Designing my set up which is best 24v 48v

RandyH

Hello, I am new and starting to design my set up. So far I have a Midnight Classic 200 and six 230 watt solar panels.(I plan to add at least 4 more panels later. I am shopping inverters but need to decide on a voltage now. Can I get opinions on which is the best 24v or 48v
I want to have a battery back up system that will run about 6 circuts in my home from a gen transfer switch. looking at outback 3500w or magnum 4000w but noticed the magnum 4400w 48v is same price as 24v 4000w. It seems to make since to get a extra 400 watts by stepping up to 48v for the same cost.
TV, Fridge, LED lights, Microwave. Not looking to run any 240v stuff but would be nice to have the option. Seems like the Magnum 4400 is overkill but would be ready if I wanted to expand. Is there any drawback to going 48v? It seems most people use 24v and just curious why not go with higher volts less amps smaller wire less cost?
My system will be hard wired into my home. Thanks

Cariboocoot

Re: Designing my set up which is best 24v 48v

Welcome to the forum.

There are reasons for choosing one system Voltage over another. Some of which are explained here: http://forum.solar-electric.com/showthread.php?15989-Battery-System-Voltages-and-equivalent-power

A big consideration is how much power you need to store up. The more Watt hours you need to have on hand, the more sense it makes to go with higher Voltage as opposed to larger Amp hour capacity.

There is a downside to 48 Volts, and it has to do with equipment selection. Starting with batteries. If you don't need the greater Watt hours you buy batteries you don't need simply to get enough in series to make the 48 Volts. Then there is the panel issue; higher system Voltage = higher array Voltage and you can then run into trouble with high Voc issues in cold weather depending on which panels you use and where you are located. Finally there is the difficulty in handling 48 Volts: many circuit breakers and switches are not rated for the 60+ Volts that is present when charging the system, making them unsuitable. Thus you have to find ones that will work, and that inevitably means more expensive ones.

My advice is to, as always, get a good number for your loads both in terms of peak Watts and total daily Watt hours. Then you will see if your system would work better at 48 than 24. Otherwise the "savings" on the inverter and wire may be consumed by extra batteries that aren't actually needed and more expensive circuit protection/switching.

It's never as simple as "this" is automatically better than "that".

Photowhit

Re: Designing my set up which is best 24v 48v

RandyH wrote: »

... starting to design my set up. So far I have a Midnight Classic 200 and six 230 watt solar panels.(I plan to add at least 4 more panels later...

Well design first then start buying things! This will save you money! like if you had purchased all your panels together you would save a lot on freight 10 panels wouldn't cost much more than 6 to ship! Also adding 4 panels might not work since you've gone witha classic 200 I would assume your array is a ways away from your battery location, but for the number of panels in series will likely change dependent on voltage output.

For a pure off grid, supporting a home I would prefer 48 once you get past about 2200 to 2400 Watts of array, since that is about the break for needing another charge controller for a 24 volt system. Like you I ended up having componets of my system before I knew my needs, in my case I purchased a new home and already had a large 24volt forklift/traction battery for my existing system.

The extra 400 watts of inverter really isn't a big deal, it's very unlikely you will run at this high level very much or very long. As a backup system you should define your loads, figure out how much energy you will be using and for how long. 240volt stuff you might not have thought of like well pump and water heater (though most water heaters will be out of this range). A kill-A-Watt meter is handy for checking out your loads!

Merry Christmas
-Whit (waiting on a christmas video chat with my family)

BB.

Re: Designing my set up which is best 24v 48v

Not commenting on the Magnum in a positive or negative tone (the Magnum series are great inverters)--Just observations for you to answer...

Regarding 120 vs 120/240 VAC usage--check how much power your planned inverter can put into 1 leg of 120 VAC--For the Magnum sine wave series, it is a maximum of 81% of rated output into 120 volt load.

Battery charging from genset... Can the inverter take a 120 VAC only input (smaller genset). What is the maximum charging current limitation into the battery bank with a 120 VAC genset (quick read of the manual--I am not sure if it is 50% of charger capacity or if y0ou can parallel 120 VAC into the inverter and get 120/240 VAC out.

This inverter family is accurate to +/- 0.1 Hz... Plenty accurate enough for running any loads. However, clocks that use AC line for timing may not be very accurate (1.44 minutes per day drift maximum).

Battery Wiring... 48 volts is going to cut your wiring costs in 1/2... 4/O vs 2/O wire AWG. Less voltage drop, can use longer wire (if needed) between battery bank and Inverter at 48 Volts.

Battery Bank Capacity--Check your options for battery sizes that you may use for your bank. Personally, I do not like to parallel battery strings--Or at least, limit the number of parallel strings to 2-3 maximum. It requires more effort on your side to balance wiring, check current sharing once in a while with a DC current clamp meter, more battery cells to water, etc. at higher voltage (12 cells vs 24 cells for 24 vs 48 volt bank--paralleling strings 2-3x more cells to check), more wiring/fuses/breaker (per string) should be used for paralleling strings.

The ultimate size (AH/bank voltage) really depends on your maximum continuous/surge power (and solar array) needed. You may have a 4kW inverter, but if you plan on only using 2kW peak loads, the battery bank could be 1/2 the capacity.

Very roughly, the AH capacity of the battery bank should be around 100 AH per 1kW of inverter loading/solar array rating @ 48 volts (or 200 AH per 1 kW for 24 volt battery bank).

Battery choice--AGM are about the nearest you will get to a "perfect" lead acid battery type. Very high surge current, no water to check, much cleaner (no leaks, acid/hydrogen fumes in normal operation, etc.). Drawbacks include ~2x as expensive, may not last quite as long as similar flooded cell batteries, easier to damage if over charged.

If this is your first off grid experience, you may want to stick with flooded cell (and maybe less than "premium" batteries)... Most people here have probably "murdered" their first battery bank or two due to mistakes (servicing issues, failed wiring or failed cell that damages bank,, somebody left a load on when nobody was home, guests/kids use too much power and kill bank, etc.). It also lets you "adjust the size" of your battery bank down the road for less costs (we recommend not installing a "massive battery bank"--We try to "balance" the battery bank against planned loads and available charging current for cost effective usage and long life--A "too large" large battery bank with too small of charging sources can take years off a battery bank's life, for example--and be very expensive to replace 5-10 years down the road).

Before you buy any hardware--Do your paper design and costing. And check the capabilities of the (several?) paper design(s) against your planned needs.

More or less, figure on replacing batteries every 5-10 years and electronics (inverter, charge controllers, etc.) every ~10+ years (they can last longer, but have money in bank for repairs/replacements). So avoiding "over design" can help keep your future costs down too.

-Bill

mike95490

Re: Designing my set up which is best 24v 48v

Here's another vote for 48V, if you can configure your panels for it, you may even need 2 strings to make it work when you add more panels.

Half the copper costs, less ohmic loss, easier battery configurations.

But, depending on the inverter, you may need a balancing transformer, to keep the 2, 120V legs happy and within amp specs.

Battery size, comes real easy, 100ah, 200ah, 400ah are common battery case sizes. 6V , 200ah golf cart batteries from a discount store, should last you 3-5 years. 1300 watts of panel, at 55V (recharge voltage) is a little over 20 amps, so the panels indicate a 200ah battery. More battery, and you will have a tougher time keeping them charged when they get real low.
200A @ 48V gives you 4,800 watt hours @ 50% discharge.

And a generator & charger, for backup charging. I've been running my generator 2 hours a day, this last week, because of cloudy weather, to keep the batteries alive.

Photowhit

Re: Designing my set up which is best 24v 48v

I think in a purely backup situation that you could shoot for a 5-6% charge rate hence a greater reserve, the solar just does what you could do from the grid cheaper, but I suspect the OP might have other outage issues.

Can you wire a house for this "...I want to have a battery back up system that will run about 6 circuts in my home from a gen transfer switch..." I would think to do this all 6 ciruits would have to be on a subpanel? usually don't you just have a transfer switch running into your main panel?

Cariboocoot

Re: Designing my set up which is best 24v 48v

Photowhit wrote: »

I think in a purely backup situation that you could shoot for a 5-6% charge rate hence a greater reserve, the solar just does what you could do from the grid cheaper, but I suspect the OP might have other outage issues.

Can you wire a house for this "...I want to have a battery back up system that will run about 6 circuts in my home from a gen transfer switch..." I would think to do this all 6 ciruits would have to be on a subpanel? usually don't you just have a transfer switch running into your main panel?

Kind of a matter of "what circuits?": how much power do they use and how much generator is available to back them up.

It's not uncommon to do this with an off-grid inverter-charger, feeding AC through it to the critical loads and allowing it to keep the batteries up while grid is present. Grid goes down, loads are seamlessly switched to battery power. Sizing the inverter & CP to match the load needs is a matter of simple math. Solar can be an optional extra, as can generator input (which would require a separate transfer switch so that the inverter and loads are fed either from mains or generator but those two sources never connect to each other).

So many ways to do this stuff! It's dependent on the particular install, of course.

mike95490

Re: Designing my set up which is best 24v 48v

Even better, for "emergency backup" just have a generator. Skip the batteries & panels. Look at the auto-throttle inverter generators, for some really fuel stingy power generation. rotate a couple cans of gas, as much as you think you need to last the typical "disaster". Or have expensive batteries, inverter, charge controller, and glass solar panels (check them out after a hurricane rips all the power poles down, or a 3 day ice storm with cloud cover and idle PV panels).

waynefromnscanada

Re: Designing my set up which is best 24v 48v

mike95490 wrote: »

Even better, for "emergency backup" just have a generator. Skip the batteries & panels. Look at the auto-throttle inverter generators, for some really fuel stingy power generation. rotate a couple cans of gas, as much as you think you need to last the typical "disaster". Or have expensive batteries, inverter, charge controller, and glass solar panels (check them out after a hurricane rips all the power poles down, or a 3 day ice storm with cloud cover and idle PV panels).

Since you did mention was only for backup in case of power outages, I totally agree with Mike. And for many reasons.
Good luck what ever you do.