Off grid PV sizing - Cali High Desert

Sailorjerry

Hi!

I'm new here and came seeking some answers as this forum seems to have the widest knowledge base on the subject.

I'm hoping that someone will be able to offer some solid advice on proper sizing of a off-grid PV system and choice of the individual components best suited for the climate I'm in.
Also need advice on sizing for the power needs that I have in order to run the system properly without starving the batteries or doing other damage.
My budget on this is somewhat meager, but I'm willing to pony up for better value in relation to performance.

The location is the California high desert. I do have full sun for about 320-330 days of the year. Temps vary from 110ºF in the summer to 25ºF in the winter.
There is also some wind at this location, 15-25mph for approx. 40% of the year. I may consider adding wind later, for now focusing on sun.
I'm looking to install this myself and actually do have some experience as I installed an off-grid system at my mom's house in 1999.
The entire system was laid out for me and all I had to do was physically install and wire it, I didn't have to do any of the math.
The system then was a Trace inverter with Siemens panels and Trojan T-105's. I can see that the technology has come much further in the mean time
and so thus I'm a lot more unsure about how to calculate a system that will work the best it can.

The purpose of the system is to power my workshop that I will be using to build a house on the property. The entire system will be contained in and on the workshop.
I expect the wire run to the panels to be 10 feet at the longest. for the batteries it will be very short, 2 feet at the most.
I would like to keep the system 12V on both sides, i.e. 12V panels, 12V battery bank.
If this is not ideal I would love to hear how performance and efficiency can be improved by going with higher voltage panels.
Definitely open to ideas about performance and efficiency.


This is the class of components I would like to use:

Modified sinewave inverter - looking at this one: http://www.whistlergroup.com/power-inverters-pro-catalog/pro-3000w-power-inverters-product.aspx?id=33

The documentation for this inverter states that it needs to be able to draw 300amps in order to perform at the upper limit of it's specs.
Should this inverter crap out or prove insufficient I'm willing to upgrade.


MPPT charge controller - looking at this one: http://www.solar-electric.com/blskysbchco3.html


Mono-crystalline panels - no idea what I need here as far as watts and amps to charge the battery bank in order to power an inverter of the size I want.
I'm thinking a ballpark figure here is around 400 watts??


Batteries - Looking at flooded or AGM's. Really not sure what to go with in regards to storage capacity. Also need some guidance on what type will be best for
hot climate and intermittent use. I know it's a bad idea to mothball your batteries without a trickle charge. The reason I need more guidance on the type of battery
to use is that I may design this system so that the panels will be secured against theft and vandalism when I am not there working on the place and thus the panels
will not be providing a charge to maintain the batteries as they will receive no sun. If this is the way I ultimately design the panel configuration the whole system
could sit idle for as long as 3-4 months at a time. The other option is to leave a smaller panel out to do the maintenance chores.
I have a 50W that could be used for this purpose. How many watts should this panel be based on size of battery bank??

I should add that as far as load on the system goes I need to be able to power individual tools, one at a time, that are rated at 2500W max surge.
Most of my tools are battery powered but I do expect to be able to run items like a small air compressor or medium size miter/chop saw.


Thanks much in advance, let's make some juice!

Cariboocoot

Re: Off grid PV sizing - Cali High Desert

Welcome to the forum.

Okay, what you have working against you: it's hot there, so panel Voltge is going to be lower than normal and batteries are going to be unhappy too; your loads are highly variable as "shop" implies different tools run for different amounts of time on different days.

What you have working for you: higher elevation that will improve panel output; us.

If there is no specific need for 12 VDC, don't go with a 12 Volt system. 24 Volts is more efficient. A need-to-read thread: http://forum.solar-electric.com/showthread.php?15989-Battery-System-Voltages-and-equivalent-power

Modified Sine Wave inverter: skip it. It's a guessing game as to if it will run your equipment or how well. Not worth the risk. Battery chargers, motors ... both are not friends with MSW.

An MPPT charge controller is a good choice, as it will grant you maximum flexibility in array design such as using "grid tie" panels which have odd Vmp but are much cheaper per Watt.

Monocrystaline panels? No point in choosing them specifically unless you have space limitations. Otherwise it's purely a matter of $ per Watt for whatever panels meet the design specs (once you have them).

Batteries; skip the AGM's. More money per stored Watt hour and pickier about charging. Do not worry about leaving batteries sit for extended times; as long as they are charged they'll be fine. In the desert heat you can dial back the Voltages and Absorb time when you go away to reduce water loss. A remote temp sensor for the controller is an absolute must.

If I were you the first thing I'd buy is a Kill-A-Watt meter and check the actual demands of the tools you plan to use. Some will be easier than others to run. I've checked a bunch myself (posted somewhere on the forum - I'll look later) and found the air compressor was the most miserable. I'm trying to use it now in winter and the gen won't start it cold! Once it warms up it's okay. Saws start easy, but draw much more once you load them with some wood.

BTW, you'll not only need a maximum power figure but also a daily Watt hour target.

Okay, how are we doing so far?

BB.

Re: Off grid PV sizing - Cali High Desert

Sailorjerry wrote: »

There is also some wind at this location, 15-25mph for approx. 40% of the year. I may consider adding wind later, for now focusing on sun.

You might have a good site for wind--You are not going to get any useful power from a 10 mph or less wind. A tall tower (60-90+ feet tall) will also be a big help (and well clear / above any buildings/obstructions).

I would suggest solar first--Get stable, then start research wind.

I'm looking to install this myself and actually do have some experience as I installed an off-grid system at my mom's house in 1999.
The entire system was laid out for me and all I had to do was physically install and wire it, I didn't have to do any of the math.
The system then was a Trace inverter with Siemens panels and Trojan T-105's. I can see that the technology has come much further in the mean time
and so thus I'm a lot more unsure about how to calculate a system that will work the best it can.

It has not changed that much--the big deal is that solar panels went from $10 per Watt to under $2 per Watt--That has allowed folks to run smaller battery banks+larger solar array for optimum performance (longer battery life). Batteries are getting expensive now too--So the old saw of adding battery bank capacity for more "energy", which was always wrong, is now easier to address by adding more panels.

The purpose of the system is to power my workshop that I will be using to build a house on the property. The entire system will be contained in and on the workshop.
I expect the wire run to the panels to be 10 feet at the longest. for the batteries it will be very short, 2 feet at the most.
I would like to keep the system 12V on both sides, i.e. 12V panels, 12V battery bank.
If this is not ideal I would love to hear how performance and efficiency can be improved by going with higher voltage panels.
Definitely open to ideas about performance and efficiency.

Depending on your power needs and ability to keep the site secure when you are not there--It is possible that you may have a short term and long term solution.

The short term is get one or two good generators (A Honda eu2000i, Yamaha 2xxx watt inverter/generator class) and a larger 3-5kw genset/noise maker/fuel gulper or whatever you need to run the big tools (when/if needed).

And build a small off grid solar power system (2-4 golf cart batteries plus X00 watts of solar panels) to give you stable power for lights, computer, cell phone, charging tool batteries, quiet time at night, etc. with a 300 Watt TSW AC Inverter.

I understand the issues of noise and fuel costs (and transportation)--But it is likely that whatever solar power system you design for building your home will not really be "optimal" for running your home.

The smaller inverter/Generator gensets will only burn 1-2 gallons of fuel during the work day (and even into the evening). And can power a skill saw (about the largest tool load). The larger gensets will burn a 1/2 to 1 gallon of fuel per hour--And if you can run those only when "batch sawing"--It should not be too bad.

The problem with solar is it is "most cost effective" if you are using stable amounts of power for 9+ months at a time. If you are there on weekends, only work a few months out of the year, then the solar power system just "sits there" doing nothing useful (other than draining your bank account).

Also, if you have special needs (such as large surge/peak wattage) to run large power tools--That can cause you to run a larger AH battery bank + Solar array--Or have to use AGM batteries to support surge current (AGMs are probably 2x or more expensive).

We can certainly help you size/cost the various systems (small, shop loads, home loads)--And I would suggest you do that (three different designs) and see what the costs are and what they will support a couple years down the road when you are living there full time.

It is pretty difficult to "expand or grow" a solar power system. Mixing old/new batteries, finding new panels that are compatible with existing panels, having to go from 12 volt to 24 or 48 volt battery bank, etc. all can make things difficult.

This is the class of components I would like to use:

Modified sinewave inverter - looking at this one: http://www.whistlergroup.com/power-inverters-pro-catalog/pro-3000w-power-inverters-product.aspx?id=33

The documentation for this inverter states that it needs to be able to draw 300amps in order to perform at the upper limit of it's specs.
Should this inverter crap out or prove insufficient I'm willing to upgrade.

Boy--that is difficult... The issue with 12 volt at 3,000 watt inverters is they pull a lot of current. Using typical numbers, the wiring/fuses should be designed to support

• 3,000 Watt * 1/0.85 inverter eff * 1/10.5 battery cutoff voltage * 1.25 NEC derating = 420 amp minimum 12 VDC circuit

That is a lot of very heavy copper cable to support that amount of current.

Anyway--Lets slow down a minute. You need to define your power needs first, then the battery bank, then the solar array. Once you have the basics, then we can look at picking the hardware.

MPPT charge controller - looking at this one: http://www.solar-electric.com/blskysbchco3.html

Don't even bother looking at charge controllers yet... More then likely, if you want enough power to run a 3kW inverter, it will not be this controller.

Mono-crystalline panels - no idea what I need here as far as watts and amps to charge the battery bank in order to power an inverter of the size I want.
I'm thinking a ballpark figure here is around 400 watts??

No idea either... It all depends on your loads (Watt*Hour per day), and size of battery bank. Mono or Poly Crystalline panels are both fine--Mono panels are a bit more efficient and have a smaller foot print--But cost significantly more money per watt. If you have roof/ground area for a large array--just purchase on quality and $$$/Watt. I would not look at thin film/amorphous panels (lifetime is usually shorter than crystalline panels).

Batteries - Looking at flooded or AGM's. Really not sure what to go with in regards to storage capacity. Also need some guidance on what type will be best for hot climate and intermittent use. I know it's a bad idea to mothball your batteries without a trickle charge. The reason I need more guidance on the type of battery to use is that I may design this system so that the panels will be secured against theft and vandalism when I am not there working on the place and thus the panels will not be providing a charge to maintain the batteries as they will receive no sun. If this is the way I ultimately design the panel configuration the whole system could sit idle for as long as 3-4 months at a time. The other option is to leave a smaller panel out to do the maintenance chores.

AGM are very nice lead acid batteries--But they tend to be 2x as expensive as flooded cell, and their life may not be quite as long.

If this is your first off grid system--I would start with a "cheap" flooded cell bank... You don't know your loads yet, and most people "kill/murder" there first battery bank or two.

I have a 50W that could be used for this purpose. How many watts should this panel be based on size of battery bank??

A 400 watt solar panel in the desert will see around 5-7+ hours of sun per day (during good weather). Such as system would supply:

• 400 watts * 0.52 system efficiency * 5 hours = 1,040 Watt*Hour per day (minimum sunny day)

That will run a skill saw about 30-60 minutes per day...

If you have no idea of how much power you will need now, or in the future--I would suggest getting a Honda eu2000i to run the smaller tools on ~1-2 gallons of fuel per day. And a second 3.5-5+ kW genset to run the larger tools when needed (1/2 - 1 gallon of fuel per hour).

For smaller devices (120 VAC @ 15 amps), you can use a Kill-a-Watt type meter. Simply plug the device/appliance in for 24 hours and read the kWH usage--Plus write down the average Watts/Amps and Peak Watts/Amps (Kill-a-Watt meters are not great for measuring peak current). You can run a K-a-W right off the back of your gensets and size the system from actual usage.

In the end, 1-2 gensets and a "small" 400-800 watt solar array plus 2-4 golf cart batteries to power your smaller AC loads (lights, radio, cellphone/computer charger, tool chargers, etc.).

Whatever you need to build your home will probably not be a "good fit" for full time living at your home. And it is difficult to "grow" solar power systems--Mixing old/new batteries/solar panels is difficult. Larger systems usually have to increase battery bank voltage. MSW can work OK for job site power tools, but are not good for electronics, computers, lighting, well pumps, etc.

Your thoughts?

-Bill

Cariboocoot

Re: Off grid PV sizing - Cali High Desert

And if your intent is to adapt this power system into the house once it's built, plan for that now so that you don't "buy everything twice".

Photowhit

Re: Off grid PV sizing - Cali High Desert

I think 'Coot and Bill are right there with me in thinking you will eventually have an off grid home? It's important to know now so you don't waste money.

I'm not set in saying you shouldn't use a 12v system, I framed up an 'A' frame with Dewalt battery tools charged from a 200 watt array and a couple golf cart batteries. Dewalt chargers after 1998(?) are fine with a modified sine wave. Befor that and they will fry as will craftsman and many other battery chargers. That said the reason I would stick with a 12V system is there are reasonably priced 1800 watt Xantrex Pro series and ProWatt inverters that should run any tool you would run off a single household curcuit and are priced $300-400. Your likely to have problems with a big worm saw, and things that require a 20 amp 120v line. If you go to a 24v system your likely to spend closer to $1000-1500 for an entry level inverter, but if you need more than 1800 watts I'd say move on up to 24v on the DC side.

I did not finish(do the inside) the 'A' frame which may have required more cutting. If you use battery tools during a days work read the instructions or risk killing your batteries, you don't charge batteries hot form being discharged, and you don't use the batteries fresh and hot off the charge, so you'll want 3-4 batteries. You should break up long cuts so if your running the saw under load for more than a minute you want to stop for a minute or so and allow the tool and battery to cool down, This rarely comes up, but cutting 1' composit flooring it may take 3 cuts to go the 8' legnth, and most of a charge.

If you plan to build this as an off grid home, I'd go ahead an purchase a midnite classic lite, for $500 it will handle this with ease and be part of a bigger system later, and I'd look for the cheapest $ per watt panels. If your not going to use this system later, I can PM you a link to some inexpensive 12v nominal panels and I'd look into a cheap PWM charge controller. Please note that these would be very hard to import into a larger system later!

westbranch

Re: Off grid PV sizing - Cali High Desert

My view quickly, if you have a full woodworking shop, 4 hp table saw, band saw, planer and cutoff saw plus router etc, and you build a system to power these tools you can power your house.
You probably need a 4000w inverter and 220 power, there are only a few systems that can do this, work on that part later....FIRST list all your loads, in the shop and estimate in the house, then plan out the rest of the system and keep coming back here for help...Oh, 24 volt or 48V will be needed.

hth

Sailorjerry

Re: Off grid PV sizing - Cali High Desert

Hi,

Thanks so much to all who have responded! I'm thrilled to have your knowledge at my fingertips!

Most of you guessed that it would be foolish not to consider a system now that could to some degree be pulled into service in the new house.
This was my thought too when I first started thinking about solar solutions and I'm still on the fence about that strategy as I actually do have access to grid power.
I do have a power pole on the property about 50ft from where the house will be. I'm not in a big hurry to hook up to it for a number of reasons:

The first one being that the power company servicing the area will not drop a construction/temporary service line to you unless you already have an approved building permit.
The second one being that I believe in the power and utility of solar radiation as a stand alone and independent solution to my meager power needs.
The third one being that I have owned this property for almost a year now and my only power here has been supplied by the sun through the means
of one 50W panel, a rinky dink 10A charge controller, one Deka 12v Marine Master deep cycle battery and a squeaky 400W inverter I bought at PepBoys
probably 10 years ago...and I have thus far yet to run out of power. That being said, all I'm doing with that system is charging batteries:
Computer, phone, Bose powered speakers, Coffee grinder, Milwaukee 18v power tools.. so no real hard power hogs there.

My first thought was to build a system that could be transferred into the house when the time was right but I realize that grid tie systems have come so
far in efficiency that I might be well served to consider such a system once the house is built. Any thoughts on that solution would be greatly appreciated.
I have lots of room (5 acres) so could put in a lot of panels to feed back into the grid if I wanted to. There is a building code restriction on the size of PV arrays
but it's pretty large and from what I understand it's not that hard to get an exemption. I'm curious to know if anyone is actually making any money out of such
an arrangement or if it merely serves as an offset to the utility company's fees

NOTE: this is only a partial reply. this response was originally three times as long with more detailed info but my reply would not post at all and this is all that remains as recovered by the auto save feature...very annoyed with this as I hate wasting time and don't have it right now to retype. Thanks so much to all that responded to me today! I will share more as I go along. and hopefully will be able to post.

BB.

Re: Off grid PV sizing - Cali High Desert

Sorry about the lost post pieces... It happens--My laptop keeps messing up my browser too (reload, close window, etc...). Big touch pad just under keyboard--all sorts of weired hot key combinations that I did not even know existed.

Anyway--How much will they charge you for power (any connection fees/service charges, etc.)?

Also, if you don't use the power for years--Will the power company pull the line to the property (i.e., if you go to sell the property with no utility feed, will there be a huge fee to reconnect for the new buyers)?

And, any A/C for summer? Or will this be a berm home/alternative cooling/heating?

In the end--I would look into using the utility power until the home is built (if the costs are reasonable). At that point you can decide if you want to Grid Tie, Hybrid (GT with Off Grid Backup+battery bank+generator), or go full off grid and pull the utility power.

Neat thing would be that you can get months to a year on utility power and figure out how much off grid power you will need--Instead of guessing and under/over building your off grid system (or sizing out a GT system).

-Bill

Photowhit

Re: Off grid PV sizing - Cali High Desert

It sounds like your a minimal power user, unless you have huge up front costs to bring power on to or into the property or huge monthly fees it doesn't make sense not to connect to the grid. Grid tied systems are different depending on your location and make sense in an enviromental and at times a financial.

If you have neighbors, it's likely you will want to do things 'above board'(comes from keeping your cards on the table) once you have an issue with them, it can become 'less neighborly' and every thing becomes an issue.

Cariboocoot

Re: Off grid PV sizing - Cali High Desert

Another solution: since you will need a back-up generator for an off-grid system anyway, buy it first. An inverter type generator large enough to run your compressor for example. If most of your tools are battery powered (the trend these days) it can run the charger for them on economy mode and be ramped up to full power for heavier loads. Then when you have a better idea of the household loads you can design your solar power around those.