Small off grid system - does this sound right

RoySalisbury

I just found this forum and i've been looking over some of the posts, but figured I would explain my specifics and see what some of the "experts" might recommend.

I am building a small astronomical observatory in north western Arizona. Its mainly just two small buildings that I will use on the weekends. I have calculated up the power consumption of the equipment that I normally use in the field (telescope, laptops, camera, a small drink cooler, ect) and its about 155 watts.

Usually sessions last about 10 hours during the night, and then I come home and process the results (using a lot ore powerful computers), but with the remote site I will be staying the entire time. So for argument sake, lets just say that that 155 watts is used all day (24 hrs) .. so that is 3.72 kwh/day .. lets be safe and say 4 kwh.

I was looking at getting a 600 watt solar system for about $3000. And form what I have read I could get between 5-6 hours a day of exposure in the area. Again, lets be conservative and say its just 5. So that is a 3 kw/day system. Not quite enough. So I thought I could then use a small 1500 watt generator (already have it) and use that to offset power use (if and when I needed it).

Does this sound right? Or am I all messed up? And what am I missing.. this actually just seems too easy. The reason I want an off grid system (other than there is no power to the land) was that some stuff will need to be left on 24/7 (security, weather, ect).

Roy

Sheldon

Re: Small off grid system - does this sound right

You don't factor in efficiencies.

You can look up what to expect for PV to raw DC, DC in/out of batteries and DC to AC.

Figure 50% is conservative lacking any better knowledge.


Your gasoline genset can cover you if you have a rare bad collection day while you are there, but if you really want to have 24/7 coverage, you need to think about how many bad collection days are feasible to still power your security, what is your reserve?

Don't discharge your batteries more than about 25% under normal usage and rarely in the no sun for a week case up to 80%.

I have never seen a load calculation that did not eventually prove inadequate, next generation equipment, convenience items (coffee pot, microwave) appear etc.

RoySalisbury

Re: Small off grid system - does this sound right

The 600 watt system comes with 8 ea 6V 225Ah batteries. I think this is already factoring in the 50% DOD for the battery bank.

And your right, I am not factoring in loss of power for the efficiency of the system. I should do that, but it just made things more complicated. I figured that when I got to the end of what was required, I would just bump it by say 25% or let the generator pick up the slack in the short term.

Basically for the weather system it will be self powered by its own system, but will need to connect with a small PC to relay the info (cellular) and the security system will use it as well. The PC runs at about 25 watts and the security system runs maybe anther 25 watts (not sure yet on that one). So normally its running about 50 watts 24/7 ... so the 600 watt system should be able to run that when no other power is being used.

Roy

BB.

Re: Small off grid system - does this sound right

A good place to start is the PV Watts program.

Let's assume Flagstaff is close enough in location/weather. Choose 1 kW of solar panels (smallest the program lets you use). Derating of 0.52 (includes flooded cell batteries 80% efficient and 85% efficient inverter). Fixed array (no tracking) tilted to latitude.

"Station Identification"
"City:","Flagstaff"
"State:","Arizona"
"Lat (deg N):", 35.13
"Long (deg W):", 111.67
"Elev (m): ", 2135
"PV System Specifications"
"DC Rating:"," 1.0 kW"
"DC to AC Derate Factor:"," 0.520"
"AC Rating:"," 0.5 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 35.1"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 8.5 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 5.19, 84, 7.14
2, 5.92, 86, 7.31
3, 6.27, 99, 8.42
4, 6.44, 96, 8.16
5, 6.56, 99, 8.42
6, 6.61, 92, 7.82
7, 5.95, 85, 7.23
8, 5.54, 79, 6.71
9, 6.59, 94, 7.99
10, 6.19, 93, 7.91
11, 5.47, 83, 7.05
12, 5.07, 82, 6.97
"Year", 5.98, 1072, 91.12

So, usable 120 VAC power from a 1kW system would average around 79-99 kWhrs per month... Divide by 30 days per month = 2.6 to 3.3 kWhrs per day (depending on season)...

Knowing your loads is important. Measuring your real loads with a kWhr meter is a good step.

• 155 watts * 10 hours = 1,550 Watt*Hours = 1.55 kWhrs per weekend day

Assuming you will have the same load 24 hours per day, 7 days per week makes the system much more expensive:

• 155 watts * 24 hours = 3,720 Watt*Hours = 3.72 kWhrs per day

If you are there only on weekends... You can assume that you will have at least 5 days to recharge for your two days of use (note: we assume a 3 day no-sun on the battery bank with 50% maximum discharge and then you will want to recharge back above 75% for best battery life quickly--assume 5 day cycle for most cost effective battery life).

For cost effective use--you will probably want to design for "average" power usage and use the genset for cloudy weather (and get a discharged battery bank above 80% if no sun is predicted for the next few days).

Of course, you observation need clear weather--so you will unlikely to be using the equipment much during predicted bad weather (when solar production would be low).

To size the array:

• 1.55 kWhr per day * 1/2.6 kWhrs per 1 kW array (winter months) = 0.596 kWatts (exactly your planned system)

To size the battery bank (assuming 85% inverter on a 12 volt battery bank):

• 1,550 Watt*Hours per day * 3 days no sun * 1/0.50 min batt * 1/0.85 inv eff * 1/12 battery bank = 911 Amp*Hours (at 12 volts)

Now, those are the average rules of thumb. If you want more solar panels, you should think about larger battery bank. If you want less solar panels, you will have to make up with a genset (typical is to start the genset in the morning, make coffee/toast/microwave/vacuum and charge--let the solar panels finish the charging after a couple hours of genset run time).

Your battery bank, I have sized for 3 days of no-sun and 50% maximum discharge. If you are only there when the sun shines and/or plan on using the genset more if the sun does not shine, you could cut the bank size by 1/2 or 2/3 pretty easily. The bank will not last quite as long--but your initial battery costs are much less too (realistically, whether you replace 3 batteries every 3 years vs replacing 9 batteries every 10 years--it comes out to be a wash in costs many times--then you are looking at the issue of how much pain to change out batteries every three years vs every 10 or so).

Inverter wise, the Morning Star 12 True Sine Wave 300 Watt inverter would be very nice for your system (very low standby power usage too).

I also suggested a 12 volt system because your loads are small (less than 1,200 watt peak) and you can use 12 volt appliances directly. However, watch out when charging. A flooded cell deep cycle battery is charged between 14.5 and 15.5 VDC and discharge can go down to 10.5 volts (dead battery + wiring drop)... Many 12 volt car adapters assume 13-14.2 volts and a few people have found that the higher/lower voltages can cause problems (or even failures) with standard adapters (some reports of laptop 12 volt adapters failing in solar RE systems).

Using 120 VAC gives stable voltage, ability to use extension cords for long distances, and lessens the needs for adapters and issues with wide range 12 volt power from the battery bank.

Regarding the drink cooler--typically for weekend use, it may be more cost effective to use an RV style Propane refrigerator/freezer. Solar really only makes economic sense if you are using it 9+ months out of the year (capturing most of the available sun energy). A weekend type use, you only pay for the propane when you need the cold--and there are no costs when the fridge is turned off (you probably need propane for cooking anyway).

Lastly, if you are looking for a genset--Highly recommend that Honda eu2000i for your application. Very fuel efficient down to 400 watts (or less) loads, quiet, and reliable (reports of 2,000-3,000+ hours of life--rebuilding probably not worth the effort).

"Typical" gensets use the same fuel flow at 50% of rated power as at 1% of rated power--whereas the eu2000i uses about ~25% at 1/4 rated power.

-Bill

BB.

Re: Small off grid system - does this sound right

Roy,

You posted a new requirement about powering a security system... One thing that make a solar RE difficult is unattended constant loads. You need to do something that either allows for graceful shutdown (other than draining the battery bank 100% and possibly ruining the bank) or alternate power provisions (shutting down optional loads, starting backup genset, "calling home" for help, etc.).

Towards that end, I would recommend looking at a Battery Monitor--such as the Xantrex units. They (others may too) have a programmable output that you can set to open/close when the battery is at (for example) 50% discharge... That would give you a few days to get back to the site and recharge the bank (assuming bad weather/charger failure) or start up a backup genset.

• 50 watts * 24 hours per day = 1,200 Watt*Hours per day

That 1,200 Watt*Hours per day is pretty close to the average power available for your 600 watt system in August/Winter.

Finding a remote alarm system (with cell phone interface?) and a smaller laptop/industrial computer for logging with lower power usage-- may be worth looking into.

-Bill

RoySalisbury

Re: Small off grid system - does this sound right

Wow.. Thats a lot of info.. Thanks.

I measure my actual full loads using a watt meter. Basically I plugged in each device one at a time and full exercised the functions. For the PC I made sure that the video was 100% bright, the hard drive was being accessed (read and write) and the CPU was at 100%. I also made sure that the onboard battery was down to 20% so it was charing at the same time all this was going on. That showed me a peak of 25 watts on the meter. Normally when its at idle its around 12 watts. I did this for all the equipment where possible. So the total watts I added up were the max use watts. It is very unlikely that I will ever see this in real life, but it gives me worst case power usage. This is how I came up with the 155 watts. The drink cooler was the worst at 80 watts.

I was thinking 1.5 to 2 days battery bank. Reason for this is that usually when I go out it would have had 4 or 5 days to charge up, and even if the whole weekend was cloudy during the day and clear as night, I would still have enough power for the weekend. And the power for the "off days" is only for the monitoring system.

The system that I was looking at was this one (http://sunelec.com/index.php?main_page=600_watt_off_grid_system). Not saying that I would get that one, but it was may base system for estimates (panels, batteries, inverter, etc). I'm not sure about some of the equipment (will it work with a generator also, and what might I need for that interconnection), but it was a start.

Also, some (if not most) of my equipment can run off 12V power right now, but I use the AC adapters since my current portable battery back has an inverter built in. I just use my car as a "generator" to recharge the battery pack via its 12V power connection. If plug it in avery few hours during the night I can run off my battery back all night. Anyway, perhaps it would make sense to run more stuff direct to 12V and not though the inverter.

Roy

icarus

Re: Small off grid system - does this sound right

Just as an comparison fyi,

Our battery based 300 watt system, delivers net/net ~ .6-1.0day on average throughout the year. Can easily be twice that in the height of summer with 16 hour days, or double that in the cold of winter with reflection off of snow even though the days are short. The average includes partly cloudy days, haze ect.

We burn ~.6 kwh/day on average, and only after several days of little or no sun do I have to charge from the genny.

My rule of thumb is take the name plate rating of the PV, divide by to to account for all system loses including wiring, charge controller, inverter, battery charging efficiency etc, then multiply that number by the average hours of good sun you would reasonably expect to get. (seldom more than 4, but in AZ you might get closer to 6.) So in my case, 300 watts/2*4=600 wh.

Tony

RoySalisbury

Re: Small off grid system - does this sound right

Yes, your right.. I would need something to make sure the security system was going 24/7. I was thinking of looking into something that would allow the generator to start when the back got to a certain level, and shut off when it was charged. However, my current generator does not have an auto start or even an auto choke, so i need to look into a different generator for that use.

The PC that I have for the system is a micro PC with a very low power requirement and has a cellular modem built in. At idle the power is about 11 watts, and at max it is 25 watts. This does really well for the system.

Roy

BB.

Re: Small off grid system - does this sound right

Roy,

To getter characterize your loads, get a Kill-A-Watt meter to totalize a day's worth of power. For DC loads, you can do the same thing with a DC Amp*Hour/Watt*Hour meter.

Off Grid Solar power is expensive--If you assume a 20 year life + battery replacement every 4-8 years, you are looking at $1.00-$2.00+ per kWhr cost for power. You do not want to "waste" it.

-Bill

PS: That Xantrex unit is a MSW type inverter... For your needs, I would suggest that you "invest" in a TSW (True Sine Wave) type inverter system.

Also, at that wattage range, you could look at a MPPT Type solar charge controller. More expensive--but gives you lots more options as how to configure the array (in series for low amperage/long wire runs--if needed--back to the battery shed). Also, in cold weather, solar arrays do output more power (maybe 10-20% more in your cold climate) and can help in the winter days.

RoySalisbury

Re: Small off grid system - does this sound right

BB. wrote: »

Roy,

To getter characterize your loads, get a Kill-A-Watt meter to totalize a day's worth of power. For DC loads, you can do the same thing with a DC Amp*Hour/Watt*Hour meter.

I have the Kill-A-Watt meter.. that is how I came up with my max loads on the devices. I'm pretty sure that the average use over a 10 hour period would be less (from 155 to maybe 110).

Roy

BB.

Re: Small off grid system - does this sound right

Average power (kWhrs per day) is what you want to design your system too... Especially if you have cycling loads (fridge/well pump/etc.). Adding a 50% fudge factor does not hurt either.

That is why I like to show my work. It allows you to see what my assumptions and fudge factors are. I like the PV Watts website... Its output estimates seems to agree with my own GT system (near San Francisco) and with what others have observed for their areas too.

We are all pretty cheap around here, but practical too... So, we try to be conservative in our assumptions and in the estimated power output. We want "happy campers" here.

-Bill

PS: I should add for a cheap and effective backup power supply, Icarus/Tony has had good luck buying used Gensets from wrecked RV's. They are cheap, reliable, and setup for electric start with monitoring. Add that some of them are alternate fuel capable (gasoline, propane)--they might be a good unattended backup genset for you.

It may still be more fuel efficient to run a smaller Honda when you are there (RV gesets tend to be larger)... Your choice.

RoySalisbury

Re: Small off grid system - does this sound right

Yes.. adding in a buffer is good. That is why I went for the max loads on everything, estimated out what it would be for 24 hrs even though it reality it would only be between 12-16 hrs, and use 5 hours of sun vs the 6 hours I can probable get. Having a 1500 watt generator as backup of a 600 watt system is also a nice buffer.

So basically, if a 600 watt system will be "on the edge", then 1000 or 1200 watt should suit my needs fine.

Anyone have have complex or easy one of these systems is to install? I have basic power system knowledge, and am also constructing the buildings myself, so basic installation should not be an issue. But who knows..

Roy

Blayd

Re: Small off grid system - does this sound right

What you are planning sounds reasonable as far as the PV system goes. What you should concentrate on is the batteries that will store the power.

Myself for the replacement batteries that I soon will need, I am leaning towards forklift batteries (BIG ONES!! 500 pound big) as being the most cost effective, rather than the smaller L16RE B 6VDC 370Ah which need to be wired together and would require 4 batteries to equal the one forklift battery.

My humble little home is alarmed with a very nice commercial system from a former
business that I owned. It thankfully is a 12VDC so its just plugged right in with no transformer.

I have both 12VDC and 120VAC to cover all the possible situations.

icarus

Re: Small off grid system - does this sound right

A couple more questions/observations. First, is this going to be permanent installation, or do you have a project that will end in a matter of months or couple of years.
That answer my have some bearing on how big you may choose to do, or which battery bank you might choose.

As for drink cooler/fridge. In some diminishing rate of return, a choice can be made between conventional (good energy star rated) compressor fridge, and a propane fridge. As I have written here, quite often, if the system and the use is going to be used essentially full time the choice would be for the conventional fridge. If it is only going to be used occasionally LP wins. Lp fridges have a higher up front cost, (but are available used from the Wrecked RV market) and will use little propane with weekend only use. A conventional fridge will use ~.5-1 kwh/day, making for a pretty big PV system to run it. (and smaller isn't usually isn't any more efficient). An apartment sized LP will burn ~1500 btu/hour, and will run anywhere from 1/8 to 1/2 the time depending on Delta T. (1 gallon of Propane is ~ 79000 btu) A tiny 2 cu ft dometic camper fridge might average 1/4 as much if you add some rigid foam board around the case. So one can do a quick calc to figure out when an LP or a conventional makes sense.

Tiny little plug in coolers using thermal electric devices are pretty poor coolers, and they draw a fair bit of power.

Tony

RoySalisbury

Re: Small off grid system - does this sound right

This will be a permanent system. It may be upgraded over time depending on loads, new equipment, etc.. But its not going anywhere.

And if I take the drink cooler off the list for watts, I'm now down to about 125 .. so if I had to remove that I can always bring an ice chest (that will last a weekend). Or I know that to use some extra stuff I just have to have the generator working. For the t or 3 days a week that I am there I can use the generator, but for the 4 or 5 days that I am not, the system has to be able to run on its own.

Roy

icarus

Re: Small off grid system - does this sound right

Two more thoughts on a cold night, -36 at present.

Loads WILL increase with time. Count on some head room in your design.
As you have 24/7 power available without running the genny you will find all kinds of ways to use it up. (I just bought my wife a hand held cuisinart blender)!

As for growing systems, consider a bigger charge controller in the beginning. I am on Charge controller #4. Panels can be added, inverters can be stacked and indeed to some degree batteries can be added later as well, but when you run into the maximum of the charge controller you are sort of stuck. You can stack multiples,but you are better off with a big enough one (within reason).

Tony

BB.

Re: Small off grid system - does this sound right

As you pencil in the system... Some things to think about.

Look at your peak power requirements. Many times, people want to stick with 12 volt systems so they can use cheap 12 VDC appliances/tools/adapters. The issue is, as the power needs go up, so do the current needs. But, 1,200 watts at 120 VAC (inverter input) is more than a 100 amps at 12 VDC:

• DC Amps = 1,200 watts * 1/10.5 volts -- low batt * 1/0.85 inv eff * 1.25 NEC derating = 168 amp design rating

So--for a 10 amp AC load, you are looking at designing your wiring/battery bank to support >168 amps to get 1,200 watts at 120 VAC (10 amps) out your inverter...

So, I usually recommend that if your loads are 1,000-2,400 watts, look at a 24 volt battery bank, and if >2,400 watts, look at 48 volts.

For example, that same 1,200 watt load on a 48 volt battery bank:

• DC Amps = 1,200 watts * 1/42 volts -- low batt * 1/0.85 inv eff * 1.25 NEC derating = 42 amp design rating

Also, since you are looking at >400 watts of solar panels--you should be looking at MPPT charge controllers. It will not only help you with gathering 10-15% more power at lower temperatures, and you can run your solar panels quite a distance from the array to the battery shed (instead of 12-48 volts at the panels, you can run them around 100 VDC and greatly reduce the current level--using smaller copper wire to save money).

Also, many of the MPPT solar charge controllers are limited to 60 amps (+/-) maximum--but can charge anything from a 12 to 48 volts battery bank. For example, the (approximate) maximum array size for a single controller is roughly:

• 15 volts battery charging * 60 amps * 1/0.95 controller eff = 950 watts of solar for 12 volt bank
• 30 volts battery charging * 60 amps * 1/0.95 controller eff = 1,900 watts of solar for 24 volt bank
• 60 volts battery charging * 60 amps * 1/0.95 controller eff = 3,800 watts of solar for 48 volt bank

So--you can support 2-4x the solar array with a single controller with high voltage battery bank (12 vs 24; 12 vs 48 volt battery banks).

-Bill