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Solar Electricity Basics
This is an Introduction to Electricity Basics. It’s intention is to lay a foundation for learning about solar electric systems. It is an introduction to the terms and phrases used in electricity. It also discusses that installing solar isn’t a license to use as much electricity as you want. You still should look into ways to conserve the amount of electricity you use. Solar isn’t a silver bullet that solves everything, it is one important step. Conservation is another.
Electricity is often compared to water to make it easier to understand the difference in terms.
Volts, abbreviated as V or E (for electromotive force) is the amount of potential energy. It is like the water pressure in your pipes.
Amperes, more commonly called Amps, and is abbreviated as A or sometimes I. It is the rate of current flow, or electrical charge. It is often compared to water flow.
Watts, abbreviated W or P for Power, is the rate of power. It’s how fast a device is using or making power. Watts = Volts x Amps. Watts can be increased by increasing either the voltage or the amps, like the amount of power a water turbine creates can be increased by increasing the water flow or the water pressure.
Amp Hours, or AH, is the quantity of amps over time. 1 AH = running at 1 amp for 1 hour.
Watt Hours, or WH, is the quantity of power used or generated over time. 1WH is 1 watt generated or used for an hour. Kilowatt Hour, or KWH, is 1000 watt hours.
One of the most common mistakes is confusing watts and watt hours. To understand the difference, we will switch from the water comparison to driving. Watts is the rate of power consumption, like MPH. Your speedometer tells you how fast you are driving. A watt meter shows you how fast a device is using power. A 60W device is drawing 3 times as much power as a 20W device. Watt hours is the quantity of power used, like miles. Your odometer tells you how far did you drive. Using a 60W device for 2 hours uses 120WH.
Now let’s look at amps versus amp hours. Amps is the rate of electrical charge, like MPG. How fast are you using the gas in your tank? An ammeter shows you how fast a device is drawing power. The lower the miles per gallon, the faster you are using the gas. And the sooner you’ll run out of gas and be hitchhiking on the side of the road. The higher the amps, the faster you are using the power in the battery, the sooner your battery will go dead. Amp hours is the amount of electrical charge, like gallons of gas. How much gas is in your tank? Your gas gauge tells you that. Deep cycle batteries, the type used in solar systems, are rated in both volts and amp hours. This tells you how much power the battery can store.
What we are trying to do in solar systems is produce and sometimes store electrical energy. We’ll want to keep in mind the basic principle that voltage will always flow from an object with higher voltage to one with lower voltage. In the case of battery based systems, the voltage entering the battery needs to be higher than that of the battery to charge it. For simplicity, we often talk about a 12V solar system. In fact, none of the items in the system are actually 12V. Remember that 12V is more of a category than actual voltage. It is called nominal voltage. When connected, the solar panel is generally about 17V, and a full battery is around 14V. If the voltage output of the solar panel was equal to or less than the battery voltage, it would not be able to charge the battery.
SERIES AND PARALLEL
A solar power system can have one solar panel and one battery. Larger systems have multiples of both panels and batteries. The wiring determines the voltage and current output. Wiring panels in series, where the negative of one wire connects to the positive of the other, results in the current staying the same, but the voltage increases. Two panels in parallel, with the two positives wired together and the two negatives wired together, results in the voltage staying the same and current increasing. It’s important to note that regardless of the way it is wired, the power, or watts, remains the same. Since watts equals volts x amps, it doesn’t matter if it is series or parallel.
Likewise, when batteries are wired in series, their voltage increases, and when wired in parallel, amp hours are increased.
You’ll often see multiple rows of panels or batteries wired in series, each row is a string. You can then wire multiple strings in parallel. This allows you to get both higher voltage (with series strings) and higher amps or amp hours (with parallel). 2 parallel rows of 4 batteries wired in series is called 2 strings of 4. Wiring four (4) 12V 80AH batteries in series equals 48V 80AH, and wiring 2 strings in parallel doubles the amp hours to 160AH. The power is 160AH x 48V, which equals 7680 WATTHOURS.
HOW MUCH POWER ARE YOU USING?
So now that we know the basic terms, let’s talk about how much power you are using. Most electric companies give you a bill that shows how many kilowatt hours you used that month. They often will show you a 12 month history, so you can compare your usage month by month. You can detect seasonal trends from the bill. Higher summer usage usually indicates air conditioner or fans used. Higher winter bills usually are for electric heaters. You may also note that the charge for the actual electricity is a fraction of the bill. You are also charged many fees and taxes for transmitting the power, funding special programs, and other services. Many electric companies charge a different rate based on how much power you use and what time of day it is used. When trying to figure out where your electricity is being used, note that different devices use electricity at different rates. Anything that makes heat and cold, like an airconditioner or a water heater or stove, use a lot of power. In the past decade, a lot of work has been done to reduce the power consumption of most electronics.
Energy Star has done a lot to bring awareness to consumers so they can make educated buying choices. Most items you buy today have an Energy Guide label available for it. You can compare different models to determine which model uses the least amount of power. You can see on the label where this particular model lies with comparable models with similar ratings, and on average how much electricity it will use in a year. Note that because a device has an Energy Guide label, doesn’t mean it is Energy Star. Having the Energy Star rating indicates that it is more energy efficient than other models. Always look for the Energy Star label.
So, what’s the big deal? Let us look at some examples to see how this pertains to solar. Refrigerators have come a long way in this century. A typical fridge from before the year 2000 used about 850kwh a year. An average Energy Star fridge these days uses less than 400 kwh a year. That’s less than half of the power from 14 years earlier. You’ve all been hearing for years about swapping out your old incandescent light bulbs. A lot of people didn’t like the CFL bulbs that were the only alternative. These days, LEDs have come a long way. Let’s compare using 60W incandescent bulbs with 10W LEDs. Granted, LED bulbs cost at least 20x more than an incandescent bulb, but it makes sense to swap to an LED bulb next time your incandescent goes out. Both bulbs put out the same amount of light, both are dimmable, immediate on, and work in the cold. If you replace 10 incandescent bulbs with LED bulbs, it would cost you about $100. But if you used those bulbs for 4 hours a day, that would save 2000WH a day, or 730kwh a year. At 11 cents a kwh, that’s an $80 a year savings. You’d have those 10 bulbs paid for in savings in 15 months, and even less time in places with higher electricity rates.
So, you may still be wondering, what’s this got to do with solar? If I make my own power with solar, I can run my old inefficient fridge with my incandescent lights on for free. Well, sure, but let’s step back to the size of your solar system. The less power you use, the less you need to make. The less you need to make, the smaller the system you need to buy. I’m not going to go into the math to determine what size solar system you need, but I’ll give you the short answer. The example of swapping out 10 light bulbs for $100 would need a solar system that is 650W smaller than providing the electricity for the old bulbs. So not only did those 10 bulbs save you $80 a year, but at an average installed cost of $4 a watt, you also saved about $2600 on your solar system. Now let’s talk about that nice new fridge you got. Swapping that old fridge with the new Energy Star fridge requires 400W less of solar. That’s a $1600 savings in solar. You could buy a nice new fridge, and a few more new appliances with that, further increasing your savings. Now that you have the basic understanding, what’s next? Look at your electric bill and see how much power you use each month. Look for seasonal trends to help you determine biggest use, and your first steps to reduce the use. There are many resources available to help you find ways to reduce waste. You can start at energy.gov.