does size matter

System

hello everyone , I have question on panel size, i would like to build a solar system that will enable me to run my table saw off the grid , also and not at the same time a 12,000 btu a.c. unit both of which draw a start up amperage of around 13 amps . i have decided on a power bright pw3500 power inverter to handle the start up. now i need to decide on the size of the panel ,charge controller and batteries. I am leaning toward a mono crystalline 185 watt panel ,36 volt and 5.15 amps . An mppt charge controller size i do not know and at least two 100 amp hour agm batteries. Now all that being said is my panel going to be to big and what size charge controller should i be looking at , also should i be looking at more battery back up . I use my saw for a couple hours at a time once or twice a week . thank-you in advance for any information you can give ,tater head

Windsun

Re: does size matter

While your proposed setup would probably work for a small table saw, it would not even come close to powering up the A/C for more than a few minutes per day. The compressor load on an A/C is pretty high, while the motor load on the saw is only high when starting and in heavy cutting. But even for the saw you would probably need more battery - depends on how often the saw is actually running in that 2 hours.

LandKurt

Re: does size matter

I'm going to guess that your air conditioner draws about 1200 watts while running. Given that a 185 watt panel will produce about 370 watt hours of stored power a day (given four hours of full sun and a 50% full system derating) that means a single panel will allow you to run the air conditioner for less than 20 minutes a day. Given the Peukert Effect on batteries at high discharge rates, probably only 10 minutes.

You're going to need some really heavy wiring and heavy duty batteries to run a 3500 watt inverter off of 12 volt batteries. Don't rely on the information in the Power Bright manual on calculating amperage on the 12 volt side:

I quote: "A shortcut method is to divide the continuous AC load wattage by 20. For example, the continuous AC load is 2000 watts. The current (amps) is: 2000/20 or 100 amps at 12 VDC."

That is just wrong. divide wattage by 10 to get amperage for 12V DC. So that would be 200 amps for their example; twice as much. If you want the whole 3500 watts out of the inverter that's going to be 350 amps. Thus the need for serious wiring and a serious battery bank. Not usually recommended to draw that much wattage from a 12V system.

Note that this is a MSW inverter. The A/C might not be entirely happy running off it. Pure sine wave would be a better and more expensive choice.

BB.

Re: does size matter

The short answer--if this is your only power usage (a few hours a week of very heavy electrical loads), then you would be better off with a good quality genset. I like to suggest two generators... One smaller one (like the Honda eu2000i--1,600 watt) which will give you >9 hours on a gallon of gasoline with random/low average loads. And a second larger genset (3,500 to 5,000 watt) that you can use to power large tools / pumps / etc. for a few hours a week, when needed (may push 1/2 gallon of fuel per hour).

If you still want to go solar--Then there are several numbers you need. First is the maximum power / type of load. The most that you will need to power a 15 branch circuit is around 1,800 watts continuous load. HOWEVER, most induction motors take upwards of 5x or more for starting. There are some things you can do to "soft start" these tools, but that adds costs/complexity too.

And, as Kurt typed, the second important number is average Watts * Hours per day of use. For example:

• 100 watts * 10 hours a day = 1,000 WH = 1 kWH per day
• 1,800 watts * 1/2 hour per day = 900 WH = 0.9 kWH per day

For 120 VAC 15 Amp branch circuits, a Kill-a-Watt or similar meter is great for measuring your AC loads (does not do a good job measuring starting surge current).

So, your battery bank ends up being sized by your daily load, times 1-3 days of no sun backup, and 50% maximum discharge (x2) for good battery life:

• 1 day * 1/0.50 max discharge = 2x daily load
• 2 day * 1/0.50 = 4x daily load (usually a good place to start your planning)
• 3 day * 1/0.50 = 6x daily load

And, when you know your battery bank size, you can figure out your surge capacity (batteries can only output a limited amount of surge current... If you have low average power and high surge currents, you may need to up-size the battery bank to support it).

Flooded cell are typically able to supply around C/2.5 maximum surge current. Some brands/model of AGM can support C*4 surge current (typicallly used in computer UPS uninterruptable power systems--because they can suppy ~15 minutes of power before going dead).

But, you need to look at the maximum current. Again, as Kurt says, the "real" current is much higher than many 12 VDC inverter companies try to convince people. For example a 2,000 watt 12 volt inverter would need a circuit capable of this much current:

• 2,000 watts * 1/0.85 inverter eff * 1/10.5 volts cutoff voltage * 1.25 NEC safety factor = 280 Amp rated branch circuit

That is some pretty heavy copper cable. Add that a "Good Inverter" will surge ~2x rated power for a fraction of a second, you are looking at:

• 4,000 watts * 1/0.85 inv eff * 1/10.5 cutoff = 448 amps of surge current

And the wiring has to have less than 1 volt (11.5 volt battery - 1 volt drop = 10.5 volt cutoff) drop or the inverter may drop the load.

Battery wise:

• 4,000 watt surge * 1/12 volt bank * 2.5 flooded cell surge = 833 AH @ 12 volt flooded cell battery bank
• 4,000 watt surge * 1/12 volt bank * 1/4 AGM cell surge = 83 ah @ 12 volt AGM cell

But, designing wiring to pull 448 amp surge from either flooded cell or a 83 AH AGM battery is not trivial. Lots of heavy copper.

Typically, I like to suggest that for >1,200 watt loads, you use a 24 volt battery bank. And for >2,400 watts, you look at a 48 volt battery bank.

In the end, for an off-grid solar PV system that will make you happy--Need to understand/measure/estimate the needs of your loads first, then go from there.

If you are starting with only a few days of use, a good place to start (many times) is a genset to support your large loads, and a small battery bank + inverter + AC charger to support your small loads (quiet time in evening/morning to run lights, laptop computer, cell phone, radio, etc.). Use battery for non-work hours, and use the genset to run your shop tools and recharge your battery bank during the day.

You can add solar panels later (or even a few now) to help keep the battery bank up without needing a lot of generator runtime.

-Bill

waynefromnscanada

Re: does size matter

Table saws and air conditioners are relatively huge loads for a solar system, and will demand a likewise large solar system/charge controller, inverter, battery pack, heavy cabling and lots of money. A system to run a few CFL lights and a smaller, efficient TV is a totally different thing, and more in line with a first time "learning system".
Good luck which ever way you go.

niel

Re: does size matter

i'll agree for you to get a generator for that. i don't believe it would be worthwhile sinking lots of $ to do this via solar unless you have circumstances that would necessitate it. you would need more than you state anyway as you have severely underestimated what is needed.