Solar Electricity at Home

by Dr. Tom Lombardo

Generating electricity from sunlight is more practical than you might think. Germany is one of the world leaders in solar power production, and it gets less sunlight than most places in the northern United States. Energy prices continue to rise and the cost of solar panels keeps going down. If you want to go completely off-grid, solar is the most cost-effective way of generating your own electricity. And if you just want to reduce your electric bill and hedge your bets against rising electric rates, solar is probably the best option. Either way, solar power is a good investment for the homestead, as long as you think of it as a long-term investment. (For more about going off-grid, see my overview of renewable energy.)

Please be aware that even if you’re a do-it-yourselfer, installing a solar power system should be done only by qualified professionals, including a licensed electrician who’s familiar with all the codes and standards that pertain to solar power. We’re talking about electricity levels that can kill a person or cause a fire, so remember: safety first!

In fact, this topic is far too technical to cover thoroughly in one article. There are entire books written and courses offered on designing solar power systems, so if you’re planning to design your own system, get a book or take a course – preferably both. But for now, let’s look at what you can expect when you contact a solar energy provider.

Photovoltaic Basics

Photo: OSHA.gov

Photo: OSHA.gov

Photovoltaic (PV) panels convert light directly into electricity. (There are other ways to convert the sun’s energy into electricity, but those are more appropriate for utility-scale systems, not residential systems.) The most common PV panels are made of silicon, the same material that’s used to make computer chips; they offer the best balance between price and performance. Today’s panels are about 20% efficient, which means that 20% of the sunlight that hits the panel will be converted into electricity. The rest is turned to heat. That may not sound great, but remember that sunlight is free – you don’t have to mine it, drill for it, refine it, or transport it. (Side note: if you drive a gasoline-powered car, your engine is only 20% efficient and it uses fossil fuels and creates pollution. So don’t complain about solar panels, okay?) One solar panel doesn’t generate nearly enough for a house, so you’ll need a solar array – a group of panels wired together to produce a lot of energy.

 

Site Assessment

Before we design a PV system, we always conduct a thorough site assessment. This can start remotely, with the assessor asking you about your goals for your system. For example, do you want to generate all of the electricity that you need or just reduce your energy bill by generating some of your own electricity? If it’s the latter, about what percentage do you want to generate? And of course, what is your budget?

Next the assessor will conduct an electric load analysis to determine your electricity usage. This includes examining one or two years worth of electric bills and looking at the energy efficiency of your home and appliances. She will probably make recommendations for conserving electricity because it’s always less expensive to reduce consumption than to produce more.

A site assessment also includes a visual inspection of your property and roof (assuming you want the panels on your roof) to determine whether the roof can handle the added weight and stress of panels, where to place the panels for optimal production, and where to locate other equipment.

Next we perform a shading analysis. We start by getting data from the National Renewable Energy Laboratory (NREL). This tells the sunlight that can be expected at a given location. It’s based on thirty years of weather data, so it takes into account your latitude as well as local cloud conditions. Once we know how much sunlight falls on the location itself, we look for the best place to locate the array, and we determine how shady it is at different times of day throughout the year. This helps estimate the actual production that you can expect from your system.

shading

Image: US Department of Energy

In the above picture, you can see why the designer put panels on the garage but not the main house: too much late afternoon shade on the house.

Here’s an example of a shading analysis report:

ShadingAnalysisReport

The unshaded AC energy production numbers (2nd column) come from NREL. As you can see, there’s less sunlight in the winter. That’s because the sun is lower to the horizon and the days are shorter. The percent unshaded (3rd column) is based on obstacles at your site, such as trees, buildings, chimneys, etc.

Speaking of south, in the northern hemisphere it’s common to point solar panels due south to capture the most sunlight throughout the day. That’s good for overall production, but if your power company charges more for electricity during peak hours, which usually happen in the mid to late afternoon, then you might find it beneficial to point your panels west. If your array will be roof mounted then you don’t have much choice, but don’t be discouraged if your roof faces east-west instead of south – you can still generate a lot of solar electricity.

Now that the assessor knows where the array will be located and its shading properties, she can determine the optimal size of the system so it gives you the amount of energy that you need. The design includes the number of panels and how they’ll be connected to each other, and what other components are needed and where they’ll go.

Cost Analysis

A site assessment also includes a cost analysis: how much will the system cost, how much money per year will you save on your electric bill, and how long will it take for the savings to balance out the cost of the system. The payback period is the amount of time it takes for the system to pay for itself. Depending on your location and your electric rate, a typical payback period may be somewhere between ten years and thirty years. (Remember when I said it’s a long-term investment?) The only reason we don’t have PV panels on our house is because we’re planning to relocate in about five years.  Our next house is where we plan to retire, and you can bet that we’ll have a PV array on that one!

A cost analysis will also include a list of incentives and tax breaks that can help defray the cost of your system. In some cases, you could cut your cost in half by taking advantage of these incentives.

Power Purchase Agreements

A PV system could cost tens of thousands of dollars. If you don’t have that much money to invest, see if there are companies in your area that offer power purchase agreements (PPAs). With a PPA the company will install a PV system on your house at no cost to you, and you agree to pay them for the electricity that it generates, usually at a rate that’s much lower than your power company charges. They own and maintain the equipment, and you get a lower electric bill with no up-front costs. It’s a win-win for the consumer!

 

Well that’s a run-down of buying a PV system. Before signing any contracts, be sure the company is reputable, licensed, and insured. Get references if possible. They should do all of the steps that I’ve listed above, and also obtain the required permits, design and install the system, and schedule inspections before going live. If you get several estimates from competing companies, don’t just go with the lowest price – look for quality. You’re spending a lot of money on this, so stay involved in the process.

Do you have a PV system? Would you like to share your experience with us?

 

 

 

 

 

 

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