Building my first system

Re: Building my first system

A few folks here were believing you to be taking the 50 vs 60 Hz and split phase 120/240 vs 230 VAC differences a bit lightly.

We try to keep everyone safe when making recommendations. The 50/60 Hz and split phase vs non-split phase are one of those issues that boarder on the edge of being a safety hazard if done incorrectly and/or with the "wrong" equipment (in North America).

But after a couple of warnings--And somebody saying they know what they are doing--Folks are going to back off (perhaps with one more warning from a different angle).

We all really do care here and 1) don't want people to get injured and 2) not waste money on mistakes that (many) of us have made in our younger lives.

Please take it that we do care and were very concerned that you may not know the level of risk doing this.

The problem for us on the other-side of the Internet Screen is we don't know the level of knowledge of the other writers.

There are many things that can be done safely with electricity/electronics (and even neat ideas that can save money/problems)--But they usually do require a more detailed discussion. The "what ifs" that come with power and safety can quickly become quite complex (ask about ground bonding neutrals and generator sets--That will be good for another 20 posts and a few links to other sources/threads :roll:).

In the end, if you have equipment that can work well with 230 VAC and 50 Hz--Not a problem. And sometimes, some of these inverters designed for the "world market" can be reconfigured between voltages and frequencies.

But one of the problems with "world suppliers"--Specifications and documentation is sometimes confusing--For example from the Ebay.au listing:

**Note: this item is for 220~240vac output only.
Adjustable AC input voltage 90~280VAC
Input Voltage Range 90~280VAC (Appliance mode), 170~280VAC (UPS mode)

It can get confusing when you mix all of the equipment/solar-AC charging/etc... It is a complex piece of gear.

-Bill

Re: Building my first system

OK--There are two ways to design your system... One way is based on the loads you want to support, and another way is to design the system around some key component you may already have.

Since I do not know your power needs, but you already have 4x D batteries (I am guessing around 12 volts @ 225 AH)--We can start there. The battery bank is the "heart" of your power system. All of the other "stuff" hangs off the batteries and you can look at each circuit (solar charger, wind charger, AC inverter, DC loads, etc.) as a completely independent circuit (from an initial design point of view). All of the circuits connect to the battery bus through (typically) fuses/breakers on the + bus connection (to limit short circuit current to all of the various branch circuits).

Home running your wiring back to the battery bank is also a good idea for other reasons--Many chargers/loads create electrical noise (120 Hz for 60 Hz AC inverter, 100s-1,000's of Hz for solar charge controllers, etc.) and running the branch circuits to the battery bus helps reduce "crosstalk" noise between devices (i.e., electrical noise on the battery bus can confuse charge controllers).

Next--Picking a Battery Bus voltage--I think you are looking at 24 VDC bus (from your other inverter choice)--So 2x D batteries in series, and two parallel strings is just fine. Suggest that you review the wiring suggestions from this website to ensure that they "share" current correctly.

Next, the size of solar array to support a 24 volt @ 450 AH battery bank at C₂₀ battery rate (that is a good size bank for a cabin/small home/emergency backup power). We suggest 5% to 13% rate of charge. No less than 5% (for a backup system, weekend system) to 10-13% for a daily use system (all based on good battery life/not too much maintenance headaches with under charging system). Over 13% is OK--Just usually a "waste" of solar panels and you should have a remote battery temperature sensor to ensure the batteries do not overheat during charging with higher currents (all based on our rules of thumbs for generic battery installations).

• 450 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 847 Watt array minimum
• 450 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 1,695 Watt array nominal
• 450 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 2,203 Watt array "cost effective" maximum

So--We now have a range of solar array that "make sense" for your battery bank.

You have picked your panels:

Up Solar (China) Solar cell
Maximum Power 240 Wp + - 3%
Maximum Power Voltage 30.2 volts
Maximum Power Current 7.9 Amps
Open Circuit Voltage 37.6 Volts
Short Circuit Current 8.40

Note that this panels are Vmp~0.2 VDC. That is sometimes called a "24 volt" panel--But it is not really a "24 volt" panel as we would use the term for off grid power systems.

A Flooded Cell battery bank (24 volts) usually needs ~29-30 volts to fully/quickly recharge, and for equalization can use 30-31+ volts. Solar panels Vmp is a "marketing" rating at ~77F/25C (simulated sun for less than a minute). Solar panels mounted on your roof under the sun will operate at much higher temperatures (sometimes as much as 60F over ambient--Hot summer day anyone?). That can reduce Vmp by as much as 20% or (30 volts * 0.80 24 VDC.

So under hot sun, with controller and wiring drops, we normally suggest a Vmp~35-39 volts or so for charging a 24 volt battery bank with a PWM charge controller (like the Xantrex/Schneider Cxx series).

If you want to use these panels, you really need to put 2-3 of them in series and get a MPPT type charge controller. These are "buck mode" digital converters that can "efficiently" take high voltage/low current from the solar array and down convert to low voltage/high current needed to charge the battery bank (~95% efficient).

For a "nominal array" of 1,695 Watts / 240 Watt panel = 7.06 panels. Since you need 2-3 in series--That would be 4 strings * 2 parallel (8 panels) or 2 parallel strings of 3 in series (6 panels).

And MPPT charge controllers are much more expensive than PWM--~$300-$600 each for a good quality MPPT charge controller.

Another choice would be to get true "24 volt" panels (if you can find some for a good price). They typically cost more these days (lower production volume?) than "GT Panels"... You have to pencil out the costs between The China Solar+MPPT controller vs "24 volt panels" + PWM charge controller.

In general, for many reasons, it works out better to use PWM controllers for systems under ~400 watts, and MPPT for systems over ~800 watt arrays. But purely your choice.

Next, need to choose an AC inverter... We look at what a typical flooded cell battery is capable of outputting (again, since we do not know your loads). For an off grid home, we would be looking at a C₂₀ discharge rate "nominal". C₈ rate for maximum continuous; C₅ rate max short term continuous; and C_2.5 discharge rate for maximum surge/short term.

• 450 AH * 24 volts * 1/20 * 0.85 inverter eff = 459 "nominal" average load
• 450 AH * 24 volts * 1/8 * 0.85 inverter eff = 1,148 "max" average load
• 450 AH * 24 volts * 1/5 * 0.85 inverter eff = 1,836 "short term" load
• 450 AH * 24 volts * 1/20.5 * 0.85 inverter eff = 3,672 "surge" load

So, from the above, I would suggest a ~1kW to ~1.5-1.8kW AC inverter based on your battery bank (assuming the AC inverter can support 2x rated output for surge).

A 2kW AC inverter is a bit on the large side--But if you do not expect 6kW surge from the battery bank--You should be OK.

I know nothing about the AC inverter or the company. And 6kW sounds like a lot of surge for a 2kW inverter--But if you do not operate near the 2kW/4kW surge limits--The battery bank should be OK.

Your thoughts/questions?

-Bill

Re: Building my first system

More or less, 1,200 watts or less, a 12 volt system is fine. 2,400 watts load or less, 24 volts is recommended. Over 2,400 watts, look at 48 volts.

Electrically speaking, you are correct, using a higher voltage battery bank has the advantages of lower current and for some items like solar charge controllers--They are rated for 12/24/48 volts at XX amps. A 48 volt battery bank, the charge controller can manage a 2x larger array vs the same controller on a 24 volt batter bank.

For example, an 80 amp MPPT charge controller can manage:

80 amps * 29 volts charging * 1/0.77 panel+controller derating = 3,013 Watt array "cost effective" maximum
80 amps * 58 volts charging * 1/0.77 panel+controller derating = 6,026 Watt array "cost effective" maximum

Another advantage is the ability to use few strings of batteries. My personal suggestion is to aim for one series string of batteries. And a maximum of 3 parallel strings of batteries--More than 3 is difficult to ensure current sharing and the chances of "hidden failures" (shorted/open cells, open cables/connections, etc.) becomes more likely (and can kill a string of batteries or entire banks before you catch the failure). Also, having so many battery cells to check water levels on monthly--It becomes a real pain for maintenance...

In your case, you have the same amount of battery cells to check (two strings of 12 caps, vs one string of 24 caps). The one downside is 48 VDC (really 60+ VDC) rated switches/fuses/breakers is a smaller subset of hardware. You need to get the correct rated components to reduce the risk of fire.

Here is an example of a typical home breaker on ~300 VDC simply being switched off:

And here is another video of sending heavy current through a nail/screw/bolt in a DC short circuit (like your battery bank):

We are very careful here in our recommendations regarding safety. Solar systems have a lot of "interesting failures":

Panel Fire Question

Since you are leaving the system in your home, unattended at times, you want/need to be extremely careful about wiring and using the correct components.

Regarding solar charge controllers, the C60 is a PWM controller... MPPT controllers from the US are not cheap--And many from Ebay/overseas are not very good (or even what they claim--Some have been PWM controllers with a MPPT label slapped on).

Sorry for some of the warnings--I am sure you know what many of the requirements/dangers are--But to error on the side of caution. :-)

-Bill

Re: Building my first system

With the correctly rated components, 48 VDC systems are fine. Remember, I am "sketching" your system out around the batteries you currently have and sizing a "plausible" system around them.

We really like to define a system around your loads (using a Kill-a-Watt type meter is usually very handy, plus it is great to use around the home and find the appliances that are drawing more power than they are worth, turning off electronics that are on "standby", but still use a lot of power like DVRs, older electronics, etc.).

In the end, it really depends on what you want to do. If you want, for example, to run your refrigerator, washer, laptop, well pump, TV/Radio, etc. for an extended time (weeks, months, etc.)--That would be around a 3.3 kWH per day system for many people (a very efficient home/cabin with a "near normal" electrical existence)--Then you would want to look at a 48 volt system (and larger battery bank, etc.).

If you do not run larger loads (well pump) or extended run-time loads (refrigerator) and can live with ~2.3 kWH per day (plus or minus, depending on seasons/weather)--Then a 24 volt system would be very nice for you. And if/when you eventually replace your battery bank, you could look at larger AH rated batteries (cells) and have a single string of ~450 AH cells (2/4/6 volt depending on your choice of battery brand/models/etc.). That would cut your bank wiring issues, and get you down to 12 cells (1/2 the amount of checking/watering).

Much of this is really a matter of choice. For some folks who like to tinker--It is a hobby and the maintenance/puttering around is 1/2 the fun of the system.

For others, off grid solar is the only way they can have a "near normal" modern life miles (or hundreds of miles) from power lines/stores/service. And they need a (as close as possible) a 100% reliable system with few surprises and not a lot of maintenance.

The problem with emergency backup power systems is that Solar usually only makes economic sense if you are running from solar ~9+ months of the year. If you are running for a few weeks every couple of years (ice storms, etc.)--Then many times a couple of smaller gensets, + fuel storage, and backup food/cooking supplies is enough.

Fro me, I live in a major metro area... Even if I did a full off grid capable system--The city water and sewer will be out after a few days/week of regional power failure... Our "big one" is earthquakes (some areas are wild fires)--And while true knocking homes over is actually quite rare in California, having an off grid solar power system bolted to a red tagged home is not going to be helpful either. Several of the reasons I went with a small (1,600 watt) inverter-generator and 20 gallons of stored fuel. Enough to get me 1-2 weeks out without too much problems. And the extra fuel may be helpful for cooking (gasoline powered camp stove) and bugging out if needed (extra fuel for the car when the service stations are shut down/no deliveries).

Long answer--Short answer is it is usually expensive/difficult to add onto (or "grow") an existing off grid power system. If you chose to grow by more than a factor of 2x, then you usually have to go to a larger (higher voltage) AC inverter, need a completely new/redesigned battery bank, and a whole bunch more solar panels (getting matching solar panels a few years later can be difficult--So you end up getting a second charge controller and building out a new array because you cannot mix/match your old+new panels into a single array).

Don't make your decissions on a single statement (60 VDC rated breakers/fuses are a bit more expensive/fewer "cheap" sources available).

Do a paper design of several systems--See what component issues you may have (sizing of wire, breakers, etc.) and how much power your system is capable of (using my rules of thumbs example)--See which gives you the best bank for the buck.

For many off grid people, they actually will put the power system in a separate shed--And if there is a genset, put the genset+fuel supply in yet a second shed--You don't want a single failure to take out your entire off grid system and leave you with nothing (genset fire, batter+solar still works; battery fire, still have generator+fuel, etc.).

-Bill