Newbie Here - Basic Setup Questions

[Cedric Vann]

Greetings All,

I apologize if I missed a "sticky" for basics/newbies or if I am posting this question in the incorrect forum.

I'm completely new to solar power, just purchased a Xantrex 1800 Pure Sine Inverter 24V and the Xantrex C60, looking for batteries and panels now.

For the panels I see that panels operate at voltages higher than their respective rating, with my inverter being 24V do I need 24V panels?

Additionally can 12V panels be wired in series to be 24V?

Thanks in advance for the help!

[Joe Blake]

Hi Cedric,

If you look closely at your panels you may see that the "voltage" is without a load. When you add a load that will bring it down, and so there may be a voltage which is "under load", and that should guide you as to size of batteries.

[Dave Schwartz]

There probably is a basic setup sticky somewhere but the jist of it is that since your battery bank voltage is set at 24v by the inverter, you will need 24v of battery (either 2-12v or 4-6v in series [there are also options with industrial single cells but you probably don't need or want to go there]).

You will need to determine your total daily draw in kilowatt-hours (and multiply by a factor of 2, 3 or more to account for cloudy days where you won't be able to fully recharge). After conversion of kilowatt-hours to 24v amp-hours, and accounting for efficiency losses and the maximum depth-of-discharge you don't want to take your batteries below - varies by type, that will give you the amp-hour capacity your battery bank needs to have.

You are also limited to a maximum of 1800 watts instantaneous load by the inverter so that means a bit more than 75 amps maximum draw from your batteries so you'll also have to choose batteries capable of this discharge rate if you ever intend to go there.

Since the C60 is not an MPPT charge controller, you will have to build panel strings that, when loaded, will be somewhere over 30 volts but which do not have an open-circuit voltage higher than 55 volts (the C60's maximum input voltage before the magic smoke comes out). Then you will parallel multiple strings to get the current up to where it will fully charge whatever batteries you calculate you need on a clear day with whatever number of full-sun equivalent hours you get for your location and time of year. The C60 can only do 60 maps charge current so the point of diminishing returns in buying panels is about 1600 watts. More panel will allow more charge to be harvested on cloudy days or either side of noon but the controller will not use more than 60 amps (plus a bit for conversion losses).

I'm sure we can help you further but it starts with you having to know your maximum instantaneous and whole-day cumulative AC load.

[Cedric Vann]

Thanks for the detailed advice guys, and thanks for detailed math Dave.

The basics of what I'm trying to accomplish:

Looking to run my detached garage off-grid at first, with this less critical system I hope to learn the do's and don'ts, and refine my knowledge before taking on the more critical living areas of my home.

My garage currently runs on a single 20A circuit, with a number florescent light fixtures (4' T8's) that I plan to upgrade to LED as part of this project, 6 to 8 Dewalt battery chargers, the occasional air compressor (very light use), a refrigerator/freezer that may be downsized to fridge only, a radio, garage door opener, occasionally a shop vac, and I think that's about it.

I found a local source for Trojan T-105 GC2 6V 225Ah Deep Cycle Flooded Lead Acid Batteries, at $158 per battery seems to be the most cost effective battery in my area, still looking and researching, and I'm certainly open to suggestions here.

With this battery I planned to go with 4 to get the 24V that I need, but still researching to make sure I buy the best battery for my needs.

Thanks again for the advice Joe & Dave.

[Rob Beckers]

To further Dave's comments: With the charge controller you have you'll need either 72-cell panels (1,500 Watt total maximum) or strings of two 36-cell panels in series. It takes 72 solar cells to reliably charge a 24 Volt battery bank. The most common panels nowadays are 60-cell, and even though their name-plate states that they run at 30 Volt they will not reliably charge a 24 Volt battery; the panel Voltage goes down as they get warm, and battery charge Voltage goes up when they get cold.

I've written a small article about charge controllers and panels that may help explain things a bit more: http://www.solacity.com/mppt-vs-pwm-charge-controllers-whats-difference/

-RoB-

[Cedric Vann]

I appreciate the additional data Rob, now I'm regretting the purchase of the C60, and after some additional reading I'm thinking that I'll either sell this unit or save it for another purpose.

Some additional notes on my project, I have now purchased 3 used Xantrex 1800 Pure Sine inverters, 2 of which showing no signs of use, the 3rd showing light signs of use, feel like I got a good deal on these, hoping it wasn't a mistake buying used inverters. Regardless I'm willing to take the risk and truck forward, and replace these inverters down the road if they prove unreliable.

Next step: find a budget friendly MPPT charge controller.

Also thinking about solar and wind, I live in Southern WI, thinking wind would be a good addition to the 4 hours or so of good sunlight for my area.

Any recommendations on a budget friendly MPPT charge controller that can handle a <2000 watt wind turbine and 800 to 1000 watts worth of panels? Hoping to spend less than $1000 on the new controller.

Thanks again for the advice guys!

[Dave Schwartz]

Those are nice batteries for R.E. use. They are capable of the 83-ish amps you need to give 1800 watts (90% efficiency in the inverter specs). However, that's still 600 watts less than your existing 20A circuit is capable of so management of those high-draw items will be key. ShopVacs brag about being 10 to 12 amp and I don't know how big your compressor is (and they sometimes come on randomly). This is what I meant by saying you have to figure out your peak load and how you're going to manage it to stay under 1800 watts continuous (the 2700 surge is only good for a few seconds for when a motor is starting).

If you look at the battery specs, it says they should have a comfortable 1.5kWh capacity so you'll have a total 6kWh. Since you have mains power, you may be able to get away with using 3kWh of that a day and plug in a hefty charger on the third sun-less day (real off-grid people would need a generator). To see if 3kWh is enough for you, you need to sum up all the load-hours of all the items you will be using. If 3kWh is not enough you may need to go with 8 batteries (2 parallel strings of 4).

The 60A charge controller should be capable of charging one or two strings of those batteries with enough solar panel behind them and Rob's given you some good information there. You'll just need certain minimum number of them to give you the bit-more-than-60A to achieve maximum charge current. As I said, more panel than the minimum will lengthen the max-charge-rate window, which you may need depending on how long you need to fully recharge the battery system you decide on.

[Dave Schwartz]

I guess you posted while I was typing...

If you have multiple of those inverters, you could create multiple 15A AC circuits with them, paralleled off the batteries. You might not get away with more than one inverter paralleled on a single string of batteries... they might not like the potential 150A DC draw if you might ever have both running full out, but two paralleled banks would be fine with it. Essentially you can treat the components (inverter, 24V string of batteries, charge controller) like building blocks with everything tied together on the DC side. Just keep the capacities matched.

Then you have to figure out how you're going to charge the batteries.