Watt vs. Watt-hour
When you begin exploring the fascinating world of off-grid living and renewable energy, you will quickly come across two terms, watt (w) and watt-hour (Wh). Both will be used frequently in marketing material, product descriptions and any how-tos on specifying a property’s energy needs.
So what are they and watt’s the difference?
A watt (w) is a unit of power. This unit describes the flow of energy through a device in a moment in time. That device could be an appliance, lightbulb or anything that consumes electricity.
The higher the number in watts, the more energy will flow through that device. For example, a 45w LED light bulb will use over twice the ‘flow’ of electricity as a 20w LED bulb.
You can calculate watts using – Watts = Volts x Amps.
A watt-hour (Wh) is a measurement of power over a given time. In this case, 1 hour. One watt-hour (1Wh) is the equivalent of one watt of flow for 60 minutes. Five watt-hours (5Wh) is one watt of flow for 300 minutes.
You can calculate watt-hours using – Watt-hour = Watts × Hours.
A kilowatt-hour is the equivalent of 1,000 watt-hours and is just a higher level of measurement suitable for larger capacity devices.
You can calculate kilowatt-hours using – kWh = Watt-hour x 1,000.
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What’s the difference between watt and watt-hour?
As you have probably figured out, a watt is a measure of power at a given time while watt-hour is a measure of power for a set period of time.
Both can be useful when specifying an off-grid solar system or calculating your power requirements. While you can use calculators to give you an idea of how much you use in a day, it is much more exact if you identify everything that uses power and perform your own calculations.
How does this relate to appliances?
Let’s go through a couple of examples of watt and watt-hour calculation so you can see its relevance when assessing your energy needs.
A kettle. Every house and caravan needs one, so let’s start there. It’s also an excellent opportunity to perform part hour calculations.
A 1,100w kettle used for 10 minutes per day would use 0.18kWh. How do we get that?
1,100w x 10 (minutes used) ÷ 60 (minutes per hour) = Daily watt-hour consumption.
1,100w x 10 ÷ 60 = 183 watt hours which is 0.18kWh per day
Let’s try another. A refrigerator. Another appliance most houses, caravans and campers use.
Refrigerators use different rates of energy. There’s the constant rate to maintain temperature and the cycle rate which is when the system actively cools down the interior of the fridge.
Most modern refrigerators are active around 33% of the time with the rest being minimal.
A typical 80w camper fridge ÷ 3 (time in active mode) = 27 watts
27w x 24 (hours in a day) = 648 watt hours which is 0.65kWh per day
As you can see, while it may look a little complicated at first glance, it really isn’t. It just takes time and the breaking down of each into its constituent parts. Once you know the maths, you can apply this to any appliance and any device in your home!
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