A conundrum

Clyde · March 2013

Hi all,

New to the site and have read and read and have not found a situation like the one I have. I have an off grid cabin that until recently was only used in the winter. There is no electricity. Light is furnished with lamps and lanterns, heat by wood stove. Its a week end destination, no washer/dryer/dishwasher etc. and we go there to get away from the computers/TV's/telephones etc.

Now the problem. Due to a need to be closer to home I am going to be staying in it some in the summer. I work at night and therefore sleep during the day. It gets HOT here in the summer and I need air conditioning. I was going to get the local power company to run a line but that is going to be north of 5k. With my extremely limited understanding of solar energy I figure it would be an easy fix with some batteries, an inverter couple of panels and viola, instant AC! As stated above after much reading here and elsewhere I do not even know where to start but I do know NOT to start until I have a much better understanding of what it is that I am doing.

I have a window unit AC that is 110v, 750watts at 7amps. The cabin is roughly 350 sqft single room affair. Insulation is reasonable but will be increased first as this will help during the winter as well.

How large a battery bank do I need?
Will a cheapo 1K watt auto inverter work? (I already have one in the shop somewhere)
What kind of problems am I going to encounter trying to run the AC while the array is recharging the batteries?

I do have a generator that I can employ and if I need to get a different AC unit that would not be an issue either.

Thanks

Clyde

icarus · March 2013

Re: A conundrum

I have a very small window unit that I run on a Honda eu1000 very occasionally. The only way the A/C wil start is if I run the genny full bore,start theA/C on fan only, let it run up to speed, and then switch to low cool. Remember motor starting current is often ten times running current, even more for compressors!

My guess is you are going to need an iverter in the 2 kw size. As for the batteries and the PV required,you are going to have to estimate the run time over the course of a day. I would bet if you have say a 10 hour run time, at 700 running watts would be 7 kwh/day. For that you would need ~4 kw of PV, and a large battery bank, maybe 1000 AH at 24 vdc, or 500 at 48 vdc.

That 7kwh would be roughly ~150 ah at 48 vdc.

You can soon see, that the $5k front he power company begins to look cheap(er)! A 4 kw battery based PV system, might cost $4-8/watt, or $16-32,000. Two general rules of thumb. The first is, the grid is often your best solar resource, ie grid tie.
Te second is a simple planning equation for off grid is this: take the name plate rating of the PV, divide that number in half to account for all cumulative system loses, th en multiply that number by four to account for the average daily hours of GOOD sun you can expect, per day, over the course of the year. So, 4000/2=2000*4=8000wh/day. Your dilemma is interesting because as you get more sun, your need for power also rises, and vise versa.

So, I think you can see, calculate your loading, and all other hardware choices become clear(er)

A couple of other points,, a "cheapo" auto inverter is a really bad idea. First, it is very inefficient, and second, it is most likely a MSW inverter, and many motor loads don't like MSW and in fact begin to emit magic smoke when run on MSW for long. T

Second, there are other A/c options that MIGHT be more efficient, look into Mini split systems. Not cheap, but significantly more efficient per btu.

Finally, don't buy ANYTHING until you thoroughly understnd your loads, and size the hard ware for those loads. Avoid the most expensive pitfall of PV..."Ready, Fire, Aim!"

Tony

Welcom, and keep in touch,

Tony

mike95490 · March 2013

Re: A conundrum

Sometimes, the power company wants to see the house permit, before they will attach wires !!

Otherwise $5K for power, that's less than a generator/battery/inverter/solar will cost you. Unless you want to be grid independent, then spring for the inverter and buying batteries every 7 years.

Clyde · April 2013

Re: A conundrum

Ok so I found the mini split a/c that works at 300 watts on low. After looking at the numbers you gave me would this mean I could cut everything in half? I am really not sure where I would put 4kw of panels any ways. I am pretty sure I could easily get by with 8 hours of run time on AC. just for clairity:

8 hours at 300 watts = 2.4kwh/day. Run that with 500a/h 24volt battery bank. 2.4kwh would work out to 100ah at 24v(20% of total). Charge all that with a generator instead of a pv array because I really dont see the need as most of the electrical demand will come when the pvs are charging most. I realize I may change my mind once summer is over but could I not get the pv array and charge controller at that point?

And then I could have electric light in winter (wouldnt that be odd), and probably run a computer or other such things that I go there to get away from. OMG! I think you have just corrupted my cabin! j/k

whatcha think?

Clyde

icarus · April 2013

Re: A conundrum

I don't undstand. Are you saying, run the A\C on the genny alone?

On the other hand, you could probably do a a calc that allows hot run the A/C on apv an a much smaller battery only when the sun is out, such the the batteries never get discharged. I suspect y could therefore downsize the PV nd th battery considerbly if you have no need for non daylight power.

Tony

Susido · April 2013

Re: A conundrum

I'd consider $5K for grid power to be a bargain. I can't see any solar system capable of running air conditioning costing any less.

I was quoted $29K just to run grid power to my yard (1 kilometer distance) and several more thousand for a meter and certified installation.

Photowhit · April 2013

Re: A conundrum

Cheapest way is a genny and a smaller window unit and sectioning off a sleeping room in the cabin.

I'm in Missouri and we get heat with Sun, Just to sleep in A/C I built a small cabin 10x16 (9x15 inside) in the shade with very good insulation, 6" walls 8+" in floors. I could manage about 8 comfortable hours on a 24 volt system with a single set of golf cart batteries and about a 1000 watt array. Originally used a 1400watt hybrid inverter. 5300 btu A/C draws about 480 watts on energy saver. I could run it roughly 4-5 hours on a 220 amp hour battery bank... but I was sleeping at night.

I later added another 1000 watts of array to allow for more daytime use, and increased tha battery just before finding a good deal on another property.

If you are where you have good and regular sun in the summer, you just might be able to pull it off for near $5000 after taking a 30% tax credit. (so $6300) figuring 2000-2400 watt array at around $2000 delivered, a 1500 watt true sine inverter at $7-800(meanwell or Samlex, $1100 would get you a better Exeltech 2000watt), a midnight solar classic lite charge controller at $500, Small forklift battery @$2500 and DC disconnect and fuse/breaker box, wires, mounts, wood, etc @$1000-1500.

It would take some time to put together and a bit of research, a forklift battery would almost require you be around once a month to check on it, but might last you 15 years. 2 strings of L16's (8 batts) would cost you nearly as much and last 6-9 years.

With your peak use in the summer you have the advantage of having more sun, hence power to work with. With the Federal Tax credit (assuming your in the US and this cabin is yours and not rented) I think it's doable. I'll point you at some sources if you want to look at this.

Skippy · April 2013

Re: A conundrum

A friend of mine was giving me a hard time about the money I am spending for getting my kitchen off grid, so I told him :

panels and c.c = 2,000 $
inverter and batteries = 3,000 $
Telling the hydro company to kiss my butt = priceless.

I have a window a/c unit as well, but it's a non essential load.

Clyde · April 2013

Re: A conundrum

Ok, so here is the end story.

Power company after much berateing (sp) lowered price a little and agreed to spread it over 12 months. Well now I am going to have grid power and pay the "man". Not happy about the last part and the cabin will NEVER be the same. my son tells me welcome to the 21st century

Clyde

icarus · April 2013

Re: A conundrum

You are right, cabins with 24/7 power are never the same. The next stop, a TV set! I have held off for 15 years on that subject, but we do movies on the laptop.

T

techntrek · April 2013

Re: A conundrum

Since you said you would be staying in it "some [of the time]", I would still go with Photowit's suggestion. Get a good inverter-genset which will save you on gas, and a window A/C. That will save you thousands over the grid option up-front, and it would take years to catch up with the running costs. You might never catch up since the grid will cost you that $4000-5000 upfront, plus the monthly connection fee, plus actual use, forever.

westbranch · April 2013

Re: A conundrum

And with the 'extra' power so come the added loads, tv, Sat internet, sat phone etc....

boomer · April 2013

Re: A conundrum

I'm a newbie here myself (lurker, first-time poster). It does seem to me that this particular user might have been a candidate for the Kingtec dedicated solar air-conditioner product.

I noticed that the vendor's announcement got a lot of pushback in this forum, which I can understand. But for this particular application, where the off-grid user basically just wants air conditioning, a product like this might be appropriate. If I understand it, after adding the solar panels this could be done for under $4k.

Cariboocoot · April 2013

Re: A conundrum

And $4k will buy a lot of solar. Enough to run a conventional A/C unit.

boomer · April 2013

Re: A conundrum

Cariboocoot wrote: »

And $4k will buy a lot of solar. Enough to run a conventional A/C unit.

Not to be argumentative, and I really have no dog in this fight. When I read about the Kingtec product, I questioned the wisdom of dedicating all that solar hardware to air conditioning when it would go to waste during cooler months. But for this particular user's application, it seemed to make more sense because he only wants solar for air conditioning in the summer.

I was going by the estimate of icarus above, which indicated that to handle the peak loads, it might be a lot more expensive than $4k with a general-purpose solar installation and an AC-powered air conditioner.

Photowhit · April 2013

Re: A conundrum

boomer wrote: »

I noticed that the vendor's announcement got a lot of pushback in this forum, which I can understand. But for this particular application, where the off-grid user basically just wants air conditioning, a product like this might be appropriate. If I understand it, after adding the solar panels this could be done for under $4k.

The KingTech has other limitations, I does have a minimal battery bank but only enough for about an hour run time, so in areas where heat is steady 24 hours a day, it would be a difficult sell, for high desert where if cools off quickly at night (or so I hear) or an office that would only be occupied during the day if might have so benefits..

inetdog · April 2013

Re: A conundrum

Photowhit wrote: »

The KingTech has other limitations, I does have a minimal battery bank but only enough for about an hour run time, so in areas where heat is steady 24 hours a day, it would be a difficult sell, for high desert where if cools off quickly at night (or so I hear) or an office that would only be occupied during the day if might have so benefits..

You can add optional additional batteries, up to four times the original capacity, I believe. Beyond that whatever a separate solar charger can support.

Cariboocoot · April 2013

Re: A conundrum

inetdog wrote: »

You can add optional additional batteries, up to four times the original capacity, I believe. Beyond that whatever a separate solar charger can support.

Hmm. What does that sound like?

boomer · April 2013

Re: A conundrum

Photowhit wrote: »

The KingTech has other limitations, I does have a minimal battery bank but only enough for about an hour run time, so in areas where heat is steady 24 hours a day, it would be a difficult sell, for high desert where if cools off quickly at night (or so I hear) or an office that would only be occupied during the day if might have so benefits..

Yes, I noticed that. I think that a prudent setup would include the addition of the optional external battery expansion (dunno the price for that) and probably upsized PV panels to feed it. So overall, the KingTec products cannot be called a low-priced solution -- especially since that capital investment might lie fallow for half the year in a temperate climate. (Of course, nothing stops him from adding on something like a general-purpose inverter or DC appliances to make use the power during the winter. But I understood that he wants to keep the cabin rustic in that season.)

In the face of opinion from several experienced posters above to the effect that general-purpose solar-powered A/C in this price range is simply not feasible at all, I thought the product merits consideration for this user's situation. On the one hand, he can pay the utility to build out the grid, and the ongoing bills to run it. I suspect that would increase the value of his resort property, which is a consideration. But if he really likes the feeling of independence in this retreat, maybe he would prefer the minimal off-grid upgrade instead.

inetdog · May 2013

Re: A conundrum

Cariboocoot wrote: »

Hmm. What does that sound like?

"Quack?"
...........

Photowhit · May 2013

Re: A conundrum

boomer wrote: »

Yes, I noticed that. I think that a prudent setup would include the addition of the optional external battery expansion (dunno the price for that) and probably upsized PV panels to feed it. So overall, the KingTec products cannot be called a low-priced solution -- especially since that capital investment might lie fallow for half the year in a temperate climate. (Of course, nothing stops him from adding on something like a general-purpose inverter or DC appliances to make use the power during the winter. But I understood that he wants to keep the cabin rustic in that season.)

In the face of opinion from several experienced posters above to the effect that general-purpose solar-powered A/C in this price range is simply not feasible at all, I thought the product merits consideration for this user's situation. On the one hand, he can pay the utility to build out the grid, and the ongoing bills to run it. I suspect that would increase the value of his resort property, which is a consideration. But if he really likes the feeling of independence in this retreat, maybe he would prefer the minimal off-grid upgrade instead.

Once you start adding things, I think my proposed system in the 7th post in the thread at $5000 would be a better solution offering more usage and uses. If you just wanted a cheaper system you could skip the forklift battery and go with 8 golf cart batteries and get near or under the $4000 mark all in, though likely to last 3-5 years rather than close to 15 before replacing the batteries.

The OP is on the grid now, so this info would be for others checking this out, I'm not sure if the stand alone AC includes or qualifies for the federal tax credit.

boomer · May 2013

Re: A conundrum

Photowhit wrote: »

Once you start adding things, I think my proposed system in the 7th post in the thread at $5000 would be a better solution offering more usage and uses. If you just wanted a cheaper system you could skip the forklift battery and go with 8 golf cart batteries and get near or under the $4000 mark all in, though likely to last 3-5 years rather than close to 15 before replacing the batteries.

Just to keep it apples to apples, your system is really a $6300 system. The Kingtec folks says their system is eligible for the tax credit, too. (The IRS gets the last word on that one.)

And your system yielded 5300 BTUs of cooling for 4-5 hours. Kingtec claims 16000 BTUs for 9 hours, although I think they are lowballing on the stock battery of the base unit. Their better performance, presumably, is based on the efficiency of the DC air conditioner.

Cariboocoot · May 2013

Re: A conundrum

boomer wrote: »

Their better performance, presumably, is based on the efficiency of the DC air conditioner.

The presumption that the DC unit is more efficient than an AC unit is not necessarily so. It can be, but by the same token one AC unit can be more efficient than another (mini-splits vs. window rattlers, for example).

We have tested a few different A/C units around here. The results aren't as black-and-white as any of us would like. For instance one portable 11,000 btu unit I tried used only a bit more power than another 5,000 btu unit!

If only it were simple. I could sit around all day watching movies!

Photowhit · May 2013

Re: A conundrum

boomer wrote: »

And your system yielded 5300 BTUs of cooling for 4-5 hours. Kingtec claims 16000 BTUs for 9 hours, although I think they are lowballing on the stock battery of the base unit. Their better performance, presumably, is based on the efficiency of the DC air conditioner.

I don't think you read it correctly or perhaps you didn't understand, that rate was with 4 golf cart batteries, with a fork lift battery I think you could run around the clock on thermostat on my old place (built in shade, thick insulated walls) ...but that is for Missouri where our heat comes with sun...

FWIW even 4-5 hours when you need it would be better than 1 hour when you need it if you need it for sleeping at night, all things relative to the environment your situation is in...

A conundrum

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