Utility payouts per KWH

GreenerPower

Re: Utility payouts per KWH

tallgirl wrote: »

The point is that battery-less inverters, such as the SMAs, don't have the same MPPT efficiency as something like an FM-80 -- and properly sized an FM-80 is well into the mid 90's on efficiency.

Yep, that wouldn't be apple to apple comparison. That's where the value-added of a good solar installer or consultant - to offer a cost effective solution for the user situation.

tallgirl wrote:

As BB pointed out, 0.77 is typical for straight grid-tie, and what I've seen for "Hybrid" OutBack systems is closer to 0.74 than the 0.71 that BB mentioned. 0.74 / 0.77 is 96%. 0.71 / 0.77 is 92%. That's not a bad percentage rate, either way, for the capabilities that battery-backed gives.

The 0.77 factor Bill cited, I guess was from this site. It assumes a "typical" case of 92% inverter efficiency. From this report, the GT is 95% and the OB GX is 91%. With typical MX/FM efficiency of 95%, the battery-base in my "case #1" is 87% (0.91 * 0.95), about 90% (0.87/0.95) of the battery-less (all else equal in the chain i.e. PV eff. , MPPT eff. , wiring loss ...)
So far, we are at least ball-park agreeing on the component efficiencies i.e. battery-based (my user case #1) is ~90% of the efficiency of battery-less system.

tallgirl wrote:

Taking the difference in cost, which for "installed costs" is about $1 per watt DC with most of my clients, the "cost efficiency" is (0.92 / (8 / 7) or 80%. If "buy" cost is 125% or more of "sell", battery-backed is the correct solution since it's the only solution that allows power shifting. The greater the spread between "buy" and "sell" prices the more inefficiency is acceptable on purely economic grounds.

In cases like yours or similarly elsewhere where the average monthly PV energy is more than AC usage (i.e. "extra" to sell), and not to give it away at a loss (due to the buy/sell price spread), you opt to cycle the "extra" energy through battery. That's where the extra battery efficiency comes in, and the average cost is 43c/KWhr in my "case #2" as cited in my post ealier in this thread and in agreement ball-park with Bill's estimate when "Just cycling X kWhrs per day through a battery bank" (I kind'a fudge all other eff. for PV, MPPT, wiring .... into the number of sun hrs/day in my calculations)

tallgirl wrote:

I'm very surprised that you came out with a cost for power that was that high. Care to share how you calculated that?

Could you show your numbers if you think what Bill and I estimated were high.
GP

Cariboocoot

Re: Utility payouts per KWH

You all know that "cost for power" varies greatly depending on location, don't you? Doesn't matter if you're talking Grid, Grid-Tie, or Off Grid. BC Hydro's rates are ridiculously low, whereas the cost of solar equipment here is mind-bogglingly high. Go to Arizona or Florida and the economic picture is quite different.

No offense, folks, but it's a silly argument.

BB.

Re: Utility payouts per KWH

There is a reason that I am not too excited about arguing about 10 percentage points or less differences in system output power--And I just simply use nice big round numbers and tend to "error" on the conservative side.

From Solar Guppy on testing done in spring of 2009 [note: I believe the below is in direct reference to the Xantrex MPPT 60 amp charge controller and before the announcement of the new MPPT Morning Star controllers--but should also apply to the Xantrex GT mppt tracking design too--I am not trying to put words in SG's posts]:

Solar Guppy wrote: »

The Xantrex is true continuous dynamic tracking, the only one on the market that from my testing works, but this is from march/april testing of all available controllers on the market and manufactures are always working to improve their products

The FM60/80 have some type of tracking, doesn't look like true dynamic tracking to my eye. The units I tested this spring had issues ( I saw many days with 20% harvest losses due to mistracking ). It was recently posted Outback has new firmware which I haven't sent our units in to be upgraded ( can't field upgrade ). I won't be able to retest for a few months as I'm swamped with other work.

The MX60 was sweep and sleep and could get stuck on clouds and other environment changes but could be programmed to sweep at a user selected interval. Personally, I'd take a MX60 over the FM series, for starters you could program a MX60 for 70 amps, not so for a FM60, also with the reported amps reporting not quite on, the two FM units I tested couldn't do rated actual amps ( FM60 limited to about 59 actual as it was reporting 60 ) , part of apparent optimistic battery watts reporting that every single Outback charger I have ever tested does, that's 6 or 7 now that I've tested.

Add in the fact that Outback is "known" (from other posts here involving testing of a handful(?) of various Outback charge controllers over time) to consistently error around the 3-5+% optimistic side of harvest numbers (Xantrex has tended to average around zero +/- 2% or so--if I recall correctly)--getting down to 4% difference arguments between manufacturers/setups/assumptions is pretty pointless for me (I don't have a lab--the one person with a lab and has been pretty open about posting results--within the limits of his business model has been Solar Guppy).

I am not saying that 4-10-20% extra losses does not matter (it matters a lot in hardware design and it may be--at times--the difference between firing up a genset for backup power or not)--It is just not something that I can argue from authority as I don't have the test results and those results can vary over time, mfg. lots, design changes, etc.

Also, Efficient Numbers are "cherry picked" in the sense that the efficiency curve various over voltage, current, and probably even has a temperature component. To pick the 92% or 95.5% maximum efficiency point does not reflect that most users will be operating there devices under a wide variety of loads and conditions. To derate the estimates back 5-10 percentage points, in my humble opinion gives a more realistic number that will "average out" to represent closer to what the "average customer" will see.

We also have variability in batteries, battery technologies, and aging... I try to give estimates (by low-balling battery efficiencies) to say what a system can produce over the life of the batteries (and reflect that some people are going to do "energy wasting" charging of long absorb cycles and lots of equalization, etc. where others may not).

Some customers will see more and a few may see less power from their system--but that is the best one can do without a whole bunch of modeling--And that few percent better accuracy based on SWAGs (scientific wild a$$ guesses) will fly right in the face that it is not unusual for some areas to have up to +/- 20% variability in year to year average changes of available sunlight over time due to local weather conditions.

In the end--I try to show my work and people can plug in their own assumptions. And make their decisions from there. And I (we) welcome the discussions about the various trade-offs and SWAG's we make... God knows I am not perfect. :roll:

-Bill

tallgirl

Re: Utility payouts per KWH

GreenerPower wrote: »

Could you show your numbers if you think what Bill and I estimated were high.
GP

Certainly.

1). I use final cost, including all rebates and incentives because rebates and incentives artificially inflate the cost of systems. Although rebates and incentives reduce the cost, that's what customers wind up paying in the long run. To me, using list price is akin to buying something on sale and saying "Look how much money I saved!" rather than "Look how much I spent".

2). I don't assume worst-case by treating the system as an off-grid installation. If the best-case scenario is 10KWh AC per day, that's what I use as the power output for KWh costs. If some other scenario yields 8KWh, the loss is attributed to whatever scenario is causing the system to be operated in a less than ideal manner.

3). I attribute the cost of time-shifted power only to the cost difference between battery-full and battery-less systems. I believe that more accurately apportions the cost of being able to do something, like store power by day and use it by night, to the increased cost of equipment responsible for doing that.

As an aside, 3) is how I justified my initial decision to go with a battery-full system -- back in '06 / '07, I'd been pricing a Generac as a standby generator due to frequent power outages. If I wanted to have a system that was capable of handling an outage, it would either have to be a generator and battery-less system, or no generator and a battery-full system.

In my experience, breaking the costs down that way helps people view the costs as "functionality" rather than a large pile of money.

As regards the efficiency numbers, they aren't based solely on what the equipment reports, unless the equipment has a meter that is utility grade.

Based on utility grade measurements, an actively managed battery-full system =can= be within 10% of a battery-less system, while still providing both time-shifting and backup power capabilities. The key is that the system needs to be actively managed, not just run as an off-grid system. The reason is that just dropping the grid means you have less control over charging and discharging voltages, and that's where the majority of battery efficiency losses arise. In other words, it may be more efficient, over all, to run "off-grid" from 6am to midnight and "on-grid" the rest of the time, than to run "off-grid" until the batteries are exhausted several days later -- simply by avoiding lower overnight discharge voltages and the higher currents and amp-hours needed to produce constant power.

GreenerPower

Re: Utility payouts per KWH

Ah ! good point, rebates and incentives certainly bring the cost down. By the same token, Bill's system would cost 14 c/KWhr from 29c (my "SWAG" of 30% incentives and 30% federal rebate on the remaning). He would actually earn 7c/KWhr with 21c buy low/sell high. Thanks for bringing this up, that would change the $/Watt cost in our calculations. Rebates, incentives change greatly from state to state, get it while the fund lasts. Only 30% federal rebate is consistent and mostly nill outside the US (but their govenment probably subsidizes the PV cost).

Each person has his/her own way to present/market their solution to potential buyers just as yours to your buyers. Bill and I mainly use PVWatts where the estimated "practical" PV KWhrs in an area was based on past historical data. How do you determine whether it's 10KWh vs. 8KWh due to some scenario without the equipments actually installed there and monitored over an extended period of time.

As for equipment efficiency numbers, no solar installer has (or willing to spend) resources to have setup with various solutions to gather "real life" data to objectively judge one against an alternate solution. Some vendor just uses the name plate % to swing user one way or the other. The best SWAG we can get is from credible sources like CEC, details on each inverter here and the tests are conformed to a published procedure here. You can have good data in your installation, your specific equipments and location, but how can you do the same with an alternative solution, in a different location to objectively compare the 2 without equipments actually being there ?
GP

tallgirl

Re: Utility payouts per KWH

GP,

I have a growing number of clients, both solar and energy conservation consulting, so I have a growing amount of data. FN-DCs are sweet -- they help a lot, and as you know from the OutBack forum, I'm the FN-DC Pusher Lady. I'm still waiting on my first TED 5000 to ship and that will give me even more data on the AC side. I was going to add support for E-Mon D-Mon meters, but they are just too expensive. Landis+Gyr has also been on the list -- there's a chance I can get them to send me one on loan. Oncor has been installing them all over the state and they are fairly widespread.

I =do= tinker with my system to figure out how to make OutBack systems function in better / different ways. I was up in Waco at a client's (GVFX and SB combo setup) and she saw a 32KWh spike (I average between 10 and 12KWh per day -- that 32KWh spike is =huge=) and was wondering what the heck I was doing. My answer -- charging batteries up and running them down and trying to figure some problem or other I had. For the past few days I've had some industrial strength gear I've been beating the heck out of -- stuff like turning SellRE down to 46 volts and watching how everything behaves, because very few companies do that to inverters, and few of them are talking.

How do I recommend designs? I ask people how they want their system to function. That's a different approach from the "how much power" one. I worked with a local installer on a 10KW system -- by listening to what the customer wanted the system to =do=, I worked with the installer to design a system that is a bit more expensive than "make lots of power!", but meets a number of key design points. My gear was spec'd on a 56KW system and I'm working with the lead engineer to define the functional performance.

So, that's how I do it -- I focus on functionality and then maximize efficiency with whatever design falls out from meeting the functional requirements.

GreenerPower

Re: Utility payouts per KWH

Julie, thanks for sharing your expereiences. Certainly there is an after-market for tweaking/optimizing installed systems. Wish you luck in your works.
Hope readers can take away from this thread - do whatever "SWAG" you can with your situation with formulas Bill and I et al. suggested, conserve first. Beware of your local state policies on RE. Quite a few states like Texas are using "monthly billing cycle" for net-metering i.e. the payout is done monthly if you have excess sell. Not like CA where it is done annually so that you can use your excess to cover other months usage.
Good installers (becoming endangered species, example here, so look around, do your research) would estimate and setup your system properly. There is still opportunity to tweak/optimize your setup if it was done by a "so so" installer.
GP

tallgirl

Re: Utility payouts per KWH

GreenerPower wrote: »

Julie, thanks for sharing your expereiences. Certainly there is an after-market for tweaking/optimizing installed systems. Wish you luck in your works.

Most of my "new work" is consulting on commercial designs, as well as larger scale, and more detailed management and monitoring.

Good installers (becoming endangered species, example here, so look around, do your research) would estimate and setup your system properly. There is still opportunity to tweak/optimize your setup if it was done by a "so so" installer.
GP

By my estimate, 1 in 4 installers are "Good". Another 1 in 4 is "Okay", and 2 in 4 should pick another career field. I looked at a 48 volt system recently where Vmpp was right about 60 volts. Last month I saw a 24 volt system where Vmpp was 22 volts -- the electrician who last wired the panels insisted that they couldn't be wired at a higher voltage because it would short out the batteries. In December I looked at a system where half the panels weren't even connected to the charge controller.

I'd like to spend more time working on properly installed systems. My only consolation is that by examining so many broken systems I'm able to add features to my software that will automatically diagnose breakage -- leaving me more time to do something else with my life. Like play guitar or ride my motorcycle 8)

westbranch

Re: Utility payouts per KWH

TG I noted this statement:

tallgirl wrote: »

Battery =efficiency= isn't an issue since the only losses for batteries are float power, which is dependent both on capacity and technology. Size the batteries towards the low end of requirement, use AGMs, and float losses go way down.

.

Can you please clarify just what 'the low end' means to you?


Reason for asking is we will (plan to ) be going to a new set of AGMs @ 24 v. so am evaluating the 'fit' of a 1400 Ah battery to projected use. All new appliances so this is seat of the pants at this point...

thanks
Eric

tallgirl

Re: Utility payouts per KWH

westbranch wrote: »

TG I noted this statement:

Can you please clarify just what 'the low end' means to you?

Reason for asking is we will (plan to ) be going to a new set of AGMs @ 24 v. so am evaluating the 'fit' of a 1400 Ah battery to projected use. All new appliances so this is seat of the pants at this point...

thanks
Eric

There are different ways of calculating a battery bank size. Pick the smallest value that still realistically meets your needs.

Judging from the 1400Ah bank, it sounds like you're not grid-tied, which is relevant to the recommendation. In a grid-tied configuration, batteries are typically sized to the maximum expected power outage, rather than days of autonomy. If your area hasn't experienced a three day power outage, three days of autonomy for a grid-tied system would be excessive. Rather than go with three days of autonomy that would likely never be used, size the batteries for the longest power outage you've experienced, or that you realistically expect (and then double that for 50% maximum depth of discharge). That way you won't be wasting power floating battery capacity that you'll never need.

BB.

Re: Utility payouts per KWH

I believe that during design and testing of the XW 6048 Hybrid inverter--Solar Guppy et.al. found that 600 AH @ 48 volts was the minimum "engineer" recommended battery capacity for a 6kW (12k surge) inverter (marketing recommendation was 400 AH @ 48 volts).

• 6,000 Watt max continuous load / (600 AH * 48 volts) = 4.8 ~ C/5 or 20% rate of discharge

Sounds about right... Any more load on a flooded cell battery bank and you probably will start to have heating problems and other issues with bank.

That gives you 2.5 hours at 6kW to 50% state of charge (and chose to shed load or startup the generator).

I would guess that with AGM (like the Concord Sun Extenders which can take up to C*4 or C/.25--you could use a smaller bank--but you would need a genset to start within 10 minutes or so with a 6kW load).

If you expect less continuous load out of the inverter (say 2kW)--you could probably size the bank down a bit...

Solar Guppy would have to say if there are minimum AH requirement for "float" and GT operation too (with the XW Hybrid--for example).

-Bill

tallgirl

Re: Utility payouts per KWH

BB. wrote: »

I believe that during design and testing of the XW 6048 Hybrid inverter--Solar Guppy et.al. found that 600 AH @ 48 volts was the minimum "engineer" recommended battery capacity for a 6kW (12k surge) inverter (marketing recommendation was 400 AH @ 48 volts).-Bill

That's the problem with sizing banks to continuous inverter output -- it leads to over-sized battery banks (more cost), which leads to less efficient battery use (still more cost).

Here's an example --

One of my clients has a 14.4KW (4 x VFX3648) continuous rated system. The highest load I've ever recorded on that system for longer than one second is something on the order of 4.5KW. (The highest connected load I've heard of is 50KW, so the maximum use was about 1/10th of nameplate load rating and 1/3rd of rated continuous output)

Assuming (without proof) that the same sizing rule applies -- 100AH per KW continuous rating -- the correct minimum size bank for that system would be 1440AH. Working through the actual usage, 4.5KW / 48V / .90 = 104 amps peak current. That system has 1000Ah FLA @ C/10 of capacity, 1000 / 104 = ~ C/10 as the peak discharge rate. Average discharge current is significantly lower.

mike95490

Re: Utility payouts per KWH

found that 600 AH @ 48 volts was the minimum "engineer" recommended battery capacity for a 6kW (12k surge) inverter (marketing recommendation was 400 AH @ 48 volts).-

Also has to do with the AC ripple showing up on the battery (pulsating DC), which can fool some MPPT chargers.

tallgirl

Re: Utility payouts per KWH

mike90045 wrote: »

Also has to do with the AC ripple showing up on the battery (pulsating DC), which can fool some MPPT chargers.

That would sound like a reason not to buy that product. From fuzzy memory, the minimum recommended battery bank size for a Brand O inverter is 100Ah, or 200Ah for a pair.