solar power in fern forest

[HI_Someday]

Thanks kalakoa for doing the math! My figure was within 1 of yours. I would hope to lessen my electricity usage when I make it over there. It's hard not to run the ac here when it's triple digit temps...

[kalakoa]

It's hard not to run the ac here when it's triple digit temps...

I had a similar problem: summary on the power bill includes AC (summer) or forced-air heat (winter), this is why you use a Kill-A-Watt to directly measure the actual consumption of important appliances (fridge, computer, etc).

[HI_Someday]

quote:

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Originally posted by kalakoa

It's hard not to run the ac here when it's triple digit temps...

I had a similar problem: summary on the power bill includes AC (summer) or forced-air heat (winter), this is why you use a Kill-A-Watt to directly measure the actual consumption of important appliances (fridge, computer, etc).

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We have "smart meters" and I can log into the electric company web site to see my hourly use. I can see spikes when my whole house fan comes on from 11pm-1am and then again from 5-7am. The usage is very low after that until the heat becomes too much (around 4pm) and the ac kicks on. I like the idea of the kill-a-watt but I have pretty good ideas what is eating up most of the electricity. I also work from home so that contributes to higher use as well.

My gas bill in the summer is usually $10 or so while the elec bill is around $175-$200. In the winter that reverses... Such temp swings are not seen on the BI and may make it easier to manage your elec usage?

[kander]

If your going off-grid, you will need to consider a battery bank to hold the charge at night and supplement when a cloud comes over.

You really only want to discharge your batteries no more than 25% per discharge cycle. Doing more will make your batteries not last very long and when you lay out the cash for a 48V 1500Ah battery bank you want to make sure you get 15 years (at least) of use from them.

[HI_Someday]

Ideally I think I'd like to do grid tied solar - that is still have HELCO lines but also solar. From what I've read on this and other forums trying to get that setup approved and installed is a cluster**ck,

[MarkP]

I get by with 2,100 watts of panels (nine 240 watt nominal rated panels) and eight Costco golf cart batteries in series making a 48 volt 210 amp-hr bank. This amp-hr rating is for a nominal 20 amp discharge rate. It is desirable to match panels and battery bank such that the bank is charged at a rate between 5% and 13% of the rated amp-hrs, ignoring the mismatch in units. For example 5% of 210 is 10.5 amps and 13% is 27.3 amps. When I had only 3 panels in series in a single string putting out around 8 amps I was technically below the recommended minimum. Now with 9 panels in 3 strings I am up near the recommended maximum. Apparently you can go higher but it gets to be less efficient as you can only cram so much power into the batteries. Below the minimum rate of charge the batteries don't get stirred well enough which has longevity implications even if you make up for it by charging longer.

There is a school of thought that says going with a smaller battery bank and working it harder gives a better return on investment. Within reason if you cut the battery bank in half and discharge deeper you reduce the life span of the bank but not by half so there is a sweet spot in there if you can find it. Obviously discharging your bank by 90% daily will kill it pretty quick, but surprisingly many battery banks are killed with kindness, being feebly charged because they are not really drawn from.

When sizing solar systems one quickly progresses through the lower voltage battery banks. Almost no one should be using 12 volts. 24 volts is almost always a better choice for anyone using more than a few hundred watt inverter and frankly 48 volts seems like a no-brainer to me now that I went that way myself. When putting together the battery bank it is very preferable to buy larger batteries and keep them all in series rather than make parallel strings to gain amp-hours. As such there are a fixed number of permutations, like 2, 4, or 8 costco GC batteries or 2, 4, or 8 L-16 batteries. The L-16 batteries have the same footprint as the GC batteries but are twice as tall, have not quite twice the amp-hours and are somewhat more than twice the price. The benefit comes in that they are supposedly more durable and will last longer and that you can get nearly double the amp-yours without going to parallel strings. They have more room at the top for water, which seems like a trivial thing until you ruin a set of batteries by letting the level get too low. Things like that.

The bottom line is that the batteries are going to die sometime anyway and larger batteries cost more so you must sharpen your pencil to determine what works out for the best. Another alternative is adding more panels pointed east and west to start charging earlier and keep charging later reducing the demands on the battery bank. A tracker would do the same thing but panels are relatively less expensive these days so virtual tracking becomes an option. The point is that if the panels and controller charge hard in the morning and mid-day and go into float for the last few hours you could re-aim some of the panels without actually losing amp-hours into the batteries, allowing the panels to directly carry the misc. parasitic loads of the household and inverter earlier and later into the day, reducing both the actual amount that the batteries are discharged and the time they spend while discharged.

[kalakoa]

Compare/contrast the performance of lead-acid:

http://ironedison.com/nickel-iron-ni-fe-battery

[kaialoha]

WOW!!! MarkP’s post makes very practical sense, but I still have questions.

I’m building in Fern Forest and want to install a solar array that is functional without spending more money than is necessary or practical.

My hale on Maui gets by with just 2 panels and 2 batteries for lights and PC and I have a propane fridge so is not part of the solar equation though it would be nice to have the reliability of an electric fridge. The system is definitely minimal and I have to use the generator on occasion.

I appreciate MarkP’s post because it seemed to speak to me in language I (somewhat) understand.

So, MarkP, can I ask, with your 2,100 watts, what are you powering? Are you running a refrigerator or other such larger appliance? I assume the idea is to be selective when running anything so that the draw on the batteries is minimized and spread out over a variety of appliances, lights, PC’s, etc. Is that a reasonable assumption?

I think I understand the %’s for matching panels to batteries and please pardon me being dumb, BUT, what does the ‘210’ represent? Is that total amps of the 9x240 watt panels?

You mention 9 panels in 3 strings. Why not all 9 panels in a single string?

Assumption: It appears you’re using close to one battery per panel and I wonder if that’s coincidence or a good general rule of thumb or just simply happenstance.

Another assumption: I think what you’re saying about battery life as it relates to charging and discharging is that it makes more sense to use the batteries rather than let them sit with no draw on them BUT at the same time keeping the batteries from discharging TOO much. Fair assumption?

Anyway, GREAT comments and I hope to learn more as I go ~ ~ ~

Mahalo for everyone’s time and consideration ~ ~ ~

Aloha


Punanny

[MarkP]

quote:

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Originally posted by kaialoha

WOW!!! MarkP’s post makes very practical sense, but I still have questions.

I’m building in Fern Forest and want to install a solar array that is functional without spending more money than is necessary or practical.

My hale on Maui gets by with just 2 panels and 2 batteries for lights and PC and I have a propane fridge so is not part of the solar equation though it would be nice to have the reliability of an electric fridge. The system is definitely minimal and I have to use the generator on occasion.

I appreciate MarkP’s post because it seemed to speak to me in language I (somewhat) understand.

So, MarkP, can I ask, with your 2,100 watts, what are you powering? Are you running a refrigerator or other such larger appliance? I assume the idea is to be selective when running anything so that the draw on the batteries is minimized and spread out over a variety of appliances, lights, PC’s, etc. Is that a reasonable assumption?

I have lights, a fan, a dehumidifier, a washing machine, a microwave, a TV, and a 12 volt battery charger that charges the battery that runs my water pump. And a small refrigerator. If it is sunny the led on my charge controller starts flashing by around 10 am indicating that it is throttling juice to the batteries so I do laundry then. Rainy nights is when I am most troubled by humidity so I often run the generator when I turn on the dehumidifier but sometimes I run the dehumidifier off of the batteries alone. It depends on how sunny a day it was earlier and how lucky I feel about tomorrow. The rest of the appliances I run as needed with no special regard for anything. The 12 volt battery charger is on a timer to run during the sunny part of the day.

I think I understand the %’s for matching panels to batteries and please pardon me being dumb, BUT, what does the ‘210’ represent? Is that total amps of the 9x240 watt panels?

Each battery has a rating of 210 amp-hours when discharged at 20 amps. I think this is an industry standard for expressing amp-hours. Multiply this rating by .05 or .13 and you get 10.5 amp-hours or 27.3 amp-hours. Leave off the -hours and you get the amp ratings I quoted. This is a rule of thumb for estimating the proper charge rate for your battery bank. Since the same current goes through each battery in a series configuration, my battery bank has a rating of only the 210 amp-hours of an individual battery but the volts are added up to get a nominal bank voltage of 8 x 6 = 48 volts. If I had made two strings of four batteries each the bank would have 420 amp-hour rating at 24 volts. 840 amp-hours at 12 volts is also an option. I have a 2,700 watt 48 volt inverter. At peak load it will draw close to 60 amps. A 1,000 watt 12 volt inverter will draw close to 85 amps. That means lots bigger wire, switches, fuses, etc and correspondingly greater expense for those items all for way less power.

You mention 9 panels in 3 strings. Why not all 9 panels in a single string?

The panels are nominally 24 volt panels. They have an open circuit voltage of well over 30 volts and even when pumping out power the Vmp (max power) is still often over 30 volts. I must put at least two in series to get voltage high enough to overcome the voltage of the battery bank. Because I use an MPPT (Maximum Power Point Tracking) charge controller I can string several panels in series up to the input voltage limit of the controller. In my case this is around 150 volts so I have to be careful. I might be able to run 4 panels in series. Anyway I chose 3 panels in series. The MPPT DC to DC transformer technology takes the resulting 90 to 100 volts and makes more amps at 50 to 60 volts required to charge the batteries. The simpler PWM (Pulse Width Modulation) technology would simply chop the current as needed without making greater amps and turn the rest into heat. Imagine a roofer carrying shingles up a ladder. The ladder must be tall enough (voltage high enough). If too tall he must still carry the shingles to the top of the ladder then drop them from there (not really I know but work with me here). The MPPT technology is like having an automatically adjustable ladder. As long as you select one that can go high enough it will automatically adjust to deliver the proper height/voltage for any lesser height. On cloudy days or during marginal morning/evening times the panels stay productive longer. In contrast, panels/strings perfectly sized for a PWM controller on a sunny day at noon wouldn't have enough oomph at any other time and are therefor over-sized resulting in a 50 volts needed/60 volts delivered situation most of the time, analogous to carrying the shingles higher than the roof and dropping them.

Assumption: It appears you’re using close to one battery per panel and I wonder if that’s coincidence or a good general rule of thumb or just simply happenstance.

Coincidence.

Another assumption: I think what you’re saying about battery life as it relates to charging and discharging is that it makes more sense to use the batteries rather than let them sit with no draw on them BUT at the same time keeping the batteries from discharging TOO much. Fair assumption?

Exactly or as close to it as possible when describing the black magic that is maintaining such a battery bank.

Anyway, GREAT comments and I hope to learn more as I go ~ ~ ~

Mahalo for everyone’s time and consideration ~ ~ ~

Aloha


Punanny

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[MarkP]

Charge controllers are available that will take up to 600 volts input but they are much more expensive and only pay off for commercial installations.