energy house

Home Solar Power: How much solar power do I need?

A few weeks ago I wrote about getting started with a home solar power project. That post was an overview on how much power you might need to power your home, and how much a home solar power project is likely to cost you. This week I’m going to start the deep dive, and I’ll use our home as an example.

How much solar power do I need

So, how much solar power do I need?

Well, let’s start with a quick overview of our household, so you can do a simple comparison to your own home and get an idea of if you’ll need more or less before doing your own deep dive.

Our house is a single story bungalow, a little over 1000 square feet. It is home to my husband Neil, our toddler Eli, and myself. We live in a small town in Wisconsin, so we have both cold winters, and fairly warm, humid summers. We are careful with our energy use, but we enjoy pretty much all modern amenities, with the exception of a dishwasher and a microwave. Our appliances are aging, 15 – 20 years is my guess, and we have an electric stove.

We are connected to the grid, and if we put in solar power here, we would stay connected to the grid.

Step 1: Calculate your energy use

Look up the amount of Kwh of electricity that you have used for each billing cycle over the past year. Add it all together, and divide by 365 (or 366 if it was a leap year).

You might think that you should look at your highest energy use month and use that to find your daily use. This method would help ensure that you are drawing all your energy from solar power all year long, but it is also going to cause the size of your solar project to go up quite a bit. This can be both cost and space prohibitive.

By building a solar power project based on energy use averaged over the entire year, you will probably have to buy some extra electricity during the winter, when there is less sunshine and you are possibly using more electricity. But in the summer, you will produce enough to sell back to the grid, recouping your electricity costs in the winter, and over time your solar power project costs. It will also keep your solar panel array to a more manageable size and cost.

Realistically, if you live in a city or town, you’re just not going to have the space necessary to put in enough solar power to meet your entire usage needs. But knowing how much you are using daily will put your project in scope from the beginning, and maybe also help you to think about where you could start cutting back. Remember, if you don’t use the energy in the first place you don’t have to pay for it, and we don’t have to create it by any means, saving all sorts of resources.

OK, so on to our house and our energy use. For the 7 months that we’ve been home owners thus far, I’ve been chronicling our energy use in the energy efficiency project posts. So far we’ve used 2873 Kwh over 208 days. This puts our average daily use at 13.8 Kwh. This is actually a pretty low ball estimate of our daily electricity use, because keep in mind that we were only living here part time for 5 of those months. As we continue living in this house, I’ll revisit these numbers and update as we get a more realistic idea of our average daily use.

Ok. So our goal is to produce 13.8 Kwh of electricity each day with our future solar panels.

Step 2: Find the amount of sun hours your location gets

We use 13.8 kwh of electricity each day, on average. But we don’t need to produce all that electricity at once, just over the course of the day. Because, as I’m sure you’re well aware, the sun doesn’t shine for just one hour most days. In the summer, it might be shining for 14 – 16 hours per day, and with solar panels you can turn that sunlight into electricity the entire time it’s shining. But, as I’m also sure you’re well aware, sometimes the sun really does only peak out for a short while, and some days not at all. So you need to find the average amount of time that the sun shines in your location each day.

Most companies that sell solar panels and accessories have charts and maps that can help you figure our the average hours of sunshine your location gets per day. For example, this chart on the Wholesale Solar website tells me that Madison, WI gets an average of 4.3 hours of sunlight each day. Again, this number is averaged over the entire year, so using it in my calculations means that on cloudy winter days, I’ll probably have to purchase electricity from the grid. But on sunny summer days, I’ll produce extra electricity that I’ll be able to sell back to my power company.

Step 3: Calculate the amount of solar power capacity you need. 

Solar power capacity is the amount of kilowatts your panel array can produce when the sun is shining on it at any moment. It’s pretty simple to calculate: Take your average daily electricity use, and divide it by the average daily hours of sunlight in your area.

For us, 13.8 kwh / 4.3 hours = 3.2 kilowatts.

So we want enough solar panels to produce 3.2 kilowatts of electricity.

Step 4: Factor in efficiency

You may remember from high school physics (or maybe not) that when energy is converted from one form to another, their are losses. In this case, when the light energy from the sun is converted into electrical energy by the solar panels, not all of the solar energy collected gets turned into electricity. Some of it is lost as heat, or friction between the electrons in the wires, etc. Currently, solar panels are 78% efficient. Meaning that 78% of the solar energy they collect, makes it into electricity.

In order to account for this efficiency (or inefficiency, as the case may be) we divide our solar power capacity by 0.78.

So, how much solar power do I need? 3.2 Kw * 0.78 = 4.1 Kw

In order to power our home we need enough panels to produce 4.1 Kw of electricity. How about you?

Or at least in an ideal situation. Next time we’ll take a look at other things we need to consider.

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Want to learn more about solar power? Check out how it’s made in these posts: Solar Power part 1, Solar Power part 2

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energy house

Home Solar Power: Getting Started

home solar powerA couple weekends ago I had to opportunity to attend a class entitled “Do It Yourself Photovoltaics” which was put on through our local garden center. The man who taught the class, Mr. Jon Passi, stressed that his goal was to make solar power projects as accessible as possible to others, and encouraged us to share what we learned from his class with our neighbors, so I’d like to share a bit about what I learned with you.

home solar panels 3

How Much Solar Power Do You Need

The first step to getting a home solar project going is to figure out how much power you need. Most power companies these days will provide graphs of your power use for the past year, so you can see how much electricity you use each month. When you look at this graph you’ll probably see that you have a season of the year where you use quite a bit of electricity, and a season where you use less. For my family, we use more electricity in the winter time than the summer, because we are more likely to be inside, and because it is dark during more of our waking hours. But if you might find that the opposite is true for you, depending on your habits and your home.

So, take a look at your energy use over the course of a year. With solar power, you produce the amount of electricity you use each day, and then start over again the next day. Figure out what your average monthly electricity use is. Then divide that number by 30 to get an estimate of your average daily electricity use. For the average American family, this number is somewhere in the range of 20 – 40 Kwh per day.

Your average daily electricity use is what you’re shooting to produce with a home solar power project. Yes, some days you will use more, but if you’re still connected to the power grid, you’ll be able to draw whatever extra you need from your power company. And on other days you will use less electricity than the average, and on those days you will be able to sell back any extra that you produce to the power company. It will all even out in the end, and usually in your favor – depending on your power company, you’ll be able to sell your extra electricity to the power company for more than you are paying for the little bit you need from them on cloudy days or days when your energy use is a bit higher than average.

home solar panels 2

How Much Will Home Solar Power Cost

The next step is to figure out if you can lower this number. Home solar projects are still pretty expensive, so the more you can lower your daily needs, the less you need to invest in supplying that power. Before you start shelling out dollars for solar panels, maybe it’s the right time to upgrade to a more energy efficient refrigerator, dish washer, or washer and drier. Maybe it’s time to commit to hang drying your clothes. Make sure your computer, television, and gaming systems are all on power strips that you turn off when you’re not using them to reduce the amount of phantom load your electronics are drawing. Upgrade your lightbulbs to LEDs. Before you spend $10k+ on solar panels, spend a couple months committing to lowering your energy use, and then recalculate your average daily electricity use.

A good rule of thumb right now for how much a home solar power project is going to cost you is to multiply your average daily electricity use by 1000. So if your home uses 22 Kwh of electricity per day, the cost of a project big enough to cover your entire energy needs would be about $22,000.

Before you get bug eyed at the cost of a home photovoltaic project, keep in mind that there are currently lots of opportunities for energy rebates. The federal government will give you 30% of the cost of the project in rolling tax breaks. (Rolling means that if you don’t use the whole 30% the first year, you can take the remainder the following years until you reach the full 30%.) Many power companies are also offering rebates for home solar projects right now as well. Mr. Passi gave us some examples of projects that he worked on in the past couple years, and many times the after rebate costs were around 60% of the total cost of the project.

Also, keep in mind that solar panels take up quite a bit of space. You likely won’t have enough room on your roof (especially in a city or suburb situation) to even install enough solar to cover the entirety of your daily use. When you look at how much solar power you can actually install on your house, the scope of your home solar power project may drop significantly. And even if you’re only supplying some of your home electricity needs, on sunny summer days, you will likely still produce quite a bit of extra electricity to sell back to the grid.

I have plenty more to share on this topic, but I think this is a good start for today. In the future I’ll get into more about the components you need for a home solar power project, and use our house as an example for figuring out how much electricity you need to produce and the costs of completing that project.

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3 Energy Wins to Celebrate this Fourth of July

Happy Birthday America!

While we celebrate our Independence this weekend, let’s also celebrate some of the wins we’ve made in America and around the world in the past year in terms of renewable energy growth.

3 energy wins to celebrate this fourth of july
Fireworks by Jeff Golden //CCBY

1. We’ve reached 1% of worldwide energy generation through solar power!

Ok, so 1% sounds pretty small, but nearly a quarter of that was installed just within the last year, and the prices on solar generation continue to fall, meaning that soon it will not only be the earth friendly way to get your electricity, but it will also be the wallet friendly way. This first 1% is hard won, requiring decades of work by scientists and engineers to bring the technology forward and the price down. China, Japan, and the U.S. are leading the way on solar power installation.  You can read more about this achievement here.

2. Dirty, Old coal plants are retiring across the U.S.

In the past year, 7% of coal plants have been retired. These retired plants are the oldest of the currently existing coal plants and dirtiest methods we currently have of generating electricity, so shutting them down is a good step to reducing the dangerous particulates they exhaust – like mercury, from the air. The old coal plants are retiring for a couple reasons – they are too dirty to meet current regulations, and they are being priced out by the plummeting costs of producing electricity using renewables.  You can read more about the shutting down of coal plants here.

3. We’re building battery capacity

One of the biggest challenges with the move to renewables such as wind and solar, is that the amount of energy that they generate fluctuates. On a windy day, wind turbines can be constantly generating electricity, but if the wind dies down – people will still want to use their computers, refrigerator, and air conditioning regardless. Same with solar power, the power generation fluctuates not only with day and night, but with cloud levels and season. A sunny day in Arizona can produce more energy collected by solar panels than all the people of Arizona can use in a day, but at night, that all goes away. So battery capacity is key for capturing and saving all that energy until it is needed. This means that projects like the Tesla Gigafactory will be key in meeting our energy storage needs.  You can read more about the Tesla Gigafactory here.

So there you have it, while you are celebrating independence this weekend, go ahead and light a sparkler or two for these 3 energy wins to celebrate this year.

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Solar Power part 2

learn all about solar powerWhat Solar Power is:

Solar power is electricity made from the sun’s energy that reaches the earth. Fun aside: wind power, fossil fuels and biofuels are all ultimately made from the sun’s energy. But only solar cells directly change the sun’s light into electricity.

How Solar Power is made:

See Solar Power part 1, The basics: light from the sun hits a semiconductor and provides enough energy to make free electrons jump.

How much of our current electricity is produced by solar power:

In 2013, the US used over 8,000 MW hours of solar electricity. That is about 0.2% of the total electricity in the US.

Potential energy supply:

In one year, the entire world uses about 15 terawatts (a 15 followed by 12 zeroes) of electricity. That’s the same amount of sunlight that hits the earth in one hour. In other words, enough sun light hits the earth in one hour to provide electricity for everyone for one year! However, the challenges are collecting this energy, turning it into electricity, and storing it for use when needed (and not just when it’s sunny outside). But there is hope, Germany just reported producing 74% of their energy from renewables, and solar power played a large roll.

Materials and how we get them:

Solar cells are made of silicon crystals and silicon is the most abundant element on earth. However, it is almost always bound to oxygen in a molecule commonly called “sand” or “quartz”. We use electricity to get rid of the oxygen. The silicon is then heated and stretched like taffy. This stretching  draws impurities to one end. The impure end is removed and the remainder is pure enough to make solar cells. The pure silicon is then made into single crystal wafers and other elements are added.

Silicon is very shiny and reflective, which is not useful for collecting light. Titanium dioxide is used to help the silicon absorb more light.  Titanium dioxide is mined throughout the world.

The cells are sealed into rubber, put into an aluminum frame, and covered with a glass or plexiglass protective sheet.

Waste produced and how we deal with it:

Creating silicon wafers for solar cells is quite energy intensive because of the need for purity. A lot of heat and electricity is used in the process, which does produce carbon dioxide. Additionally, the manufacturing process makes industrial sludge and toxic waste, which need to be trucked to waste management sites for cleaning and containment.

Solar power produces no carbon dioxide after the cells have been made. However, the industrial wastes and CO2 produced during manufacture of the solar cells are siginificant. It is important for us to pursue cleaner and better managed outputs, as well as producing cheaper,  more efficient solar cells.


Large scale solar power plants currently produce electricity at the cost of about $0.12-$0.17 per kwh to the customer. The cost of installing solar panels onto the roof of your house is typically about $7-$9 per watt. A 5kW (the typical household consumption) array costs about $30,000. The good news is that it is estimated that solar will cost the same as fossil fuels within the decade. However, we need to act now to start combating our use of fossil fuels.


As I’ve mentioned above, cost and waste are both currently challenges to solar electricity. These challenges will be addressed by making solar cells that can more efficiently turn light into electricity. Currently, solar cells are able to convert about 20% of sunlight into electricity. Nearly 45% efficiency has recently been achieved by researchers in Europe. Being able to mass produce solar cells with that much efficiency will drive down the cost and reduce the number of solar panels needed to produce the amount of electricity that we use, reducing waste as well.

For an introduction on sources of electricity, look here.
For an explanation of how we make electricity, look here.
Clean Coal
Nuclear Energy
Wind Power
Geothermal Electricity
Solar Power part 1

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Solar Power part 1

solar power photovoltaicsBecause the way we create electricity from solar power is completely different from all the other electricity generation I’ve written about, I’m going to break this topic into two parts. Today we’ll cover the science of how we make electricity from sunlight.


Photovoltaics or solar cells, are what we use to change the light from the sun into electricity. Solar cells are made of a semiconductor material, typically silicon. The silicon contains a small amount of phosphorus mixed into the crystal structure. This phosphorus provides free electrons. The light energy from the sun moves these free electrons. And voila, give the free electrons a pathway to move through and you have made electricity.

The trick is to get enough of those free electrons moving and to get them moving further.

Electrons move different distances depending on the energy in light that hits them. Light is composed of various wavelengths which which each have a different amount of energy. We see the wavelengths as different colors, electrons see the wavelengths as different distances to move. With solar power, we want to catch all the wavelengths of light, and we want it all to make the free electrons move. We also want the electrons to move as far as possible because this will produce the highest voltage. Using both low and high energy light is difficult, and so the structure of the semiconductor must be very well designed and controlled. As you might guess, this is a difficult and expensive process. Making even a slight increase in solar power efficiency represents a huge breakthrough in science.

If you’re interested in learning the more detailed science of solar cells, this article is a great place to start.

For an introduction on sources of electricity, look here.
For an explanation of how we make electricity, look here.
Clean Coal
Nuclear Energy
Wind Power
Geothermal Electricity

 Oh, hey, Building Earth has a facebook page now.  Keep up to date on posts and other interesting green news by liking us!