Friday, November 28, 2008

Off-Grid Solar Power Budget

When you live off grid budgeting takes on several meanings. There is of course the usual money budget which under normal grid attached circumstances is the all important American control mechanism. As long as you can pay for your power and water there is nothing limiting your consumption. You will always stay warm and be able to flush that toilet. But in the off grid world your power, water and heat budgets become the governing factor in day to day life. These finite resources must be carefully measured. Miscalculation and waste can leave you freezing in the dark unable to even find the toilet.

I measure my stored power in amp hours which is the way deep cycle batteries are rated. Let's say I have 6 deep cycle batteries each with a capacity of 270 amp hours. That would give me a pool of around 1600 amp hours to draw from when the battery array is fully charged. Remember true battery capacity can only be figured by a trained voodoo priest with a good thermometer.

Deep cycle batteries can be drained to down to 30%, this is not recommended but can be done in an emergency situation. It is nice to only draw down to 70% giving me around 480 amps to play with. Now here is the important part, whatever you pull out you must replace. If you drain 480 amps off the array each day 480 amps need to be put back either through wind, generators or solar.

I have 6 130 watt solar panels each delivering 7.38 amps back into the battery array for each hour of sunshine. (I know the numbers don't work out but that's the way it is with solar) So with my solar array I can realistically put 40 amps back into the battery array per hour of good sunshine. I base my budget on 6 hours of good sunshine per day. A sunny day will replace 240 amps back into the battery array.

The governing number for me is 240 amps. (I am battery heavy) I still don't want to draw down the full 240 amps. Being paranoid like I am I only use 120 amps per day.

In theory I have 1120 amps to use if I drain the batteries to 30%. This gives me a little over 9 days of reserve if for some reason I can not recharge the batteries. Additionally if I only use 120 amps and am replacing 240 the batteries will recharge in just a few days.

Now what does all this mean, how much power do I have. Can I run my blender and make my Pina colada smoothies? Will my 50 inch plasma TV still be able to baby sit the kids 8 hours a day? Can the SubZero keep my champagne and caviar chilled?

So now that we know how much power we have available the next step is to determine how many DC amps each AC device draws. Remember that when I am talking batteries I am referring to 12 volt DC, just like your truck.

Here is the formula:

AC Watts / 12 x 1.1 = DC Amps (Amps are measured in hours) (1.1 is inverter inefficiency)

60 Watt light bulb / 12 x 1.1 = 5.5 DC Amps per hour of use

Now multiply the DC Amps by how many hours you plan on using the device

5.5 DC Amps x 8 hours = 44 Amps daily

As you can see my one 60 Watt incandescent light bulb has already used up 1/3 of my daily available power. This is why you do not use incandescent lighting in an off-grid project. I use low wattage florescent bulbs. The bulbs I use are 11 watts but mimic a 40 watt bulb, I have several through the cabin. I figure 3 lights running 8 hours each. (this is high number of hours by the way)

33 AC Watts / 12 x 1.1 x 8 hours = 24.2 Amps daily

You could reduce the daily amps for lighting even more if you used LED bulbs. I would suggest a warm temperature LED bulb. The first bulbs that came out were a cold temperatures bright white light and I really hated them. Our eyes are used to the warmer incandescent quality lights and I prefer them.

Here is the tricky part. You can not just go by the rated input wattage listed on the device. My laptop for instance has a 95 watt power adapter. That is a power hog and I would not invite it into my house. The reality is that those numbers are the highest wattage produced not necessarily what will be used.

I use a Kill-A-Watt meter to determine the actual power consumption of any device. This little box plugs into the wall and you plug the thing being measured into the Kill-A-Watt. Here is the actual power consumption of my laptop.

Charging 60 Watt
General Use 22 Watt
Watching DVD 31 Watt
Asleep Lid Open 11 Watt
Lid Closed 1 Watt

So for me the most efficient way to work is to plug the laptop in keeping it fully charged and when I am not actively using the thing close the lid. Worst would be to sit on the deck running the laptop batteries down then plugging it every few hours.

Here are some of the other devices and roughly how long each day I will be using them.

Microwave 650 Watt/10 minutes
Coffee Maker 600-650 Watt/10 minutes
Ryobi Battery Charger 30-40 Watt/2 hours
Phone Charger 1 Watt/2 hours
XM Radio 5 Watt/8 hours
12 volt Water Pump 5 DC Amps/30 minutes

In the off-grid world a careful budgeting and understanding of your power, water and heat usage is critical if you are looking for a relative comfortable life. It is critical to know your needs and more importantly what you are willing to give up before moving down the off-grid road. You just can't use extra and pay a little more at the end of the month your reserve is fixed.

If you found this useful let me know and I will do the same thing for heat and water.

Pick up a Kill-A-Watt meter and find out just how much power you really need. Even if you are not planning a move off-grid it is important to understand your power usage.


Anonymous said...

Great post, nice setup too.

We can only count on about 4 hours of full sun a day here - and sometimes less than that. I'm assuming you get a fair amount of snow in CO, are the panels on the roof, and/or how do you keep them clear? I'm using a ladder and a broom, which ain't no fun on a two-story slippery 45% pitched roof... Been up there twice already this year.

re: finding the toilet, wind-up flashlights on the nightstand here - otherwise we're tripping over dogs and bumping into walls.

I'd like to hear about your water setup...

kymber said...

i found the post helpful and interesting as well...more posts along this line would definitely be of interest; however, i enjoy all of your posts...

in addition...and with all respect to your privacy...more pics of your house, land, etc. would be wonderfully enjoyable!

thanks for all of your excellent and informative posts!


mmpaints said...

Bear, excellent, easy to understand explanation. Please do the water and heat post....

Bullseye said...

BigBear, gonna print this page for later use. A lot of good information that is easy to understand. My people seem to think that their is an endless supply of electricty behind that wall outlet. I'm tring to show them that there is, only if you want to continue to pay for it. I guess as long as I pay for it it's not gonna matter much to them. I have often though about metering the use each day to let them see just how much they waste. Love to hear about your water and heat suggestions.

Anonymous said...

Solar seems to be pretty straight forward...what is your take on wind power?

Would be interested in a water budget. I'm looking to move up in the high desert mountains where there is only 10-14" of rain a year.

Heat concerns are also on my mind. Read about one area where everyone is dependent on a natural gas pipe for the thanks!

Anonymous said...

The Kill-A-Watt meter has failed on several people using a modified sine wave inverter. I have one and have tried it before I knew that fact. The reality is that the Kill-A-Watt shows quite a bit more power consumption for devices using a modifid sine wave inverter. Which inverter are you running?

Many battery people suggest NEVER running the batteries down below 50% DOD (Depth of discharge) or you will do permanent harm to them.

Much can be learned at which is a forum about solar electric stuff. Wonderful info and FREE! Very helpful people too, there is a arcive full of wonderful information.

Please tell about the water and heat setup too.

Thanks for sharing your life with us and telling how it is.

Skip in Kansas City

Mayberry said...

Excellent, excellent, excellent! This is the kinda stuff folks need to see! Real world numbers really put things in perspective. Thanks Big Bear!

chris said...

"So for me the most efficient way to work is to plug the laptop in keeping it fully charged and when I am not actively using the thing close the lid. Worst would be to sit on the deck running the laptop batteries down then plugging it every few hours."

try another experiment - try to run the laptop with the power cord and REMOVE the battery - the laptop WILL run just fine and possibly save some wattage as it's in use and NOT charging the battery!

Tim said...

May I ask why you're not planning on charging more of these energy devices via solar?

Coffee Maker 600-650 Watt/10 minutes
Ryobi Battery Charger 30-40 Watt/2 hours
Phone Charger 1 Watt/2 hours
XM Radio 5 Watt/8 hours

I am fairly certain you can charge the radio, computer, and phone off flexible/portable solar panels and switch to a coffee press. [I'm uncertain about the battery charger - I don't think that would work with a solar panel.]

I apologize if this sounds nit picky - it's completely up to you how you use your energy, I am asking out of the urge to know if you feel the solar chargers don't work well? Or are too expensive versus the power you would achieve?

I'm very impressed by the fact that you went completely off grid. I've been reading this steadily to take hints for the future so I appreciate that you continue to share your findings/efforts. Thank you!

BigBear said...


You could use small solar chargers made for the various units but what happens when you don't have any sun for several days in a row. Plus when you store in batteries then convert to AC it really opens up what products you can use. In other word you can use any household product hopefully seeking out the most energy efficient.

I actually use a french press coffee maker in the winter when the water can be heated on the wood stove...I am having a cup right now. But in the summer it is wasteful to start a fire or use propane to heat the morning water so I will use an electric coffee maker at that time.

Tim said...

Thank you very much for answering my questions! Hopefully, in two years I'll be headed in your direction. [Going off-grid I mean.]