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Small Cabin Forum / Off-Grid Living / How much solar is needed.
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# Posted: 3 Oct 2010 16:29

We are getting to the point of planning the cabin out and know we want solar power there. I have read quite a bit here and elsewhere and cannot figure out how many watts I really need. I will only need to power a few lights a couple fans here and there and a radio. I have tools that are cordless or gas powered so I dont see a need for anything else.

So how much do I need watt wise and the like in a system to acomplish this?


# Posted: 4 Oct 2010 16:06

Unfortunately there just isn't enough here to help you, but here at least is the process.

make a list of what you want to run and how for how long for a typical day, then actually find the item and read how many watts it eats. Multiply the numbers together to get a Watt-hour value. If a device lists "amps", multiply that number by your system voltage (120 in the US) to get watts. You should have several lines like this:

Light (60w equivelent, CFL), 13W, 4 hour, 52Wh

Now to size your system. Add up the W column of all devices that you want on at the same time. Multiply by 1.2 to be safe, and that is the size of the inverter you need. Sum up the Wh column and this is the amount of energy you need.

For true off grid, you'll want to size a battery to last a week, and average out solar or wind on a daily basis. So, if I ended up with 1000 Wh per day, for full time living, I would want (W / Battery Voltage, 12V for example) 1000Wh * 7 (days per week occupied) / 12 = 583Ah Hr. That's 6 good Deep Cycle marine batteries.

For the source, let's do solar. Call the place where you want to get your panels and ask them what your "Peak Solar Hours" are in the winter (lowest number of the year). For my area in far north eastern illinois, I get 2.5 in the darkest days of winter, that means I need 1000Wh / 2.5 = 400 watts of solar panels. Again I would multiply by 1.2 for a bit of a safety margin and 500 watts of panels.

I know that is a lot, but I hope that helps.

# Posted: 5 Oct 2010 17:30

Thank yo very much, I have some math to do.

# Posted: 11 Oct 2010 12:03

I just installed a couple of these and plan to install more. Nice white light and low usage. :IT

let me know if the link dosen't work and I'll try again.

# Posted: 21 Oct 2010 14:58

There are many off grid solar calculators online. Some are better than others, IMO. I didn't like many so wrote a spreadsheet to do the basic calcs.

The spreadsheet can be found HERE. It is oriented to PV power, not wind or hydro.

Make a list of all the items; power used and then start filling in the fields. It does help to have a basic understanding of alternate power as it is a tool not an instructional course.

# Posted: 21 Oct 2010 16:29

Nice spreadsheet MtnDon. Thank you for putting this together!

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