Monday, May 14, 2012

How to figure out solar or wind power needs.

One of the hardest things to figure out if you want to go Off grid, or be self sustaining and make your own power is how much power do you think you will actually draw or use?    You would be suprised at how much the things you own or use drink electricity, and when you have to make it yourself, suddenly everything matters.

Below is a table of items I have measured with my Kill-a-Watt meter and calculated out for a daily power use.

 1000 Watt Microwave oven.  Uses 1110Watts per hour. used 30 minutes a day.  555WattHours a day BUT it also consumes 2 watts every hour all day long. so add in another 48WattHours for it just sitting there plugged in.
Your cellphone charger,  Consumes 12Watts per hour.  a typical charge time is 4 hours.  48 Watt Hours a day BUT it also consumes 2 watts an hour just sitting there plugged in.  That's another 48 Watt Hours!

Everything that is plugged in today consumes power while sitting there.  At least if it's electronic  it does.   your toaster might not, and a old style turn knob timer Microwave also may not consume electricity when not in use. But you need to check to see.   That is why everyone even thinking of living off the grid needs to buy an energy consumption meter.  so you can check to see if it uses power when off.

You can combat the wasting of power by these devices two ways.  One is to simply unplug it when not in use.   The other is the new "timer outlets"  that you can set to only stay on for 4 hours and then it shuts off.   OR a old fashioned Light timer set to only turn on your chargers for 4 hours at night.  But remember that timer will also use power 24 hours a day.  From 24 to 36 Watt hours a day just for that timer.

All of the above not only is important to help determine how many solar panels you need, but how big of a battery bank.  Until you know how much power you will be using a day and then multiply it by 125% or 150% you cant begin to calculate what you need for solar.

So let's go with a hypothetical small home.
1 800Watt microwave with old style timer.
1 cellphone charger
1 ipad or other tablet charger.
4 5 watt LED lights
A laptop that uses 95watts
and a clock radio, 10 watts typically.  I am assuming you will probably use the clock radio as a radio at times.

440Wh for the microwave (30 minutes a day with 10% added for power loss)
48wh for the cellphone
72wh for the ipad charger
380wh for the laptop that takes 4 hours to charge a day.
264wh for the clock radio that is plugged in all day long.

This is not figuring in Lights, water pump use and any fans for heat or cooling.  Those really are important as well.

1204WattHours used per day.  so you need to make MORE than that during the 6 hours of daylight you can count on having.

Typical 100 Watt solar panels make 600WattHours per day.   2 of those panels will make 1200WattHours.  3 will make 1800Wh per day..   But wait, it's not that simple.  If I was living in Arizona, Yes that will work.  But I'm silly and want to live in Minnesota.  If I double the number of panels, they will make the same wattage in 1/2 the light.  (really general of an assumption)  The sun is not as bright up there compared to the desert, and there is more opportunity for cloudy days.   So I am closer to right by simply doubling the number of panels, on bright sunny days, I will make a lot more power than cloudy days, but you need to make power every day.

But that is 1/3rd of the system. I also need battery storage for when the solar panels are not making enough power.   At a minimum you really want to have 3 days of battery capacity.  so that is 3612Watt Hours of storage.  Watts divided by volts = amps.   301Amp Hours of battery storage is needed for the 3 days.  But you cant discharge a battery 100%, you really want to stay above 50% discharge for the longest battery life.  so let's go for 650Amp Hours of Batteries.

That is a large battery bank.  It's not going to be in the home and not easily carried.  Solar power starts looking bleak when you waste power like a typical consumer.

So look at reducing power use at home.  Laptop,  charge it at work or elsewhere.  Clock radio, fond one that uses a lot less power, a LCD model that has almost no features will use 4Watts That saves 168Watt hours right there.

You can also offset it with wind, a  small 400 watt sunforce wind generator is only $600.00 and those work when you have wind.  All night long in a windy location you are generating electricity which is typical on cloudy days.

But the thing is, you cant rely on having sun and wind 24/7/365  you have to plan for the days when you have none for long stretches of time.   This is when you either have a larger battery bank, or you supplement with a gas generator.

I probably raised more questions than I answered, and Off grid living is a complex topic.   Check out for some detailed info on off grid living as well as Home power magazine.

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