Friday, August 15, 2008

Sizing Your Off-Grid Solar Electric System

The average US home consumes about 940kwh of electricity each month. For many, electricity use could be cut in half with a serious conservation effort. But if you had to rely on a small photovoltaic (PV) system could you get by on 120kwh per month?

To get by on less you’ll first need to make sure you’re using electricity as efficiently as possible. Replacing incandescent light bulbs with compact fluorescent (CFL’s) is a good start. You’ll also benefit by eliminating phantom loads and replacing inefficient appliances. I’ve listed many more things you can do in previous posts, so I won’t repeat them here. Check this blog’s archives for that information.

Living (comfortably) off-grid on less than 120kwh of electricity per month (about 4kwh per day) may sound impossible, but you just might be able to do it. Here’s how:

Of the 20 or so 13 watt CFL lights in your home, you might use each (on the average) 1 hour per day. So, 13 times 20 times 1 = 260 watt/hours. Shown below is the total for lights, and a list of other ways you might use this limited supply of electricity.

Lights: 13 watts X 20 hours = 260 watt/hours
Refrigerator: 50 watts (average) X 24 hours = 1200 watt/hours
TV and Cable box: 125 watts X 3 hours = 375 watt/hours
Radio: 5 watts X 6 hours = 30 watt/hours
Fans: 35 watts X 16 hours = 560 watt/hours
Computer and monitor: 120 watts X 2 hours = 240 watt/hours
Microwave oven: 1000 watts X 0.5 hours = 500 watt/hours
Toaster: 900 watts X 0.1 hours = 90 watt/hours
Vacuum Cleaner: 750 watts X .2 hours = 150 watt/hours
Cell Phone Battery Charger: 25 watts X 2 hours = 50 watt/hours
Washing Machine: 500 watts X .25 hours = 125 watt/hours
Iron: 1000 watts X .25 hours = 250 watt/hours

Total: 3930 watt/hours per day

How you use the available electricity will not exactly match my list of course. This is simply an example to show how you might get by on much less electricity than you’re currently using. Off-grid doesn’t have to mean living like a caveman. A small PV system can meet most of your electrical needs, including a limited amount of cooking and climate control. As long as you have other systems in place for heating, cooling, and other high-energy devices, you could live quite comfortably on much less than you currently use.

Why is this important?

Most of us purchase electricity from our local utility company for less than 2% of our household income. Because grid-supplied electricity is inexpensive and convenient, few people have any interest in alternatives at this time. But just as gasoline prices have skyrocketed in the last two years, we’ll soon see the cost of electricity increase dramatically. Most consumers will deal with this by cutting back, but some will choose to disconnect from the grid. A PV system large enough to meet your current electricity requirements may cost 25 to 35 thousand dollars. For most, reducing usage and installing a smaller PV system will be easier and less costly than installing a system big enough to meet current demands for electricity.

What would this smaller PV system cost?

First of all it’s important to understand that a system capable of providing 4kwh of electricity a day will not provide 4kwh on a cloudy/rainy day. Typically, a lack of sunshine prompts the user to either cut back on electricity use that day, or to use another source of electricity during those times, typically a generator. It is also important to understand that we’re discussing an off-grid system, not a grid-tied system. An off-grid system includes the extra expense of batteries, and is not as efficient as a grid-tied system. Your system design might include a battery bank large enough to compensate for a day or two of cloudy conditions.

PV panels produce electricity when the sun strikes them, but are most productive during hours of peak-sunlight, or stated another way, when the sun is almost directly overhead. We’ll do our calculations based on an average of 4 hours of sunlight each day. A 100 watt solar panel can produce 400 watts/hours (100 watts times 4 hours) of power each day. It follows then that to get 4000 watt/hours (4kwh)from the panels each day, you’ll need 1000 watts of PV panels. To make up for system inefficiencies, you should shoot for at least 1200 watts of PV. That would be 12 one hundred watt panels for example. If you shop around, you’ll find solar panels for less than $4.50 per watt, so you’ll spend about $5400.00 for PV panels alone. You’ll also need a charge controller, batteries, an inverter, panel mounting hardware, wire, and safety components. These items can be bought for $2600.00 if you shop around. If you’re not able to do the installation yourself, you might spend another $3000.00 for that, making your total cost about $11,000.00. If this sounds expensive, don’t forget that it eliminates your electric bill. The system could pay for itself in 5 years, or less as electricity prices increase. And since solar panels can be expected to last in excess of 20 years, you’ll be getting many years of low-cost electricity after that.

Using your system:

Your small system may not always keep up with your needs, but you’ll learn techniques to maximize efficiency. Using energy from the sun as it’s generated (instead of storing it in batteries for later use), increases system efficiency greatly. By using the vacuum cleaner, washing machine, and other appliances during peak-sunlight hours you eliminate losses associated with converting, storing, and retrieving energy. Your goal should be to use electricity wisely, ensuring a surplus. That surplus will come in handy when it’s cloudy.


Having your own power plant means that you’ll not be affected by outages and brown-outs that grid-connected customers often experience. News reports about rate increases will no longer concern you. You’ll feel good knowing that by disconnecting from the grid you’re not contributing to the environmental problems associated with mining and burning coal to produce electricity. By installing your own PV system you’ll be taking an important step toward personal electric transportation, or as a politician might say eliminating your “addiction to oil”. Declining oil supplies will soon usher in the age of electric cars, and it’s not unreasonable to think that someday you’ll be able to drive on free energy from the sun. That’s something to get excited about!



Damon said...

Nice one SJ.

Even in our small UK home we're between 5kWh/day and 6kWh/day after conservation efforts, though maybe 1kWh/day is for my main outward-facing Internet server so not 'typical'!

But if nearly everyone could get to somewhere around these figures, on-grid or off-grid, with or without microgeneration, it would be a tremendous advance IMHO.

And as you say, making hay while the sun shines is one way to deal with intermittency.



Anonymous said...

Hi John,

Your survey sounds like it was inspired by this Wired article on the future of all-electric cars:

I'm sure you and your readers would be interested in the above article. Mr. Agassi has many good points, but in the end it sounds to me like he's maneuvering to become another Bill Gates, only far richer. See what you think. I'd still prefer a simple & inexpensive 2-passenger electric car that you could charge with your own photovoltaics or wind power or plug it in or whatever and keep your old gas guzzler exclusively for long trips.

Send me an e-mail again John. I've (finally!) got my Win Vista computer going after installing 2 gigs of memory. But things leading up to that caused me to lose e-mail addresses again. But the Vista is working good now. I don't know why Dell would ruin their reputation by selling such a disabled computer in the first place.

--- Bob

Solar Electric said...

the most interesting thing about solar electric is that it helps in maintaining a greener and healthier environment. This is because it does not use fossil fuel that the normal power source uses; fossil fuel emits greenhouse gases that cause pollution.