Before we go too far, to really understand and appreciate solar electricity [and how it can offset your fossil fuel use], you first should understand some basic concepts of electricity, and how that electricity is billed to you.

All electrical appliances tell you the number of amps, volts, and watts that they use. Their relationship is simple: Watts = Volts * Amps (W = V *A). Many battery systems run on 12V, so for example, a 12V cell phone charger might be 0.9 amps, or, 12 * 0.9 = 10.8 watts. That’s about as much electricity as can be pumped into a smartphone battery at one time.

Most devices in your home consume a lot more power than a smartphone, however. Your home (if it’s in the US) is wired at 120V Alternating Current (AC) and some appliances (dryers, water pumps) are wired in at 240V. A washing machine may have a peak load of around 12 amps at 240 volts, or, close to 2,880 watts. That’s as much current as can charge 160 smartphones!

Small appliances (even lightbulbs!) can be contributing to your electric bill in a big way!

What is running up your electric bill?

1,000 watts = 1 kilowatt. However, you are billed for electricity in kilowatt-hours, the product of a certain amount of electricity over a period of time. So for instance, that washing machine, pulling 2,880 watts, for 5 minutes, would consume 240 watt-hours (0.24 kilowatt-hours) of electricity. At today’s rate (around 15.5 cents) that amount of electricity would cost you about 4 cents.

The accumulation of ALL of the electric loads in your household, times the amount of time that you run them, culminates in your total electric bill from the utility. The energy loads that end up being big hogs can be deceiving. For example, a 100-watt incandescent light bulb, left on 24 hours a day, 365 days a year, will consume 876 kilowatt-hours each year, at a cost of over $130 (now you know why you should switch over to LEDs)!

How Solar Electricity Fits In

Solar panels are the opposite of electric loads, they generate a certain amount of electricity for every minute they are exposed to the sunshine. Solar panels are rated in watts (typically 265 to 320 watts) and collections of them for homes are rated in kilowatts (1,000 watts). For example, 20 solar electric panels rated at 250 watts results in a 5-kilowatt solar electric array (5,000 watts = 5 kilowatts).

An oversimplification is to say that a 5-kilowatt solar electric system will generate 5 kilowatt-hours (kWh) for each hour that they are exposed to sunlight. Realistically, some amount of the solar energy is lost in the wiring process and conversion from the direct current (DC) electricity generated by sunshine into the 120V alternating current (AC) electricity consumed in your home. Also, the sun is rarely constant for a full hour; any clouds or changes in sun intensity will affect the real-time performance of a solar array.

Solar-HouseSo to predict how much electricity a 5-kilowatt solar array will generate, we take the data on regional solar insolation that we saw in the last chapter and build out a model of expected solar generation. The National Renewable Energy Labs (NREL) has an excellent calculator, PV Watts, which uses 25 years of weather data to assess expected solar insolation for a location.

A full analysis of a solar array’s expected production based on climate, adjusted for the angle of the solar array, and orientation (azimuth) towards the sun, and adjusted again for any shading, results in a prediction of a system’s output over the course of the year. As a rule of thumb, each 1-kilowatt of installed grid-tied solar, on a good site will generate around 1,250 kWh/year in New England. A typical 5 kW array for a home will generate about 6,250 kWh/year. In the next chapter when we get into talking about the economics of solar, we will also talk about how to convert a solar energy system’s potential output into dollars saved on your electric bill, and how that savings justifies investing in the project.

Negawatts

Energy efficiency enthusiasts make the claim that the easiest kilowatt-hour to create is the one you don’t have to use, hence, “Negawatts!”

When we start designing a solar electric array for a system, our first step is to figure out exactly how much electricity they are using, and how much solar resource is available at their location. A homeowner with a very tight roof, who can only fit 16 panels, for instance, has a different set of decisions to make than a homeowner who has a vast south-facing barn where they could potentially install much more solar electric generation than they actually need.

Most of our customers see solar as part of a road towards energy independence, and taking reasonable aims to reduce energy consumption in tandem with a solar installation is a highly recommended strategy. It can work out to be much more expensive to produce electricity that is used relatively inefficiently – for example, powering old refrigerators and water heaters. Generally, it is cost-effective to make some (not all) energy efficiency upgrades as a path towards meeting all of your needs with solar electricity.

Using Solar Electricity to Offset Fossil Fuel Use

There are, however, “good” reasons to increase your electric bill! With heat pumps, electric cars, and electric water heating solutions available, a home can essentially eliminate oil, propane, or natural gas completely and be fully electric powered. Many of our customers add enough solar electricity to meet all of their needs initially, then upgrade the system over time to supply power for an air-source heat pump, an electric water heater, or an electric car. Solar energy can also heat your water directly with solar hot water panels.

As you can see, there are quite a few options that exist in the world of renewable, solar power. The next section, SOLAR IS AN ECONOMIC OPPORTUNITY, takes a look at the economic benefits of solar electricity and the paths that we offer to owning solar power.

To Chapter 1

To Chapter 3

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