question about solar power!

[eeerrriiinnn]

thanks for the replies! they are super helpful. i found this really cool blog post Our Simple DIY Home Solar Power System | Eartheasy Blog

this guy uses that efficient refrigerator someone mentioned and apparently only uses one solar panel and two 6 volt golf cart batteries, all for less than $1,000….! pretty good.

[hierony]

1200 watts doesn't sound too bad. From the maps, it looks like Cali gets 2.5-3.75 hrs good sun during the winter. This'd give 1200 W*2.5 hr=3000 Whr. For the battery bank, you'd need 3000 Whr/12 V=250 Ahr. But if you want your batteries to last longer you need to only discharge them so far, maybe 20-50%. So for 40% depth-of-discharge & 250 Ahr usable, that'd be 250 Ahr/.4= 625 Ahr. That'd be ~5.5 of Jafo's battery (a very common one), or ~$660. That'd likely be half the solar panels' cost. The rest would be charge controllers, inverters, mounting, wiring, and professional installation.

20 bad days sounds like pretty good weather to me! You'd probably be fine as long as they aren't all in week-long stretches. For sizing, just imagine you don't get electricity out of your solar panels for one day, so you'll have had to have made it and stored it the previous day--just multiply your panels & batteries by the number of bad days in a row. Your system size calculation may have already taken this into account though. Talking to a professional solar designer familiar with your area would give you much more accurate numbers.

There are two ways to save on solar. One is on conversions. At best, each conversion is ~90% efficient. So solar panel-battery-electricity (2 conversions) gives you 81% usable energy (unavoidable). If you then need an inverter (Dc to Ac converter), you loose some more. Plus the inverter takes power just being on (check spec sheets). So using straight 12 V or 24 V saves you some efficiency, plus you don't have to buy an inverter. However, if you have anything above 180/240 watts (15/20 amps) for a 12 V system, you pay for thicker copper wiring instead... RV and boating appliances often run off 12/24V dc though.

The other way to save is to use less electricity. LED lights do really well there. If you can replace a TV with your laptop, that'd help things a bit--and save space plus encourage snuggling :P Otherwise, get the smallest TV acceptable & make it LCD or plasma (compare wattages).

An important detail is that your peak refridgeration needs will be during peak solar generation (ie, the hot summer months). Like Jafo said, you'd likely be hard-pressed to use all the power generated then. During the winter, you could put the fridge/feezer outside where it'll have to do less work (or none!), saving power. You also won't be

heating

your fridge then.

Best of luck!

[hierony]

I like that EarthEasy setup--they did a really good job minimizing their power consumption and keeping things simple. Those Sundazer fridges/freezers are expensive though!

[djspn]

Ja,$1,200 or so.
But at 17 Amp-hrs/day at 12V, 32°C (90F), sure minimizes expense for panels and batteries.

[hierony]

That works out to about a 4-8 watt average over a day (depends on fridge vs freezer, size). Looks like most fridges/freezers come out to 40 watts or more average power. Of course, that's for large upright fridge/freezer combos.

The only other way I've seen such super-efficient fridge claims is to convert a chest freezer to a fridge by swapping the thermostat. Supposedly gets 4 watts average (runs full power 2 minutes every hour). And new/used chest freezers can be found for $100-200

[djspn]

Here's a link to a write up on a freezer fridge conversion.
Downside for off grid is the need to run it on AC and the related conversion loss, and perhaps the need for a bigger inverter.
I'd do some research comparing noise levels.

[hierony]

The public library in my town has an Energy Efficiency Kit that can be checked out--it consists of a little info pamphlet, a blank table, and a Kill-o-Watt meter. The meter measures the power (watts) and total energy (kWh or KiloWatt*hours) of anything plugged into it. So I measured the power used by a bunch of stuff around my apartment. Here are some potentially relevant ones:

18.5" LCD monitor: 10 watts
Laptop, charging: 12 watts
Tiny Fan: 15 watts --running constantly to vent humidity/bathroom
Small Fan: 39-52 watts --for cooling/moving some air
Spice/coffee grinder: 140 watts
Halogen Lamp: 260 watts
chest freezer: 80-100 watts --only while running; intermittent; 5 cu ft, 2003
Upright fridge: 100 watts --only while running; intermittent; 2003 upright fridge/freezer

Without the lamps, small fan, or fridge, I would probably size an inverter (DC-AC converter) for 300-400 watts for a little extra (lighting, alarm clock, misc accessories) and to handle the freezer compressor startup surge and be just fine.

I'd need to check the freezer daily Watt*hrs to get a good idea of how often it actually runs (1 or 5 or 15 or 30 min every hour?) for the particular temp setting and ambient temp, but I might guess a 100 watt panel (with a good 3 hours direct sun daily) could be sufficient for just the freezer. But it might be on the edge without much reserve capacity during the winter. And then there's the vent fan, lighting, the laptop/monitor, any other accessories...

[Jafo]

I am wondering how much your peak is for the startup of your freezer. I have over 200 watts of panels at my camp, and it is more than I need in the summer, just barely enough in the winter. I am not running that much. I use LED lights (see my other posts) and I charge a laptop and run some small HAM equipment. I also have a cell phone extender. I have a fan, but I don't use it often. A small radio and that's about it.

If I were you, I would get an 800 watt inverter than can peak to at least a 1,000. I have two at my camp. A small 800 watt one, and a 2,000 watt one for doing heavy stuff. The one reason why you want to oversize is, many of them come equipped with a fan that will turn on after X amount of power starts being drawn. The higher the rating, the more power you can use before the fan comes on, though some inverters, the fan stays on all the time.

In a small yurt, that fan noise can be annoying.

[hierony]

The ID plate on the freezer says it draws 5 amps, so that'd make for about 600 watts; I'd need better tools to give a more definitive answer. Some sheets suggest it'll be running at 80-100 watts anywhere from 25% to 45% of the time, temp & setting dependent.

But with startup surges, it might last anywhere from <1s to 15 seconds or so. Most inverters go by their continuous rating but usually have a 'peak' rating double that. A well-designed inverter will probably have a 1s rating, a 5s rating, and a 30s peak rating (some variability on those intervals) listed in the specs. What you're fighting there is heat buildup in the transistors--the quality of the transistors, the quality of the heat sink, ambient temp, the design safety factor all factor in there. Too much current->too much heat and you let the magic smoke out... But high current for a second or two will usually be alright as long as corners weren't cut in the making.

The short of it--get a good quality inverter from a reputable maker, not the cheap $20 knock-offs that won't meet their own specifications, if they even give you specifications. You'll also get better quality electricity (better for motors, better for electronics). Get an oscilloscope and look at the output of your inverters vs your house mains and you'll see what I mean.

[Bob Rowlands]

My dad was in radio biz from the gitgo. He had a ham set up. dah-di dah-dit --dah- dah dit- dah. That was 55 years ago and I still remember CQ. lol