Help please....

AndrewC

OK...just starting to learn about Solar because I plan on converting an old cargo trailer to an off grid "tiny home"...I have enough room on top for 7 Suniva 350 Watt panels...my question is should I use a 24 volt system or a 12 volt system?  My local batteries plus has 100 ah 12 V Deep Cycle AGM batteries...how many do I need???

westbranch

You are right on the line between a 12 and 24 volt decision..  First you should define your loads and then decide , however as you are going to make it a full time house I would really consider a 24V system.  Reason: loads grow  over time..and the more power you have the more they grow.  Wiring is cheaper as you can use the same wire as a 12 volt system and have less (by 1/2) amps flowing... also IIRC with 2400+Watts you can use smaller charge controllers than needed for the amps @ 12Volts..  like a Kid vs a Classic CC.
Batteries: consider Golf cart  6V or 2 volt cells or L 16 style cells

BB.

DO NOT BUY ANY HARDWARE JUST YET.

Do some simple paper designs (and costing) of the various options and the loads you want to run (as Westbranch said above).

It is very easy to purchase incompatible gear that you cannot use and just need to get rid of later--Or you make a "Frankenstien's Monster" design because you already have the stuff.

Read up on batteries--They are the heart of your system. And will, over time, be the thing that costs you the most (replace battery banks every 3-8 years of so with typical lead acid batteries) and spend a lot of time making sure they are happy.

For your loads, the fewest loads, the most efficient loads, will be your best bet. It almost always cost more to generate power than to conserve power.

Solar electric power costs (most people, roughly) around $1-$2+ per kWH for off grid power (all costs rolled in). Your electric power at your home costs about 1/10 that amount.

An average home that uses ~500 kWH per month and has a $50 to $100 per month power bill--The same thing off grid will cost around $500 to $1,000 per month, and you have to pay 5-10 years of "bills" up front (and more if you accidentally "murder" your first battery bank or two--Not uncommon).

For an off grid power system--If you can get full time off grid for 1kWH per day (just lights, small water pump, run a few electronics) for a summer cabin, or ~3.3 kWH per day (lights, refrigerator, well pump, washing machine, TV+Laptop) for full time off grid--That is something to aim for.

If you live in a region that does not have a lot of sun--Solar Electric power is even more costly (more solar panels, more genset fuel in winter)... Say Parsippany, NJ, fixed array tilted to 49 degrees from vertical:

http://www.solarelectricityhandbook.com/solar-irradiance.html

Parsippany
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 49° from vertical angle:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
3.23	3.89	4.45	4.68	4.83	4.98
Jul	Aug	Sep	Oct	Nov	Dec
4.99	4.88	4.73	4.31	3.30	2.94

Toss the bottom three months and use February as the "break even" month:

• 7 * 310 Watt panels * 0.52 off grid system eff * 3.89 hours per day ave sun Feb = 4,389.WH per day (4.4 kWH per day)

That is not bad... If you do a 10%/13% rate of charge (10% to 13% rate of charge recommended for full time living with off grid power system):

• 7 * 310 Watts * 0.77 panel+controller deratings * 1/0.10 rate of charge = 696 AH @ 24 volt battery bank
  
• 7 * 310 Watts * 0.77 panel+controller deratings * 1/0.13 rate of charge = 536 AH @ 24 volt battery bank (typo: should be 0.13, not 0.10 in equation)
  

12 volt bank not recommended (too large for "average" off grid components--Heavy cables, larger/extra charge controllers/not great for supplying >1,200-1,800 Watt loads at 12 volt AC inverter, etc.).

Please note--More than likely, if you choose a 24 volt battery bank, you will have to get panels in groups of 2 or 3 (i.e., 2, 4, 6.. or 3, 9, 12... to "match up" with a typical MPPT type solar charge controller).

AC inverter wise--using our rules of thumbs (200 AH per 1kW of AC inverter at 24 volts on flooded cell lead acid batteries), you can support a maximum (reliably) of:

• 696 AH / 200 AH per 1,000 Watt inverter = 3,480 Watt max inverter (minimum "makes sense" inverter ~ 1,740 Watt inverter)
  
• 536 AH / 200 AH per 1,000 Watt inverter = 2,680 Watt max inverter (minimum "makes sense" inverter ~ 1,340 Watt inverter)
  

Note, rules of thumbs designs are to get you close--These are not exact numbers and just guides to what makes sense for a reliable/cost effective installation.

Anyway--Lots to consider--Just some "numbers" to give you some ideas of what you are looking at based on what you have told us and some other guesstimates.

-Bill

PS: Fixed typo in equation (results are correct).

Photowhit

What Bill said;

DO NOT BUY ANY HARDWARE JUST YET!!!

I'd suggest you are close between a 24 volt and a 48 volt system? I would want at least a 24 volt system with an array larger than 1000 watts.  ....but we are putting the cart ahead of the horse.

First you need to define your loads, what do you hope to run off your solar system? In general, money spent on conservation will put you 'money ahead' when designing your system.

I'm not as pessimistic as Bill is, I'd say if your total cost for grid electric is more than 50 cents a Kwh, you should consider solar. With fixed panel on top of a trailer that couldn't be optimized for the angle of the sun you might increase this to 70 cents.

So if you are a particularly low user and your electric company has high user fees, you might be spending more than 50-70 cents a Kwh. I should say that my numbers work only for people willing to work within a system and with out a generator or alternate sources of power.

Loads, with panels mounted on top of a trailer, it is unlikely you would be able to have meaningful air conditioning. That in it's self my get you to rethink this endeavor. Since you would have to be parked in the sun for good generation, but that would allow for thermal gain of the trailer, it would be very hard to over come.

AndrewC

Thanks guys!!! this is very helpful info...I live in NJ and not alot of people are doing this out here...but I want to get off grid!!!

Photowhit

AndrewC said:

Thanks guys!!! this is very helpful info...I live in NJ and not alot of people are doing this out here...but I want to get off grid!!!

So do you have property? Could you ground mount the panels and put the 'Tiny Home' somewhere near by, but in the shade?

The East coast will be warm and muggy enough to want A/C for parts of the years, you might survive on fans but it wouldn't be too much fun for parts of the year. I survived Missouri for 5 years with just fans, but I worked 2nd shift and my place was in the shade, and Missouri usually cools off at night.

Being healthy helps your body adapt to warmer temps. I cycled the east coast tent camping, Florida to Maine but I couldn't do it now. I remember brain frying heat a couple nights. and I was in Maine by  June.

I can relate to being 'off grid' as a life style. There are other reasons to be off grid, saving money is usually not one of them.

Outline what your energy needs will be. Lots of people with good information here.

Photowhit

You might keep in mind that things that heat and cool will use the most energy, Solar thermal for water heating is nice, even if it's just a 'sun shower' Some cooking things are okay with your described system. Small Foreman grill, rice cookers, bread machines, crock pots. A good modern energy efficient fridge will use about a Kwh of energy perhaps a bit more in a warm/hot environment.

Without knowing your loads we can't really spec out a system, but I might suggest starting with 6 volt (208-215ah)golfcart batteries Sam's Club and Costco sell these for @$80 each. They are mass produced and tend to have the best bang for the buck. A 2100 watt array (6 - 350watt panels) would support 2 strings of 4 batteries (try to avoid more than 2 parallel strings)

You could also squeeze those panels on a 60 amp PWM charge controller, making for a pretty reasonable system cost.

westbranch

Uhmmmm. I can not find a 350 Watt Suniva panels, max they show is 340W....
this affects the choice of CC you go for, PWM or MPPT...

Photowhit

westbranch said:

Uhmmmm. I can not find a 350 Watt Suniva panels, max they show is 340W....
this affects the choice of CC you go for, PWM or MPPT...

I went on faith, Their 60 cell panels top out at 300 watts...

If OP is mistaken and talking about some other wattage, I should have said if they are 72 cell panels...

AndrewC

Photowhit:

I would definitely need AC in summer and Heat in Winter... I am looking at a 9000 BTU mini-split system...see link:

https://www.amazon.com/dp/B01DVW6BG0/ref=psdc_13397451_t1_B00M7CRZ0W?th=1

I work during the day so during peak sun hours...my "home" can be parked in direct sun and the AC would be off, as well as all other electronic devices...

AndrewC

Adding to last post:  The Refrigerator would need to stay on

AndrewC

I apologize...the panels is not 350 watts...these are the 2 panels I'm looking at:

SolarWorld 345 Watt
https://www.wholesalesolar.com/1922345/solarworld/solar-panels/solarworld-sw345-xl-silver-mono-solar-panel

or

Suniva 340 Watt
https://www.wholesalesolar.com/1524540/suniva/solar-panels/suniva-opt340-72-4-100-silver-mono-solar-panel

Which is better for my system??

AndrewC

When calculating loads...do I want to figure it in amps or watts...I know they are related very simply mathematically but it seems Batteries are rated in amp hours and panels in wattage...please advise

BB.

Stick with Watts and Watt*Hours for loads. Converter to Volts and Amp*Hours for battery bank when needed.

Watts and WH are "complete" units 100 Watts is 100 Watts at 12 volt or 120 volts. 10 amps at 12 volts (120 Watts) is not the same as 10 amps at 120 volts (1,200 Watts). It gets confusing as we move from 120/240 VAC to 12/24/48 volts if you try do do everything in Amps.

-Bill

Photowhit

AndrewC said:

Photowhit:

I would definitely need AC in summer and Heat in Winter... I am looking at a 9000 BTU mini-split system...see link:

https://www.amazon.com/dp/B01DVW6BG0/ref=psdc_13397451_t1_B00M7CRZ0W?th=1

I work during the day so during peak sun hours...my "home" can be parked in direct sun and the AC would be off, as well as all other electronic devices...

I would not consider heating with electric off grid. There is less sun and such a high energy use. 1 cloudy day would likely require running a generator. Better to use 'fire', a wood stove or small, direct vent gas heater. I did not do well with vent less gas heaters with windows cracked for oxygen. Check out cabin heaters for boats.

Even air conditioning will be a challenge, as I pointed out it's a high energy use and panels mounted on your roof will not be optimally faced for solar collection.

The same people make a higher efficiency unit here;
https://www.amazon.com/dp/B01DVW6BGU/ref=twister_B01E2ONIJO?_encoding=UTF8&psc=1

AndrewC

Is it better to buy 24 VDC appliances or just accept the losses from the inverter and stick with 120 VAC?

westbranch

That becomes your decision.... but BEST MADE AFTER you finalize your design and determined the losses of going each way 12 or 24 V DC or  120 V AC OR a hybrid where you have an inverter for those items that just are not available in DC format...  or the length of wire is very long and too costly, etc hth
BTW most LED MR16 track  lights can be found in 12V DC
Job's not finished till the paperwork is done...

Estragon

You really need to define loads.  Assuming trying to run HVAC off batteries is off the table, the next big loads to think about are water pumping and refrigeration.  Will you have a water tank and just need to run a small pressure pump/tank?  Will you want to run a regular AC fridge or a DC RV/marine unit.  

In a tiny home you probably won't have long wire runs so running DC loads is a practical possibility.  If you can run fridge and water on DC you may be able to run a small inverter like a morningstar 300w which has low self-consumption losses.

AndrewC

Maybe I'm looking at this backwards...but I know how much room I have for solar panels (7 Suniva 340 watt panels that will be tilted at the beginning of each month for optimal performance) ...So I am trying to figure out the most efficient way to capture, store and deliver that energy and then fit my loads accordingly...in other words...I guess I am letting my solar power production dictate what appliances I can have not vice versa...

AndrewC

Estragon:

I will have a water tank in the trailer for showers etc, and plan on installing an on demand propane tankless water heater with some type of water pump to deliver the goods!   oh yeah... and a composting toilet

Photowhit

I can see trying to run a DC fridge 24 volt, but the cost to benefit ratio isn't there for me. Other things just don't make enough sense to me, unless your plan is to shut down all AC loads. That's doable, but think about all the things you use AC for... Unless you can turn off the inverter, I don't see a huge advantage.

westbranch

On demand water heaters, almost all of them , have a minimum PSI and flow rate for the water that will narrow down your pump selection a bit.
What are the dimensions of the 'box' you plan on using?

AndrewC

The trailer is 20' long by 7' 7" wide by 7' 2" high...these are inside dimensions

AndrewC

Photowhit:  I think I agree with you...the DC appliances are very expensive!!!   I think I will just go with all 120 VAC stuff and make sure it is energy efficient

AndrewC

This is what I figure for wattage output from my panels based on my research:

Month	Daily W Output	Monthly Total
Jan	6524	202238
Feb	7640	213928
Mar	8669	268746
Apr	10333	309990
May	11012	341361
Jun	12369	371069
Jul	12325	382080
Aug	11209	347469
Sep	10267	308020
Oct	9063	280961
Nov	6261	187833
Dec	5692	176449
Totals	9280	3390145

BB.

Did you derate panel to load by ~52% for all of the system losses?

Yep, you only get about 1/2 solar panel name plate to your AC loads. Roughly:

81% panel derating
95% charge controller efficiency
80% flooded cell lead acid battery eff
85% inverter eff

Multiply all together, about 52% end to end useful power transfer.

-Bill

Photowhit

So you haven't lived on a solar driven system before?

Many things that people fail to understand, is off grid solar requires waste! If it produces 5692 watthours does NOT mean you have that much energy to use. In fact if you are using stored energy you can figure no more than 1/2 of that due to system waste, and that is before being sure your batteries are fully charged at least once a week.

So in December you have 3 hours of sun light on average per day, your 2380 watt array produces about 1785 watts per hour for 3 hours for 5355 watts (perhaps you have found a true NOCT value that is higher than 75%). But the way that energy comes in in the winter is you have 3 day of clouds then 2 days of sun, so you find that you've depleted your battery 50% on the cloudy days then recharge it during the first day and part of the second and waste the energy for the rest of the second.

BB.

Yes, as Photowhit says... You can only "count on" ~65-75% of your daily production for necessary (base) loads (refrigerator, lighting, water pressure, etc.). The rest of the potential energy usage, you have to decide if today/tomorrow will be sunny enough for vacuuming, running the wash, pumping to the cistern, using the electric slow cooker, etc.

On average, you will have lots of power in the summer (not excess power--Just more than you probably will need), and be squeaking by in the winter (plus genset usage during bad weather to take the bank from ~50% to 80%+ State of Charge--Keep the batteries from going to low, and not set at low levels of charge during bad weather--Batteries that "sit" for days/weeks below 75% SOC will sulfate and die).

You try to keep the battery bank "happy" by adjusting your loads, watching the sun, and use of the backup genset.

-Bill

AndrewC

Bill:

All I accounted for was inverter loss...did not realize all these other losses...thats frustrating...but good to know!!!

AndrewC

Bill:  What is Panel Derating?

AndrewC

Photowhit:

This will be my first time on Solar...I want to make a serious lifestyle change