Feedback on new system

vince

Please review and comment on my planned solar system. I think that I have done my homework but want to make sure I did not overlook something important as I'm still learning all the different variables to consider.
I am building a Tiny House on wheels and want a solar system to meet my electric needs of 1.5 KwH/day on average with optional loads on sunny days. I was planning to get the following system description and already have the solar panels:
3 Sunpower 435 W panels, parallel connected to a combiner box (Midnite) with 15A circuit breakers for each panel, Midnite solar Kid charge controller 30A, and 4 Trojan batteries T1275 (150 AH) in series for a 48 V system. Will down convert to two 12v. 10A circuits to power LED lights, pumps and a 24 v circuit for solar fridge (Grape Solar), each circuit protected by a breaker. Will use Cotek 1500 w inverter for optional 120 AC loads such as split level air conditioning, toaster, microwave, etc. Location is primary KY but as it is mobile, it needs to be adaptable to broad range of conditions.
Your feedback is appreciated. Thanks

Cariboocoot

Re: Feedback on new system

Welcome to the forum Vince.

Well it will work but ... I wonder about some of the choices made.
For example, why a 48 Volt system for 1.5 kW hours of power? Why the T1275 batteries? You can get more power for less money in other brands and the Trojans are sometimes problematic in charging (demand higher Voltage).

I don't think I've ever seen a 1500 Watt 48 Volt inverter. Nor would I be keen on using a DC to DC converter to run 12 VDC lights from a 48 Volt battery bank and the same for a 24 Volt refrigerator.

Wanting to run A/C, toaster, and microwave is a whole 'nother level of power. The toaster and microwave will have high demand for short time, whereas the A/C (even a mini split) will have a high Watt hour demand. Be sure you examine what the power needs of these will be before you finalize the system plan. It may be a good idea to go with the large capacity inverter right from the start.

As for those 435 Watt panels ... I can't imagine trying to handle something that big. I hope they are at a really good price per Watt (like <$1) because two people would have a job to put something that big in place.

Not all that nice a review I'm afraid, but I calls 'em as I sees 'em and I've had to straighten out a lot of systems 'after the fact' and it's not so easy to do then.

vince

Re: Feedback on new system

I value your response. My thinking was that the higher voltage would allow me to use the Kid and the T1275 batteries ($165 each) would give me enough capacity for 2-3 days and not be too heavy. I saw a Cotek sp1500 at a reasonable price and the 48 V was the most efficient version of the different inputs. As I indicated, the high wattage items are optional when the sun shines and the batteries have been charged and I can work with the low setting on the mini-split as the house has excellent insulation. The dc-dc converters I have not really researched, what would be the concern other then some energy loss? Installing the panels on top of a Tiny house is a one time challenge that seems very manageable and they were at a good price and allowed me the highest production on a small roof.

inetdog

Re: Feedback on new system

vince wrote: »

I value your response. My thinking was that the higher voltage would allow me to use the Kid and the T1275 batteries ($165 each) would give me enough capacity for 2-3 days and not be too heavy. I saw a Cotek sp1500 at a reasonable price and the 48 V was the most efficient version of the different inputs. As I indicated, the high wattage items are optional when the sun shines and the batteries have been charged and I can work with the low setting on the mini-split as the house has excellent insulation. The dc-dc converters I have not really researched, what would be the concern other then some energy loss? Installing the panels on top of a Tiny house is a one time challenge that seems very manageable and they were at a good price and allowed me the highest production on a small roof.

Get it done quick. In CA the required roof edge and ridge setbacks (3' for the most part) will not leave any space on the roof of a Tiny House. :-)

Photowhit

Re: Feedback on new system

First and foremost... You will find this difficult! once you have your array mounted on the roof, you simply won't find you can park it with the correct angle to the sun and spots with out shade very often, I suspect you will become frustrated with looking every time you want to move. When I purchased land for a solar cabin in a camping community. of the 1500 lots I figured there were a couple dozen that would work pretty well. This is a wooded area and the roads run along the ridges which are primarily north/south but I think you will find a lot of issues with this... Remember in mounting that if you can make the mounts adjustable, and if not mount within 10 degrees of the latitude in which you expect to spend most of your time.

I'm curious what you intend for loads in the 1.5 kwh daily demand? Did you include the 10-15% loss in the inverter? Your battery bank should be about right for a daily 1.5 kwh load with a days additional capacity for a couple days of over cast skies, if the loads are correctly assessed. It may be that you can use the A/C some, but I wouldn't count on much time. One of the problems is your fixed mount. Your fixed mount will require you to park in the sun.

In the past I built a cabin (10x16x13tall) with the intent of air conditioning it with a small system. I built the cabin in the shade, with the array in the sun. The cabin had 6 inch thick insulated walls and tall ceilings. I found I could run about 4-5 hours of A/C on thermostat (an energy star window shaker) which would cool down the thermal mass and allow for a good nights sleep. The high ceilings aided in this allowing for the heat to rise. The sleeping loft was too warm to sleep comfortably in so I moved down stairs. Between the A/C and the fridge (my other major energy draw) I would run near 2kwhs over night and it would take most of the day(with the fridge still running) to recharge the battery bank (220 Ah 24 v) during which time I would want to be somewhere else by 11-12. I slowly added 600 watts of solar panels so I could run the A/C some during the day, making life much more bearable.

You have some advantages, I suspect the fridge uses less energy than my 4cuft fridge which I guesstimated at about 1.2 kwh during the summer with the hotter temps. Maybe .7 kwh in the cooler temps. The window shaker ran about 440 watts running in eco mode SEER of 11, I think, I believe the duty cycle was something like 60-80% during the first hour and perhaps 40% after 4 hours. You'll have a higher seer but most 120v mini splits don't have the very high ratings. and you'll have the disadvantage of having to park in the sun, and I would guess a less insulation.

vince

Re: Feedback on new system

My daily demand involves led lights, circulation pumps, on-demand water pump, and a solar fridge. Comparing a solar fridge versus an RV fridge (atwood helium) the lp flexibility first seemed appealing but the electric usage was 4x the usage of the solar fridge, so I figured to stay with the Grape Solar or Sundanzer. My walls have r=21 and roof r=34, with double pane energy efficient windows and french doors. The mini-split is rated at seer of 23. The panels will be mounted within the 10 degrees lattitude during summer, although the winter may require an adjustment. My floorplan in only 120 sq ft and 900 cubic sq ft space.

vince

Re: Feedback on new system

I have thought more about the battery bank and begin to favor a 36 volt bank of Trojan 6 -6v 225 AH batteries over a 48 v, 4-12v 150 AH batteries because of a better charge and discharge compatibility with the kid controller. The bank would be a little heavier in weight but the batteries offer a 10% higher capacity for about the same price. The panels (3x 435W) come close to the max capacity of the CC but are still within range. Is my thinking correct?

Cariboocoot

Re: Feedback on new system

vince wrote: »

I have thought more about the battery bank and begin to favor a 36 volt bank of Trojan 6 -6v 225 AH batteries over a 48 v, 4-12v 150 AH batteries because of a better charge and discharge compatibility with the kid controller. The bank would be a little heavier in weight but the batteries offer a 10% higher capacity for about the same price. The panels (3x 435W) come close to the max capacity of the CC but are still within range. Is my thinking correct?

No you do not want to use a 36 Volt system. It is "non-standard" in the RE world: you will have trouble finding an inverter that operates at that Voltage.

435 Watts * 3 is 1305 Watts. On 24 Volts that's 42-ish Amps so the Kid would be 'out' there. On 48 Volts it's about 21 Amps and would work fine on a Kid.

What you probably need to adjust is your battery selection.

vince

Re: Feedback on new system

It is important for me to keep the weight down as I use it for a mobile tiny house. To get a bank in the 200-250 AH range, the 36 v. bank is 130 lbs. lighter compared to the 48 v bank with the same batteries, unless I go back to the 4x 12v batteries (T1275) but that would only be 150 AH. DC-DC converters for golf carts usually include to 36v option and inverters can also be ordered as 36V if I were to go that route. Do I still need a breaker or fuse if the dc-dc converter is protected against short or overload?

Cariboocoot wrote: »

No you do not want to use a 36 Volt system. It is "non-standard" in the RE world: you will have trouble finding an inverter that operates at that Voltage.

435 Watts * 3 is 1305 Watts. On 24 Volts that's 42-ish Amps so the Kid would be 'out' there. On 48 Volts it's about 21 Amps and would work fine on a Kid.

What you probably need to adjust is your battery selection.

AuricTech

Re: Feedback on new system

I would focus on Watt-hours, rather than Amp-hours. The 36V/225AH battery bank you're contemplating would give you a battery bank capacity of 8100 Watt-hours. The 48V/150AH battery bank you've also considered would give you a battery bank capacity of 7200 Watt-hours, which would be 88.9% of the capacity of the 36V bank. If you focus on Amp-hours, the difference appears to be much greater: the 48V bank looks as though it only has 66.7% of the capacity of the 36V bank.

ETA: Meanwhile, if you can manage the weight, a 48V/225AH battery bank would give you a battery bank capacity of 10800 Watt-hours, which is 133% of the 36V bank.

ETA2: Another 48V option would be to use four of these Crown 12V/185AH batteries. They weigh 107 pounds each, for a total of 428 pounds, and would give you a total battery bank capacity of 8880 Watt-hours. Your proposed 36V/225AH battery bank would weigh either 402 pounds or 372 pounds, depending on whether you went with the Trojan RE-series batteries (heavier) or the standard Trojan T105 batteries.

vince

Re: Feedback on new system

As Cariboocoot pointed out, I'll have difficulties finding an inverter for 36v 1500w. With shipping, the Crown batteries get expensive unless I can source them locally compared to the T1275 that I can get locally fr a decent price ($165).

zoneblue

Re: Feedback on new system

1200Wp is kind of in between land alright. Its too much for a kid at 24v, and on the light side for a classic at same. However 24v would generally be regarded as the best system voltage for your needs for lots of reasons. However as far as storage goes, if this is ever a use case for lifepo4, i dont know what is. Weight. Size, low Ri, high C rates.

Lets see. 42 odd amps tops at 24v. Tricky, as PW said possible shading, AOI issues, but yeah, my view is it would run a single kid just a bit too hard.

3 panels would also normally be considered about as flexible as a brick. One string or one string. But those panels of yours would be those 120 cell jobs i guess, vmp 70-75v. Those are some unusal panels alright. I can see the attraction of high efficiency, high vmp for the cell count. But, one thing to consider is redundacy and damage. If you get a stone spat at you on the road and lose a panel, its a large chunk off line.

Well with those things one string is out. 3 strings gets you near the low end for 48V but doable enough, and plenty for 24V.Kind of a lot of cells if you go lithium. I know s

Small roofs are tricky. After hours on autocad trying every permutation of panel sizes i eventually settled on 4 80W panels that best fit the shape and curve of our RV roof but that was a while back, and a single one of these new panels would now probably fit much better with less mounting hardware and fuss.

I guess on balance for those panels 48V does seem to be the way. 48 to 12 converters should be workable for small stuff. Stick with a decent inverter, youll never regret it.