number of watts will be different when the refrigerator first starts up versus when it has been running for a while.
It might have a start rating of 1,200 watts and a running capacity rated at 800 watts. Thus, when the refrigerator starts (when the compressor kicks on), it requires 1,200 watts at that moment. But while it’s running normally, the required wattage decreases to 800 watts.
Read on to learn how we measure watts. It’s easier than you might think!
How Do You Measure Watts?
There is a simple equation we can learn to help us measure watts in any situation. To be able to calculate the number of watts provided by a power source, you’ll need to know the number of amps and volts in that power source. Amps and volts are measured using a multimeter.
Once you have that information, the calculation is simple for a DC circuit:
Watts = Amps x Volts (or W = A x V).
So, if the current is 5 amps, and the voltage is 110 volts, then 5 x 110 = 550 watts.
You’ll also sometimes see this equation written as Power = Amps x Volts (or P = I * V). This is a more formal or technical version of the equation, which you might see in a textbook.
However, as long as you can remember watts = amps x volts, you’ll be good to go for DC circuits.
In just a moment, we’ll look at the reasons why knowing this formula is very helpful in RV and marine applications. First, though, let’s take a glance at the difference between high wattage and low wattage.
High Wattage vs Low Wattage
High wattage means more power is consumed. Because of this most of the time we are trying to build and use appliances that use less power and have lower wattages. Higher wattages however are good when you particularly need to turn electricity into heat. The higher the wattage the hotter the heater.
It is important to know, however, that high wattage can mean high heat. When you’re powering a high wattage appliance, more heat is generated and this needs to be accounted for within the context of your particular application. High wattage appliances can also cause a circuit overload and blow a fuse or trip a circuit breaker.
Alternatively, low wattage applications require less power to charge or operate. Examples of low-wattage applications are a cell phone charger, a smartphone or tablet, and an LED light bulb.
Also, keep in mind that it doesn’t matter whether we’re talking about 12-volt or 220-volt applications. Watts are watts. The voltage does not matter here. A device does not use less power because it is a 12-volt device.
Power Supply Capacity
The job of a power supply is to provide power to the connected device or appliance. So, it would make sense that the more power a device or appliance requires, the higher the wattage you’ll need from the power supply.
Higher wattage = more power.
When considering the capacity of your power supply, there’s one rule of thumb that’s important to remember. It’s better to have more power than you need because you shouldn’t run your power supply at 100% capacity.
For example, if you want to run a device that requires 100 watts, you would not use a 100-watt power supply. Why? As stated before, you don’t want to run your power supply at full capacity. Ideally, you’d want your power supply to have a wattage higher than 100 in this situation.
If you have a 250-watt power supply and you use it to run an appliance that requires 100 watts, that power supply will only put out the necessary 100 watts the appliance needs. Essentially, the power supply will not “overpower” the appliance.
Why Do Watts Matter in RVs & Boats?
When you’re traveling in an RV or on a boat, watts matter because you need to be able to calculate your daily power requirements. For marine applications, it’s helpful to know how powerful your motor needs to be based on the size of your boat. In RVs, you’ll need to understand what appliances you can power and whether you can power more than one appliance at a time.
For example, RVs are often powered using generators. If your generator puts out 4,000 watts of power, you’ll probably be able to run your RV’s air conditioner, microwave, and several small appliances, although maybe not all at once.
If you’re considering installing a solar system on your boat or RV, you need to be able to calculate how much power (how many watts) you use in 24 hours. This will help you understand how many solar panels you need (based on their wattage) to be able to sustain your daily usage. We’ll share how to calculate this at the end.
Where Will You See Watts In RVs & Boats?
RVs and boats have many applications that require power, from lights to device chargers to fans and refrigerators. It’s important to understand how many watts you’ll consume in a day, and how you will recharge your batteries to store more energy for the next day.
Using lower-wattage devices where possible is key, especially when not connected to shore power (i.e. living off-grid). Let’s use lights as an example to illustrate the variations in power consumption.
Remember, watts = power!
An incandescent light bulb that illuminates at a level of 450 lumens will use 40 watts. On the other hand, an LED light illuminating at the same level will only use 4-5 watts. If you’re living off-grid, mostly on solar power, you’re probably concerned with your poƒer consumption. In this case, which light bulb would you prefer to have in your rig or on your boat? The LED bulb that uses only a few watts, of course!
When considering appliances that use more watts, you will likely need an inverter to run them. Inverters turn DC current into AC current to run certain appliances and devices.
DC current, or direct current, is the type of electrical current you receive from a battery (think powering an RV). AC current, or alternating current, is the type of electricity you receive from a power station (think powering a residential home).
An inverter needs to have sufficient capacity to power what you want it to power. So, in order to know what inverter you need, you have to know the wattage (starting and running) of the appliances you want to power. So again – watts matter!
Let’s go back to our refrigerator example from the beginning of this article. If your refrigerator’s starting capacity is 1,200 watts and the running capacity is 800 watts, a 750-watt inverter would be insufficient to run this appliance. Instead, you’d want an inverter with a larger capacity, say 1,500-watts.
One additional thing that needs to be considered with inverters is that most of them are rated in VA or volt amps. In AC circuits there is something called power factor that causes power to appear higher than the load is to the generator or source. Its a complicated topic but to account for this its typically good to add 20% additional margin to your loads to get an accurate power consumed.
Generators & Solar Panels
Now let’s suppose you’re in the market for a portable generator to give your off-grid adventures a power boost. If you want to run an air conditioner in the summer heat, your generator is going to need to be able to deliver sufficient wattage to start and run that AC unit. This is true of everything you want to power.
And finally, if you’re looking at investing in solar panels to harness the power of the sun, you need to be able to calculate how many watts of solar energy you need to power your off-grid life. How many watts of solar you need will be determined by how many watts of solar you use.
Distinction Between Watts & Watt-Hours
The difference between watts and watt-hours is quite simple. You may recall that electrical power is measured in watts. Watts measure the rate of power at a particular moment in time.
A watt-hour measures the rate of power over a specific period of time – one hour.
Simply put, one watt-hour is equal to one watt of power flow over an hour. So, a 5-watt LED light bulb left on for one hour has used 5 watt-hours of energy.
How Many Watts of Solar Do You Need?
To illustrate the significance of watts and watt-hours, let’s figure out how many watts of solar panels you would need to power your RV or boat off-grid.
First, make a list of all the electrical devices you’ll want to run. Then, write down the wattage of each device and how long you intend to run each one. Next, total the wattage of all devices by adding them all together, then total the running time of all devices.
Lastly, multiply those two totals together (total wattage x total projected running time). The result tells you how many watt-hours are required to power all of your electrical devices with solar panels.
Let’s say you’re planning to run all of your devices for 24 hours. If you have one solar panel rated at 250-watts, your panel could produce 6,000 watt-hours (or 6 kilowatt-hours) of power during that time (250 x 24 = 6,000). Meaning, if the total watt-hour requirement you calculated for all of your devices over a 24-hour period is more than 6,000, you would need more than one solar panel to accommodate your power needs.
The above is a very simplified method to figure out solar. But, there are many more factors that come into play when determining solar needs. Mortons on the Move wrote an in–depth article explaining some more accurate ways to figure out solar needs for an RV.
Watts Really Do Matter!
Watts are clearly very significant in any application where power is needed, whether that’s in a home, business, large facility, boat, or RV. Simply stated, watts matter – and so does understanding them!
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