Backwoods Solar offers free custom design and sells custom made systems. You might want to check them out: www.backwoodssolar.com 208-263-4290 From my contact with them I think they're dedicated, reliable folks. Their web site points out there's a 30% federal tax credit for solar. You need to research how many sunny days you average there. Even without sun you get some solar gain, but this is research and math you might want to do now.

160 watt-hours per day would be 6 hours sun, which is probably about right for here, in summer. (Based on maps in a backwoods solar catalog I have.) Half it for winter.

600/160 = 3.75 days to charge the battery. 7 days in winter.

Is it okay to shop around and find the cheapest 30 watt solar panel, or are cheaper solar panels going to burn out quickly?

It has to be cheap somewhere. It could be in efficiency, longevity, build quality or all of them if you are unlucky. You know; "good, fast, cheap: pick two". I'd do some research, visit solar panel user fora etc. Check what guarantee the manufacturer offers. Take into account that you want to leave them outside year round. So moisture proofing is important, as are proper connectors.

Would it be smarter to do more research and buy the battery, inverter, and charge controller separately?

That's what I would do, I think. But then I'm a technogeek. I did a solar power project for my B.Sc. 17 years ago; that was good fun. And I've never lost my interest in solar power, so I try to keep current in the technology. The panels have improved but the fundamentals haven't changed. Feel free to ask if you have questions.

I would advise you to go and talk to someone who builds and sells these systems, and not buy the parts at wal-mart. They can probably give you a good advice on what & how much you need given your power requirements. This will cost you more, but you can profit from their knowledge and experience. It doesn't hurt to ask for a quote.

If you go down the buy-it-yourself route, keep in mind that next to a charge controller you'll also need a maximum power point tracker. (The panel needs to be kept at a certain voltage to generate its maximum power!) But that might be built into the controller.

If we bought an external charge controller and a 50 watt solar panel, would the larger panel charge our power pack faster?

It would depend on the type of battery. Some types can cope with rapid charging better than others. Keep in mind though that a 50 Watt panel means 50 Watt peak; when irradiated with a full 1000 W/m2 of sunlight perpendicular to the panel. This will probably not occur very often or long, depending on at what angle and orientation you fix the panels, and how sunny it is.

One website notes that this system would give us around 160 watt-hours per day. I'm not actually sure where people came up with that figure --- does it make sense?

To get 160 Watt-hours a day, your 25 Watt panel would have to work at peak power for 6 hours/day. That seems wildly optimistic to me. How much you really get depends on many factors;

• Direction and inclination of the solar panels.
• The season.
• Your local solar irradiation.
• How clean the panels are (you want to put them somewhere where you can clean them!).
• Are the panels completely in the sunlight (they don't like partial shade, AFAIK)
• Efficiency of the panel in less than full sunlight conditions.
• Efficiency of the maximum power point tracker and charger electronics.
• The weather
• etc.

Basically, you'd have to ask around what other people's experiences are. I don't see manufacturers providing much relevant data.

Does [160 Watt-hours] mean that I could run a single 25 watt laptop for 6 hours?

Yes.

The Duracell battery pack is 12 V, and can hold 28 Amp. hours. So that's 12x28 = 336 Wh. Considering your total load of 25+25+23+13 = 86 Watts, a 336 Wh battery would last (336/86=3.9) just under four hours. You'll need a way bigger battery pack. Especially in winter when irradiation is low, you might need several days to recharge the batteries.

Keep in mind that a battery is a replacable item! Curent Li-ion batteries are good for about a thousand charge/discharge cycles. So you'd have to replace them in a couple of years.

Daddy --- Thanks for hunting down that company for me! I'll have to take a look at them, although I suspect they're like all the other companies in that they sell multiple thousand dollar systems and nothing for a basic, cheap backup. Good reminder about the tax credit --- I knew and forgot. I should factor that into my cost and maybe spend a bit more.

Joey --- Thank you! That makes perfect sense --- I was wondering where that random number came from...

Roland --- I wrote this post mostly to get feedback from you, so I'm glad you chimed in! I need to digest your comment before I say anything more, though, so just THANKS!

Hey Anna,

Please let me know what you guys decide, I would love to know what you decide on and how it works for you.

Please post your decision

Thanks.

So Roland confirms something I thought but was not 100% sure about (converting amps and volts to watts always trips me up mentally): The battery in that "600 watt" power pack does not provide 600 watt hours of output.

In my 2006 Backwoods Solar catalog, prices for a full system start at $800 (for a 65 watt panel, a couple batteries and controller) and go way up.

There's some very clear advice in there for sizing a system:

• Take the total watt-hours you need per day, and divide by 6 (for 6 hours sunlight) to get the number of rated watts of solar panels you will need. Let's say you want 200 watts per day; you need 33.3 watts of panel.
• Add 50% more watts to deal with "solar panel derating" (ie, panels only produce their rated numbers in ideal conditions) and power conversion loss. In your example, we're now at 50 watts.
• Now add another 50% to 100% to the total to make up for cloudy and winter days. For the SW Virginia area (as well as most of the South), they suggest adding 33%. There is a troubling nearby patch over most of Tennessee (excluding the NE tip) where they suggest adding 60%. So I've been using 50%. So in your example, somewhere between 61 and 66 watts.

The numbers I've heard for solar panel degradation over time is they might lose 10% over 10 to 20 years. I'd like some more detail in this area as I will soon be using a decade old 256 watt PV array. Battery degradation is much more of a concern though.

Joey -- The 600 W rating is the maximum power output of the battery. But with the capacity of 12 V x 28 Ah = 336 Wh, pulling 600 W from the battery would deplete it in about half an hour!

Modern panels shouldn't lose more than 15% of their power of their 25 year lifespan. Batteries will indeed be a bigger worry (and costly. I figure you'll be replacing them every three to five years).

Anna -- According to a local supplier in the Netherlands, a 100 Wp (Watt peak power) panel would cost around € 300 and generate around 80kWh per year. Of course the numbers for your neck of the woods will be different (better, I suspect because you're a lot closer to the equator than we are!)

You should also consider using an inverter to generate AC and then deliver power back to the net. This would reduce your electricity bill and would use your panels when you aren't. I don't know what your local regulations are and what price you'd get for your generated energy, but it might be worthwhile to find out.