Off-Grid vs. Grid Tied Efficiency

[sub3marathonman]

I know I've read about this many, many times. I've seen it stated, as just yesterday by jcgee88 (not to say anything negative about jcgee88 ) here: http://wind-sun.com/ForumVB/showthread.php?t=12654 and quoting, "If you go off grid, you need to buy equipment for storage and charging, you have to replace the batteries every few years, and your solar power generation efficiency is cut in half." And I have no doubt it is correct about the costs being much higher.

But what I'm wondering is can anybody show the steps that reduce the efficiency to 50% of a grid-tied system? Is it that they really aren't the same "system," but instead the best scenarios with each setup? Since you can't have an inverter that works optionally with or without batteries.

Now, stevendv explained a bit here: http://wind-sun.com/ForumVB/showthre...t=10711&page=6 and said, "The problem with an HV CC is + Xantrex XW battery inverter will be the loss of efficiency in GT scenarios. If you're getting paid for every kWh, every % of efficiency counts, and with transformerless inverters reaching 97% efficiency, compared to the 96 ish of the CC + 95 ish of the battery inverter, you're losing quite a bit compared to a pure GT solution."

OK, that would get it down to about 91% efficiency. Not calculating for the battery efficiency. But I can't believe it would fall to 50% when you factor in the batteries.

Can anybody help explain this?

Thanks.

[BB.]

I do start at 77% efficiency... Roughly 80% for solar panel derating (plus a bit for dirty panels) and 95% for charge controller/inverter (plus a bit for wiring loss).

Them add 80% efficiency for flooded cell batteries (older batteries are less efficient, running above 90% state of charge is less efficient, etc.).

And I use 85% efficient for the inverter... Assuming that most people run a 2kW inverter at 300 watts average power for much of the day (tare losses) and switching/standby loss is worse than optimum efficiency.

0.77 * 0.80 * 0.85 = 0.52 end to end derating

That is where I start from. You can adjust numbers for your installation, but you are not going to get much closer than 10% anyway.

-Bill

[Cariboocoot]

In addition to what Bill said, off-grid systems lose much of their power-producing potential because once the batteries are charged there's no place for the power to go. That usually occurs right around peak production time too. Some people load shift or set up diversion loads to make use of at least some of that extra power, but you can't beat grid-tie for being able to utilize the full panel potential no matter what the conditions.

[Vic]

The data that BB Bill posted above, gets you to the 50-ish percent figure.

An additional inefficency results from a desire to use solar input to fully charge the battery bank on 90% (or so) of the days in a year. This dictates a significantly larger solar array, which means that on most days there is a large excess solar power which can be used for heating, cooking, managing A/C, water heating or pumping, clothes washing and so on.

Almost all of these tasks are manually driven. They can take a lot of a person's time to manage, and often need monitoring or intervention, so one's attention needs to be directed onto these activities. If one makes a run to town, or is away at work, some of the above can run beyond the excess solar use, into running these things from battery power, if the "excess" power is taken from an inverter, as opposed to running from the DC output of the solar array.

Most of the advanced solar Charge Controllers have a useful function that indivates when there is excess solar power available -- CC in Float mode. This would help atuomatically manage the use of excess solar input, but it is up to the person who manages the off-grid system to implement a more automated system to use this information from the CC, applying the excess onto a pritorized list of uses for this excess power (as an aside, the Xantrex XW SCC has lacked this important function).

An alternative to having a lot of this excess solar power capability, is to run a generator more often when the demands on the off grid sysem for power exceed the PV power available. This type of system could be viewed as being more efficient, and it is much easier to automate, with Automatic Generator Start, but, of course it would use more non-renewable energy.

Trade-offs, always tradeoffs ... . Vic

[Solar Guppy]

The 50% is offgrid , not offgrid capable and selling net-metering

As you point out your talking maybe 5-7% maximum efficiency difference when both technologies are selling all energy to the grid ( Electronics ).

Where things quickly diverge is when the battery's are cycled, Two issues with this, first battery's can lose 5-25% of your harvest depending on the technology ( primarily AGM -> Flooded )

And with cycling the batteries you have to count the replacement cost of the batteries into the equation.

[sub3marathonman]

I do start at 77% efficiency... Roughly 80% for solar panel derating (plus a bit for dirty panels) and 95% for charge controller/inverter (plus a bit for wiring loss).

Them add 80% efficiency for flooded cell batteries (older batteries are less efficient, running above 90% state of charge is less efficient, etc.).

And I use 85% efficient for the inverter... Assuming that most people run a 2kW inverter at 300 watts average power for much of the day (tare losses) and switching/standby loss is worse than optimum efficiency.

0.77 * 0.80 * 0.85 = 0.52 end to end derating

That is where I start from. You can adjust numbers for your installation, but you are not going to get much closer than 10% anyway.

-Bill

The 50% is offgrid , not offgrid capable and selling net-metering

As you point out your talking maybe 5-7% maximum efficiency difference when both technologies are selling all energy to the grid ( Electronics ).

Where things quickly diverge is when the battery's are cycled, Two issues with this, first battery's can lose 5-25% of your harvest depending on the technology ( primarily AGM -> Flooded )

And with cycling the batteries you have to count the replacement cost of the batteries into the equation.

Thank you BB and Solar Guppy, those two answers helped answer my question. I'm also understanding the additional logistics of being off-grid that Vic and Cariboocoot stated, which can also impact overall efficiency in the real world.

So, what I'm understanding is:

Grid Tied: 80% efficiency for the panels, and 97% efficiency for the inverter, which equals 77.6% overall efficiency

Hybrid Inverter System: 77.6% as Grid Tied, then adding the charge controller efficiency of 95% and between 95% and 75% efficiency because of the batteries and method of cycling or floating the batteries. AGM batteries floated would be 95% and flooded batteries cycled could be at 75% efficiency. (I remember Tallgirl's charging efficiency tables, which showed that "fast charging" was much less efficient than "slow charging.") So: 0.776 * 0.95 * 0.95 (best case with AGM battery on float) which equals 70.0% overall efficiency

Off-Grid: As in the Hybrid Inverter System, but instead of 95% as with an AGM battery on float, with 75% for a flooded battery cycled. So: 0.776 * 0.95 * 0.75 (flooded battery cycled) which equals 55.2% overall efficiency

[mike95490]

Off grid here, and this is why I have a timer set on my water pump, to only allow it to run mid day, while the sun can power it directly, and I save the 20% in the battery recharge. (by running off the pv, not the battery)
It slows down my battery charge some, but in summer, plenty of sun, and winter I have very little water usage because of all the rain.

[BB.]

These are rules of thumb to allow us to quickly sketch out a system... I used to go into much greater detail--but it is almost useless with confusing details.

Few people can operate a inverter at its 95% efficiency point 24x7. AGM's cost ~2x as much and may not have as much life as a good quality flooded cell battery/fork lift battery (and fork lift batteries seem to have more losses). Solar panels in cold climates on mountains will collect more energy -- especially when connected to an MPPT controller (which is several times more expensive than a PWM controller)... Batteries are more efficient if cycled more deeply (less energy losses when spending time charging that last 10% to full)--but do not last as long when deeply cycled... And charging that last 5% into a battery bank to 100% full (and equalization) is also hard on battery banks too.

Add that weather can vary year to year averages by +/- 10-20% or more and you can get a 2+x difference between winter and summer output.

Do a few off grid people here get better performance from their systems than the rough rules of thumb predict... Yes. But as a few people have said--It usually requires special conditions (where the system is installed) and lots of close monitoring/shifting of loads) to get that optimum performance.

In the end, most people end up using more power than they originally planned. If nothing else, having off-grid electricity in a remote location just is too attractive. More lights, neat electronics, electric tools, reduction in generator use/fuel costs, Microwave, etc. will usually happen too.

-Bill

[Dave Sparks]

Nice Bill ! very nice! On AGMs the old saying of twice the price for 1/2 the cycles and 1/2 the warranty is improving. Someone was telling me that Toyota is rebuilding the Prius Li-ion batteries, is that old news?

My supplier Solar Depot told me last month that BP is out of the solar panel business and that the Chinese plant where the 4175's are built is shut down!
Did you hear anything about that? I am so out of touch these days...

[BB.]

I do not keep up on the industry and their rumors--other than what I read here.

I have heard that BP is getting out of solar, but nothing about the Chinese plant being shuttered (not that I would disbelieve it).

Keeping my fingers crossed... Today they put another Xantrex GT inverter on my home to replace the one that failed last month. So I am generating power again.

-Bill