Is anybody looking?

Wednesday, August 31, 2011

In the beginning we had an idea...

Hello!

    Welcome to my trials at setting up an off-grid power system. I live in the mountains where we don't have power. PG&E never connected power to the property and my woman never bothered to pay the $40k+ to have them connect power. Now I am in the picture and I come with an electrical load that has to be met.

    So I decided to build an off-grid power system. I mean I was a mechanical engineer in the Navy and had many years experience with main propulsion systems and auxiliary power generation, this should be a breeze! Ah... no, it's actually a lot more difficult than many realize.

    Well I am not one that is easily deterred and generally when I am challenged or told "no you can't do that" I embrace the challenge and head into the fray! Well, let us say this has not been without it's own painful lessons, but I figure might as well share them so that others don't have to suffer as I have. Learn from my mistakes.

    One thing I have noticed is there are very few sites out there that actually show, document or tell what is going on. Hence this blog!

   Okay to start this is an OFF-GRID system. That means there is no power coming in from any utility company at all. This is an important thing to keep in mind as that means you get no rebates, to my current knowledge NO tax breaks and essentially no financial help for setting up a solar battery system.

   To start your load requirements. This is something that will mean sacrifices will be needed on your part if you want to build a realistic off grid system that doesn't require you to hawk your first born child and one testicle. Look at all the electrical gadgets you have and ask yourself "do I really need this item"? Seriously you would be surprised at how many thing you have that you really don't need and can quite easily live without.

    Once you have gone through your electrical gadgets and sold off or discarded the ones you don't need, your next step is to check how efficient your current items are. Generally the newer your electrical components the more energy efficient they are, this equates to a lower power foot print. And really this is a prime key to success.

   Now that you have updated or validated your electrical components it's time to check their power consumption. This is where a lot of confusion can happen and it's an area that got me several times before I finally "got it!".

   Every item you have that plugs into a wall socket has a label or tag that will tell you the following information,Voltage and Amps. Now some items will actually post the watts which is great! But that's what you need, is the watts and eventually kVa (which will be converted to kWh).

   So to determine the watts of a particular item you take the Watts = Volts x Amps

   Here I will show the total watts my refrigerator uses when running (this does not include start up which is much higher initially). So I have a 5 year old Kenmore refrigerator the data plate says the following:

        Volts = 120
        Amps = 4.5

     So the calculated Watts would be 120 x 4.5 = 540.

    Awesome! But we are not done yet, no now we need to calculate kVa and the formula for that is kVa = Watts / 1000.

    So my calculated kVa is 540 / 1000 = .54

     Bored yet? Head hurt? Don't worry, if it does then you are most assuredly not alone! Okay now we still have more to do. Run times, how long are your items going to run for? This is important as it's part of the kWh calculation, which in turn is used to determine the amount of Amp Hours (AH) you will need from your batteries. The best way to find this out is to use a Kill-A-Watt to obtain equipment run times. You can gather other information as well, such as validating your energy calculations and so forth.

    Now before I proceed, remember I said that you also need to include start up power. Now this applies to items such as refrigerators, freezers, fans and microwaves. This is also a hard one to figure out (although the Kill-A-Watt can help!) but it's important as it will affect your battery performance. What I found by talking to my friends that are electricians is that generally the Amp surge to start a motor is generally 1.5 time operational Amps. But keep in mind that it's instantaneous, which means when you apply a time to it generally .25 is sufficient.

    What? Okay in layman's terms, when my refrigerator starts (this is the compressor pump) it will surge at about 7.5-12Amps for less than a second. So using the Watt formula my total watts consumed would be:

        120 * 7.5 = 900W
        900 / 1000 = .9kVa
        900 * .25 = 225kWh OR .23kVh

    So make sure you include start ups in your calculations. I will include a link later to the spreadsheet I set up for my power calculations. If you find it helpful great, if you find errors... shut up! LOL! Kidding, if you note errors please feel free to let me know and I will correct as I validate. If you find what I have set up helps you with your system design, awesome, please send some credit my way.

    Okay back on task. To calculate kWh or kVh it's simple as the formulas below!

        kWh = Watts * Time
        kVh = kVa * Time

    See how easy that was?! Now as you build your power use list you want to have a running total of power consumed. This will later be converted to Amp Hours, which we will from here on refer to as AH.

No comments:

Post a Comment