So you look at a light bulb and it says “100 Watts.” What the hell does that mean? What the Heck is the Difference Between a Kilowatt and a Kilowatt-hour?

Your power company charges your for “Kilowatt-hours” that you use… What the hell is a Kilowatt-hour?!

For starters, a Kilowatt (kW) is just one thousand watts, just like a megawatt (MW) is one million watts. A “watt” (W) is a measurement of POWER. To be precise it’s using one joule per second. However, since no one has uttered the word “joule” since that physics class they took fifteen years ago, we use watts instead of Joules per second, because they’re printed on light bulbs and people have an idea of what they mean. To put it in perspective, a car engine cruising uses about 25,000 watts, which is about 30 horsepower.

But that’s a car… Mechanical energy. We want to talk about electricity since we’re concerned with solar power. With electricity, POWER is voltage time amperage. Or 1watt = 1volt * 1ampere.

A good way to think about electricity is that it’s a lot like water. Voltage is the pushing, or the pressure, and amperage is the flow. A dammed up lake, although it has a hell of a lot of pressure (voltage), doesn’t flow anywhere (zero amperage) so there’s no power (lots of volts * zero amps = zero watts). On the other hand take a super soaker water gun. Super fast stream (lots of amps) but a tiny little compressed reservoir (not a lot of volts). I wouldn’t go so far as to call that “powerful.” Finally, look at a rushing river. Lots of volts, lots of amps, huge power. So wait, Dave, you got off subject, we’re talking about Watts here…

Oh yah, Watts. Ok, so a Watt is energy burned per second. If you flick on a 100 Watt light bulb it’s eating up 100 Joules of energy every second (interestingly, a standard candle is exactly 1 Watt). So now what’s a watt-hour (wH)? Well, don’t get confused when you see a confusing or seemingly incorrect usage of watt or watt-hour in mass media. The two terms are often interchanged and misused.

“Watts per hour” doesn’t make sense because it’s already a measurement of “joules per second.” Does “Joules per second multiplied by one hour” make sense? No. A Watt-hour is what your power company uses to charge you, and it’s a way of removing the “per second” from Watts. So now instead of talking about Joules per unit of time, you’re just talking about Joules period…. but we call them Watt-hours because no one knows what the hell a “joule” is.

Think of Watts as the speed you’re running and Watt-hours as how far you’ve actually run. A kilowatt-hour is the amount of energy equivalent to a power of 1 kilowatt running for 1 hour. If you leave a 100 Watt light bulb on for 1 hour, you’ve done gone and used up 100 watt-hours.

But your electric company will bill you by the kilowatt-hour, so you’ll get a bill for .1 kWh, multiplied by your per-kWh rate. That means if you run a 100 watt bulb for an hour a day for 30 days in a month, and you pay $.10/kWh, your bill will be for .1 kWh x 30 days x $.10, or $.30.

Ok, I feel like I’ve made that explanation 100 times longer than it should have been. Hope that helped.

Last modified: June 24, 2019

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I’m getting a 19.43 KWh system here in Bakersfield ca. with about 9 hours of sunlight with the newest panels with inverters on each panels i think i said that right. i was told it will produce over 19,000 kwh in a year is the correct. and how much will it produce daily. In June,July,August i used over 3,200 kwh for each of those months. Winter average 2,000 kwh is this system big enough for me so i don’t have a true up cost at the end of the year

Hey Eric- Thanks for the question. Without looking at your actual usage, I can’t tell you with 100% certainty, but I think I can give you a ballpark idea. A 19.4-kW system in Bakersfield should generate a great deal more than 19,000 kWh. Using the National Renewable Energy Laboratory’s PVWatts tool, it looks like you’ll be getting about 33,000 kWh per year from your system, and yes, nearly 2,000 kWh in the winter months of December and January. Average per day is 90 kWh, with a low of 58 kWh/day in January and a high of 110 kWh/day in June.… Read more »

Thank you Ben,

That sound about right. I wanted a system large for future usage like electric car and electric patio heaters etc. My average electric cost is about $600.00 and the PPA I signed up for is $486.00 so below avg. I have a large pool and spa that is used year round. Just don’t want no surprises at the end of the year for going over what I can produce. Again thanks for the quick response

Wow, if you could try that w/o the four-letter profanity every other line, you might even come across as a credible professional.

No guarantees, however.

If offended, then you could pay some one to explain it to you instead reading it and then complaining.

if a watt is energy burned per second, then wouldn’t a 100 watt bulb use (60×100) 6000 watts per minute and then 360,000 watts in an hour? Something i’m not getting…….”Oh yah, Watts. Ok, so a Watt is energy burned per second. If you flick on a 100 Watt light bulb it’s eating up 100 Joules of energy every second”

Jesse-

Watts are a measure of the constant energy needed to keep the lightbulb lit. Extending that amount over time doesn’t add watts; it just adds time. So a 100 watt lightbulb lit for 1 hour uses 100 “watt hours,” because you’ve kept that energy constant for that time period.

An apartment building with 14,400 square feet parking garage rooftop surface — how much KWH solar electricity can it generate on average day of 12 hours daylight if the entire rooftop is covered with good solar panels? Approximate answer?

Good enjoyable light hearted explanation. The part that reads “Joules per second per hour” should read, “Joules per second multiplied by one hour. It’s simply a measure of energy capacity provided by the solar panel.

Thanks, anon. On it.

how many panels would be needed to produce 200 kwh per day?

Well, depending on your location on the planet, you get about 4-5 kWh per kW of solar panels per day, on average. So you’re looking at a 40-50kW system, and you’d need about 125 of today’s most efficient 400W solar panels to make that system.

This was a Real long explanation….Kilowatt is 1000 watts. Kilowatt hours is 1000 watts used in an hour.

Damn, I made a mistake myself: 1 MW=1,000,000 W, not a thousand. The article is correct on that, although it still uses “mW” mistakenly.

WRONG; There are 24hrs in a day, not 6 hours. 1000 watts generated; (120v x 8.33amps) 1000 watts x 24 hrs=24kwh; 24kwh x 30 days= 720kwh for one month; cost if bought at $0.10 /khw is: 720kwh x 0.10 = $72.00 cost if bought at $0.30 /khw is: 720kwh x 0.30 = $216.00 @ current San Diego rip-off rates.

Yeah, but… the sun doesn’t shine for 24 hours. The number of peak-sun equivalent hours for San Diego is about 5.7 per day, on average (round up to 6), and you get 6 kWh per kW of solar per day, per kW of panels installed. After losses from wiring and inversion, you’re at about 5 kWh/kW. The average home in San Diego needs about 6,000 kWh per year, or about 16.5 per day, so you’re looking at a 3.3-kW system to make 6,000 kWh per year. Incidentally, you’re right about San Diego rip-off rates. SDG&E charges an INSANE price for… Read more »

Ben…please explain how you have arrived to 3.3 Kw system and the previous 40-50 kw system for the earlier comment that needed 200kwh per day..i am yet to understand that concept.