Sizing your backup: how to judge risk vs cost tradeoffs

[Cariboocoot]

Even in a purely off grid situation there is no sense in having a 6kW inverter with only 200 Amp hours of battery. If you ever came close to the inverter's total output capacity the battery bank would fail very quickly: 6000 Watts @ 48 Volts is 125 Amps. That would be like trying to suck half the battery capacity in an instant (the higher the discharge rate, the lower the total actual battery capacity).

And yes, it is complicated.

[westbranch]

choose to disable selling excess power (from these panels) to the grid.

Ugh, this is complicated :)

I don't comprehend how you would be 'selling' during an outage as GT inverters need incoming power before they can sell...???

Oh, I see you corrected that in the second post...

[Cariboocoot]

I don't comprehend how you would be 'selling' during an outage as GT inverters need incoming power before they can sell...???

Not during an outage: when the grid is up. The XW with a tiny battery bank has fits trying to Sell to Grid because there's not enough buffering.

What I'm confused about is needing UPS for laptop computers.

[mtdoc]

I am defining "critical loads as "loads that I want to have available in an outage"
and "UPS loads" as "loads I want to have protected against transient events"
In particular, the 5 computers and wide screen are in the "UPS loads" category. During any kind of outage (that lasts more than 10 minutes), they would be turned off.
The microwave is in between; it is a convenient cooking option (a lot less power needed than the electric range). We also have the grille, and cook stoves, if we want to cook
something more elaborate.

I want the inverter to provide both functions. So it should be able to deliver enough power to deal with transient events, but it doesn't have to deliver that kind of power
over an extended period of time.

That's why I was hoping the Xantrex would do the job. BUT, Dusty warns me that 200 AH and 2kw of panels and 6kw of Xantrex inverter is likely to stress your batteries,
or make you choose to disable selling excess power (from these panels) to the grid.

Ugh, this is complicated :)

Based on my experience, the Outack GVFX 3648 would handle this job fine. Unless you're running supercomputers and microwaving at 5KW at the same time, I would think it's 6000 watt surge capacity would handle this. It handles my baseline loads for a 3000 sf modern home (200-300 watts) and occasional very high surges -e.g. toaster oven on when 1 hp septic pump - (with 30 amp inrush current) kicks in and runs for 15 seconds.

Worst case I suspect is if you happened to have almost all of your "critical and UPS loads" on while simultaneously running microwave on high when the grid power goes out- you might trip a 60 amp breaker or fault the inverter and lose power briefly - but really what are the odds that the grid power goes down at exactly that time.

No doubt the XW6048 would handle even that scenario.

You really need to measure your "critical and UPS loads" and have a well defined power need to know for sure which inverter would cover you.

The battery needs are another issue. The AH capacity will be defined by the "non UPS" loads you need to run during an extended outage. Once you know how much battery you need for that purpose - you can limit the number of panels you devote to this purpose to avoid the AC ripple issues (using the 100AH per KW of PV rule of thumb). i.e. it's not really the inverter size that determines the size of battery bank you need - but the size of battery bank you need to maintain your critical loads will determine the number of panels you devote to this back-up function. Your situation is a bit different than most in that you have plenty of panels to start with and are working backwards from there.

[BB.]

There alternative configurations.

You can do a XW6048 inverter + 600 AH battery bank and no solar panels. Let the AC mains/generator keep the batteries up (and add solar panels later as funds/needs permit).

In theory, you can have your 6kW XW + 600 AH battery bank and up to ~6kW of solar panels and GT inverter.

Put the GT inverter on the protected side of the XW. Normally, XW operates as a battery charger and passes power from the grid to the protected panel, and the GT inverter will also feed the local loads and push any excess power out to the grid (and turn your meter backwards).

During a power failure, the XW will switch to off grid mode (and switch off from grid supply). At that point the XW will supply power or absorb power form the GT inverters. When the battery bank is full, the XW will shift frequency from 60 Hz to 62 Hz which will "knock" the GT inverter off line (or micro GT inverters) for 5 minutes, drop frequency back to 60Hz for GT inverter(s) to restart... Repeat cycle as battery state of charge dictates.

And still works with a backup genset (more than likely, the GT inverter will not turn on when AC generator is operating--Standard generators cannot hold 60Hz +/- 0.5 Hz (inverter generators may sync--which raises a whole more questions about maximum AC voltage, back feeding inverter/generator, etc.--much of it potentially not good).

Anyway, when I did some sample cost of power before (capital costs+solar array+electronics with 10 year life, battery bank with 8 year life, 20 year life on panels+wiring), I got a cost of around $0.45 per kWH, even with Net Metering and grid power--So you are not saving money (but, as always, I suggest you do the design/math yourself for your needs--my numbers are just "ball" park to see what your threshold of pain is).

If you have a few computers and only need short term (15 minutes of power) backup--You may be better off with standard computer UPS systems... Overall, less cost and less energy losses. UPS systems do have their own maintenance issues (small UPS for desktop systems may last 2 years before replacement--usually not even worth replacing the battery).

-Bill

[danielh]

Based on my experience, the Outack GVFX 3648 would handle this job fine. Unless you're running supercomputers and microwaving at 5KW at the same time, I would think it's 6000 watt surge capacity would handle this. It handles my baseline loads for a 3000 sf modern home (200-300 watts) and occasional very high surges -e.g. toaster oven on when 1 hp septic pump - (with 30 amp inrush current) kicks in and runs for 15 seconds.

Worst case I suspect is if you happened to have almost all of your "critical and UPS loads" on while simultaneously running microwave on high when the grid power goes out- you might trip a 60 amp breaker or fault the inverter and lose power briefly - but really what are the odds that the grid power goes down at exactly that time.

No doubt the XW6048 would handle even that scenario.

You really need to measure your "critical and UPS loads" and have a well defined power need to know for sure which inverter would cover you.

The battery needs are another issue. The AH capacity will be defined by the "non UPS" loads you need to run during an extended outage. Once you know how much battery you need for that purpose - you can limit the number of panels you devote to this purpose to avoid the AC ripple issues (using the 100AH per KW of PV rule of thumb). i.e. it's not really the inverter size that determines the size of battery bank you need - but the size of battery bank you need to maintain your critical loads will determine the number of panels you devote to this back-up function. Your situation is a bit different than most in that you have plenty of panels to start with and are working backwards from there.

I think I have a handle on the "non ups" loads; it is similar to what Bill sketched out earlier. Basically, 200AH of battery fed by a 2kw of pv panel, with 6500w generator as a supplement. Or, if I am careful and/or willing to be more frugal, a honda 2000eu as the generator.

The question is then how to provide UPS to a larger set of loads, perhaps (on occassion) a lot larger. As you succinctly point out, should the UPS be needed (a transient grid event) during one of these much larger load occasions, a breaker might be tripped. Oh well, that's a rare event with minor consequences, and can be easily lived with.

However, there is a concernt: will this strategy, where battery capacity is undersized relative to inverter size, lead to rippling and other battery stress.
I might be misunderstanding, but it seems that this is likely to be the case with the Xantrex, at least if you want to sell excess PV production to the grid.
And to your knowledge, the Outback does not suffer from these same dilemnas

[danielh]

There alternative configurations.

Put the GT inverter on the protected side of the XW. Normally, XW operates as a battery charger and passes power from the grid to the protected panel, and the GT inverter will also feed the local loads and push any excess power out to the grid (and turn your meter backwards).

During a power failure, the XW will switch to off grid mode (and switch off from grid supply). At that point the XW will supply power or absorb power form the GT inverters. When the battery bank is full, the XW will shift frequency from 60 Hz to 62 Hz which will "knock" the GT inverter off line (or micro GT inverters) for 5 minutes, drop frequency back to 60Hz for GT inverter(s) to restart... Repeat cycle as battery state of charge dictates.

And still works with a backup genset (more than likely, the GT inverter will not turn on when AC generator is operating--Standard generators cannot hold 60Hz +/- 0.5 Hz (inverter generators may sync--which raises a whole more questions about maximum AC voltage, back feeding inverter/generator, etc.--much of it potentially not good).

This sounds like AC coupling? Is the xantrex frequency switching dependable enough (or do you still need load diverters).

Anyway, when I did some sample cost of power before (capital costs+solar array+electronics with 10 year life, battery bank with 8 year life, 20 year life on panels+wiring), I got a cost of around $0.45 per kWH, even with Net Metering and grid power--So you are not saving money (but, as always, I suggest you do the design/math yourself for your needs--my numbers are just "ball" park to see what your threshold of pain is).

If you have a few computers and only need short term (15 minutes of power) backup--You may be better off with standard computer UPS systems... Overall, less cost and less energy losses. UPS systems do have their own maintenance issues (small UPS for desktop systems may last 2 years before replacement--usually not even worth replacing the battery).

Thats what I have now... and what I am hoping to improve upon: avoiding replacing the desktop ups, getting a longer run time if needed, and having a broader arrange of equipment covered).

The question in my mind is: consider a battery backed inverter capable of providing more than 1kw to 100AH of battery capacity. Is that an invitation to trouble, even if the times when this excessive output (say, > then 2kw of output with a 200AH batterY) will almost always be when the grid is up (and the batteries aren't needed).

[Cariboocoot]

I don't think you're getting the hang of this.

Assuming that battery bank is 200 Amp hours @ 48 Volts you won't need 2kW of array to recharge it. 1200 Watts is more likely. Putting too large of an array on a battery system is a waste of money.

An inverter will put out what it is capable of and no more, and even then only if there is enough battery to supply the power demands. If you have more than 3500 Watts (actually Volt Amps) of stuff on a 3500 VA inverter it will fault. If it's less than that and there's not enough battery to supply the power the input Voltage will sag and the inverter will shut down.

The trick is to size the inverter and battery bank to accommodate the loads. You can't get away with scrimping on either and expect it to work. It does not matter which inverter you choose.

The same thing goes for hybrid GT: not enough battery, it doesn't work. No matter which inverter you choose.

The same thing goes for AC coupling to a battery-based inverter: too small a battery bank (even with dump loads) and you run the risk of failure.

For all these applications you need to be able to store a certain amount of power. In the case of the hybrid GT it is "short term storage" as the batteries act as filters but must be large enough to do the job. In the case of emergency power supply it is "long term storage" as they must be able to handle the loads until power returns.

Your trying to do it without batteries, so to speak, and that does not work.

[mtdoc]

However, there is a concernt: will this strategy, where battery capacity is undersized relative to inverter size, lead to rippling and other battery stress.
I might be misunderstanding, but it seems that this is likely to be the case with the Xantrex, at least if you want to sell excess PV production to the grid.
And to your knowledge, the Outback does not suffer from these same dilemnas

Ah, I see. BB or Cariboocoot would be better to answer that question, but my admittedly limited understanding of AC ripple concerns is that it's not a rare brief surge of inverter output beyond the 100 AH/1KW panel rule that is a concern it's more the regular "too high" inverting/selling to the grid that would come with too many panels relative to battery bank size that causes problems. I could have that all wrong though and will let the experts answer.

[BB.]

I am going to punt on the XW answer... The hardware appears to do what most people need. The support and firmware updating (if needed) tend to be a bit iffy at the moment (from what I have read here).

And, while peak/surge power is important, most of the time, what drives the design of the system is average power * hours of operation per day. Running a 1,500 watt inverter 20 minutes per day is usually much less of an issue (overall) vs running a 250 Watt desk top system 12 hours per day:

• 1,500 watts * 20 min/60 min per hour = 500 Watt*Hours = 0.5 kWH per day
• 250 watts * 12 hours = 3,000 WH = 3 kWH per day

And on and on...

The rules of thumb for battery/load sizing (100 AH @ 48 volts for 1kW of load/charging) is one rule of thumb.

Obviously, there is the important rule for Watt*Hours of load per day have to be replaced the next time the sun rises. So you need to meet both requirements to have a satisfactorily operating system. If you have a low peak power running many hours per day--The size of the array will probably be defined by hours of sun per day to recharge the battery bank.

If you have a low average loads but a lot of heavy power surges (pumping, cooking, etc.), then you may be forced to have a large battery bank to support the surge current--and a larger solar array to properly recharge the battery bank.

For example, a 3.3 kWH per day (~100 kWH per month) system is about right for a full off grid home, lots of conservation, yet still have an "electric home" experience (refrigerator, well pump, washer, lights, TV/computer, etc.).

Using PV Watts for Sterling Virginia, fixed array tilted up 39 degrees from horizontal, we get hours of sun per day by month:

Month    Solar Radiation (kWh/m 2/day)
1      3.59     
2      4.28     
3      4.80     
4      5.34     
5      5.32     
6      5.66     
7      5.46     
8      5.38     
9      5.07     
10      4.72     
11      3.56     
12      3.03     
Year      4.68

Lets say run without a genset for 9 month of the year... 4.28 hours of sun minimum for February:

• 3,300 WH per day * 1/0.52 * 1/0.77 panel+controller derating * 1/4.28 hours of sun per day = 1,925 Watt Solar Array Minimum

That is one simple array calculation and assumes that you will have to run a genset during bad/cloudy weather in the winter to make up for poor production days.

Next, sizing the panel based on the battery bank... Basically, we suggest 1-3 days of "no sun" and 50% maximum discharge. Use 2 days as a "nominal" setup:

• 3,300 WH per day * 1/0.85 inverter eff * 2 days no sun * 1/0.50 max discharge * 1/48 volt battery bank = 324 AH battery bank @ 48 volts

Now, assuming a C/8 typical maximum continuous discharge, the AC inverter rating would be:

• 324 AH * 48 volts * C/8 discharge rate * 0.85 inverter efficiency = 1,652 watt rated inverter "nominal max continuous power"

And assuming a flooded cell battery bank that can support C/2.5 rate of discharge:

• 324 AH * 48 volts * C/2.5 discharge rate * 0.85 inverter efficiency = 5,288 watt rated inverter (max surge for flooded cell)

Since many "good" inverters support surge at 2x of rated power (Watts or Volt*Amps -- VA):

• 5,288 Watt surge / 2 surge rating = 2,644 Watt max inverter rating based on battery surge capabilities

Now we need to check the Rate of Charge that we would suggest for such a battery bank--5% to 13% rate of charge:

• 324 AH * 59 volts charging * 1/0.77 panel+charge controller derating * 0.05 rate of charge = 1,241 Watt Array "minimum"
• 324 AH * 59 volts charging * 1/0.77 panel+charge controller derating * 0.10 rate of charge = 2,483 Watt Array "healthy nominal"
• 324 AH * 59 volts charging * 1/0.77 panel+charge controller derating * 0.13 rate of charge = 3,227 Watt Array "max cost effective array" (my humble opinion)

So, combined with the minimum array to supply 3.3kWH for 9 months of the year of 1,925 Watt Solar Array, and the above 1,200 to 3,200 watt array based on battery capacity... The recommend range for "your system" would be around 1,925 watt to 3,227 watt solar array...

Notice the minimum array of 1,925 Watt and the "healthy" nominal array based on battery size is 2,483 watts--That is a "balanced" system... If you go to 1 day of "no sun" and can live with less surge current--Then the array size based on battery charging goes down... However, if you need 3 days of "no sun" and/or higher surge current then you need a larger array to properly recharge the battery bank...

Anyway--I hope the above makes some sense... You really need to be "iron fisted" on your loads--both in what you will want to power and how many hours per day (vs available sunlight, generator fuel, etc.). Solar PV power (true off grid capable) is not a "fire and forget" type system. You will need to monitor it closely when first installed. And you will need to train others (spouse, kids, guests) on the basics (and how not to leave things on when nobody is around) to keep from "killing/murdering" the battery bank.

-Bill

PS: The above is just my opinion and based on feedback I have read here from others who actually live off grid... I am on grid with GT power--So my daily power is pretty simple. The utility is my main power, and the GT inverter helps keep the bill low ($6 per month, with excess balance at the end of the year--to allow room for growth later).

Also I carry out to 3/4 places for math so you can reproduce (and catch any errors). In reality, if you are within 10-20% of the numbers, that is "good enough" for solar calculations (10-20% variation in solar energy averages is normal--weather patterns play a large role in day to day operation).

And, this is a starting point... Suggestion/questions/changes are always good for another few posts.