Newbie needs help with backup solar power system design

Re: Newbie needs help with backup solar power system design

If you are going to be dealing with DC loads--You can also get something like one of these DC Amp*Hour/Watt*hour meters for your test bench.

-Bill

Re: Newbie needs help with backup solar power system design

The devil is in the details...

Regarding small devices (cell phone charger, etc.)--It simply may be that you are below the k-a-w meter's ability to log accurately (many devices put a floor it to stop "drift" customer calls--i.e., why is my KaW meter registering 0.001 kWH per when nothing is plugged in).

Another issue with many small electronic devices is that they have "poor" power factor...

Basically, we are all used to seeing the equation:

• Power = Volts * Amps

And, for steady state DC and simply AC power (electric heaters, filament lamps, etc.)--it is true.

However, for many AC devices, the power supplies do not take nice uniform current in phase with the AC voltage sine wave--Instead, they take little chunks of high current at the peak of the voltage sine wave...

One way to deal with it is to use a fudge factor--Aka the "Power Factor". This number represents the amount of measured current (RMS or root mean square energy content) and say how "efficiently" it is being converted into useful work:

• Power = Voltage * Amp * Power Factor
• Power = Voltage * Amp * Cosine of the phase angle between AC sine wave voltage/current

So, for many small electronics, the PF may be 0.6 or even 0.5 --- Basically:

• 120 VAC * 0.10 Amps = 12 Watts not correct
• 120 VAC * 0.10 Amps = 12 VA (volt*amps)
• 120 VAC * 0.10 Amps * 0.5 PF = 6 Watts

You need the 12 VA as the part of the load calculations for the inverter. Most inverter Watt ratings = VA ratings.

So, a 300 Watt limit inverter is really a 300 VA limit inverter; at PF=0.5, then that would be a 150 watt load.

And the battery would be supplying energy (power) at the rate of ~150 watts.

So, when sizing your system--A PF of 0.5 (just made up worst case numbers) your 300 VA load will only consume 150 watts from your battery bank...

And, you do need to add other losses... For example, the inverter is probably around 85% efficient:

• 150 watt AC load * 1/0.85 eff = 176 watt load on the battery bank

There are lots of losses in a Solar PV system--and one quick rule of thumb, if you have at least 4 hours of sun per day (not winter, no coastal fog), and assume 0.52 end to end efficiency (yes, about 1/2 is lost from solar panel Watt spec. to AC inverter Watt output):

• 135 Watt panel * 4 hours of sun * 0.52 = 281 Watt*Hours of 120 VAC power available on a good day

If you are going to have a 24x7 PV system + loads, and it is "unmanned"--there are a lot of details we need to talk about... Like what happens in winter with a stretch of bad weather... You can easily kill your expensive battery bank if you draw it to near zero state of charge.

-Bill

Re: Newbie needs help with backup solar power system design

tombronx wrote: »

So, if I have a total of 510VA daily, would this be a correct calculation?

If I understand your calculations--No...

510 VA is a peak usage (this is a rate, like miles per hour, gallons per minute, etc.). So, yes that is part of the inverter sizing (maximum Watts/VA draw of loads).

You also need to take into account for time (hours) of use.

For example, if your average draw is 250 watts for 5 hours a day:

• 250 watts * 5 hours = 1,250 Watt*Hours per day

That is your "daily" energy usage (miles driven, gallons pumped).

Total VA: 510
Minimum inverter Wattage (VA): 510
Minimum battery (12v) Wattage: 510 x 0.5 (PF) x 1/0.85 = 300 Wh
=> 300Wh/12v = 250 Ah
with 2 days of bad weather: 250 Ah x 3 = 750 Ah

Using my make believe number above of 1,250 Watt*Hours per day--Sizing the battery for 2 days of bad weather and 50% maximum depth of discharge:

• 1,250 Watt*Hours per day * 1/12 volt battery * 1/0.85 inverter eff * 2 days * 1/0.50 discharge = 490 AH battery bank @ 12 volts

Panel: 240w * 4h * 0.5 = 480 Wh

No, we need the Watt*Hours per day of your loads... Again using my made-up loads:

• 1,250 WH * 1/0.52 system efficiency * 1/4 hours per day = 601 Watts of solar panels

In case of a long stretch of bad weather (> 2 days) , I'd not use solar power until i can afford a larger system and bigger battery bank. Really, it is rare to have 3-4 days without sun in this area (90W/35N), even in winter.

A small backup genset + AC battery charger usually is a good thing to have--both for solar backup/emergencies, and portable power.

We need to go through your load numbers again to understand your requirements.

-Bill

Re: Newbie needs help with backup solar power system design

tombronx wrote: »

Sorry for my ignorance ... I still got a lot of reading to do for this project.

Not a problem--we all started knowing nothing ourselves. One step at a time.

Everyone here wants to help--No "just read the FAQ" or "have you tried the search function" answers from us. :-)

At the peak, my total usage is 53 VA. So, (cut and paste from http://www.leonics.com/support/article2_12j/articles2_12j_en.php) “For stand-alone systems, the inverter must be large enough to handle the total amount of Watts you will be using at one time. The inverter size should be 25-30% bigger than total Watts of appliances.”, I'd need an inverter rated at least 70W.

My required Wh/day is 362 Wh, so: 362 Wh * 1/12v * 1/0.85 * 2 * 1/0.50 = 144 Ah battery (12v)

You got it!

362Wh * 1/0.52 * 1/4hours = 174 W solar panel → I think a 240W panel would be fine.

Remember, the 4 hours per day is a rough starting point for many folks that live in regions with reasonable amounts of sun--and typically does not include the ~3 months of winter.

In general, you would plan on using about 50-75% of the expected daily power collection... Do not plan on using 100% every day--there is just too much variability in day to day solar availability--plus most people add to their loads (new fun stuff to have at the cabin).

The other thing I like to do is check the solar array by matching the 5-13% rule of thumb for charging current for battery banks... Note that adding lots of AH means that people will need to add more panels. Assuming 0.77 solar panel + charge controller derating/losses:

• 144 AH * 14.5 volts charging * 1/0.77 derating * 0.05 rate of charge = 136 watts minimum
• 144 AH * 14.5 volts charging * 1/0.77 derating * 0.10 rate of charge = 271 watts good sized nominal array
• 144 AH * 14.5 volts charging * 1/0.77 derating * 0.13 rate of charge = 353 watts typical cost effective maximum

So, the reasonable array size for you (assuming 4 hours of sun per day) would be around 174 to 353 watt solar array.

Note: close in solar is +/- 10-20% -- I am including a bunch of digits so you can confirm my math (Trust But Verify).

My total loads are:

laptop (5 hrs): 90 VA, 100Wh
webcams (2 * 24 hrs): 288 VA, 192Wh
cell phone (2 hrs): 14.4 VA, 20Wh
Light (5 hrs): 108 VA, 50Wh

looks OK... Inverter wise, I would recommend that you choose a TSW (True Sine Wave) Inverter--They are many times more expensive than a MSW (Modified Square Wave) Inverter, but you will probably have less problem with your wall transformers, small power supplies, etc.

All About Inverters
Choosing an inverter for water pumping

A really neat 12 volt 300 watt TSW inverter is the MorningStar model...

With any inverter, understand that they tend to waste a lot of power when idling--and TSW have higher idle losses than MSW. Try to size the inverters to your loads and turn off the DC when not needed -- Or use the "search mode" feature available on some inverters (typically larger inverters--but the above MorningStar has search mode and remote DC off too--basically they turn on once every second or so looking for >6 watts of AC load-then turn on 100% until the load goes away).

-Bill

Re: Newbie needs help with backup solar power system design

I am always hesitant to recommend running for 12 VDC directly from a solar RE system...

Many of those devices are "optimized" to run from a car system--Which is around 13.8-14.2 with the engine running, or possibly down to 12.5 volts or so.

A solar RE system operating normally can range from below 11.5 volts to to upwards of 15-16 VDC (charging/equalizing).

If this is "critical equipment"--the owner needs to understand the limits of their 12 VDC equipment. A 11-16 volt range is actually quite wide and can damage some equipment.

That is one reason I like to recommend a good quality AC inverter instead--the ~15% because of the inverter can be addressed by a little more solar panel + battery storage.

-Bill

Re: Newbie needs help with backup solar power system design

Certainly possible--More research is needed on the OP's part. Things that can be done--either a manual or automatic load disconnect when battery voltage is out of range--such as (some controllers have a high limit on the LVD too).

-Bill