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.