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One of the most frequent questions we get from customers is how much power (watts) or energy (watt hours) do I use and how much do I need to generate with my home solar system to meet my needs? To answer we need to make sure we are using the right definitions for terms like watts, watt hours, kilowatt, and kilowatt hours.

Watts in Regards to Solar Panel Energy

Watts (W) and Watt hours (Wh) while similar in some ways they are often confused. A watt is the immediate measurement of power and often abbreviated as (W). Power is a means of measuring the rate at which energy flows, and is measured in watts with regards to electrical systems. Watts are the MPH of all things electrical and simply tell us how quick the electrons are racing down the highway.

Example: Watts is the rate at which energy is produced or utilized. A 40W light bulb is using 40 watts of power whenever it is turned on.

So What Are Watt Hours?

Watt hours (Wh) is the accumulation of energy over a span of time. Let’s look at something in your house that runs continuously like your fridge which on average requires 150W of power, and it runs 24 hours a day you are using 3600Wh and 1,314,000 per year.

Here are a few examples of different household appliances and their typical power consumption.

  • Fridge / Freezer 150W
  • Home Air Conditioner 1000W
  • 100W light bulb (Incandescent) 100W
  • Ceiling Fan 25W
  • Coffee Maker 800W

Watts measure power and kilowatt hours measure energy

When you receive your monthly utility bill, the power you’ve used is measured in kilowatt hours (kWh). We know from our earlier discussion that a watt is the measure of power, and a kWh is the measure of energy. Energy is defined as the power obtained from the utilization of physical or chemical means, principally to provide light, heat or to power machines. So, if you run your 1000-watt air conditioner for one hour, you’ve used 1000 watt-hours, or 1 kilowatt-hours, since a kWh is 1,000 watt-hours. In other words, 1 kWh is the amount of energy you need to run an air conditioner for an hour.

In your home, you are charged for the electricity you use, which is measured in kWh. But many commercial and industrial buildings pay also pay “demand charges,” usually measured in megawatts, or MW and are determined based on their peak power draw, which pays the electric company for guaranteeing that it has enough power ready to meet the demand.

What Does This Mean For Solar Power Systems?

When designing a solar energy system, you need to know how many watt hours you use in a 24 hour period to make the right amount of power for your home or business needs. If you are drawing 4000 watts total from all lights, devices, and appliances, without understanding for how long and what day parts, there is no way to know exactly what size solar PV system you need.

So let’s take a look at a real world situation. If you are running 4000W for 1 hour, then you would need to generate 4000Wh (4000W x 1.0 hour), or 4kWh, or if it’s operating for 12 hours, you will need 32kWh (4000W x 8 hours). In both examples, the load is 4000W, but the resulting energy use is entirely different, 2kWh vs. 32kWh. A 600-watt solar panel could produce the first scenario, and the other will need 9,500 watts of solar.

You also need to consider which day part is your heaviest power usage unless you are going to build a solar system with battery storage to make sure you are generating enough sun power during high usage times.

As you can see understanding the terminology is important and is just one of the areas we cover when helping customers figure out if solar is right for them and how best to design a system that meets their needs.

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