Sizing your backup: how to judge risk vs cost tradeoffs

[BB.]

As usual, an informative pleasure to read your posts.
Ugh, happened to me this morning :<
I am in Montgomery county MD (so probably same pv environment as you)

Nope--that was my typing/computer's location of touch pad (sometimes I turn the touch pad off when typing longs posts). Also, the cntrl/alt keys seem be pressed by a ghost...

I would envision I would be at the other vertex of the triangle. Near 0% discharge most of the time, and near 50% a few days a year.

That is pretty much the definition of a UPS system.

Was it you who said (in some other forum) about a generational difference in off-gridders. The old notion was "batteries cheap, panels expensive" , but now its the exact opposite

Not me--but probably true to a degree... Although, it is almost people are handicapped by old ideas. Leads them to a large battery bank with small charging resources (less than 5% even). And when they "ran out of power", the recommendation was a larger battery bank (yes, that was common advise not too many years ago, and probably still true in some areas of the Internet/off grid resources). Adding batteries instead of charging resources--was still a bad idea back then, and still is now.

What about the fancier battery monitors?

Presently battery monitors are (for the most part) not integrated with charge controllers... So, other than a programmable alarm (for limited type of events), the typical battery monitor is to inform the system owner about the present state of charge of the battery bank. I don't know of any Battery Monitor that can warn of "over charging events"... (not that I know much in details about monitors or what is available).

A battery monitor will not be a substitute for a remote battery temperature sensor controller charge controller.

Another reason for AGM batteries for the setup I describe (better for providing possibly large wattage during transient events)

I tend towards recommending small AC UPS for computers (UPS's have losses--I don't want to waste the power to provide a UPS for my fridge and microwave/toaster ovens so I don't have to reset the clock)--and a "balanced" off grid solar PV system for balance of home (with conservation/power management for motors via choice of motors/systems and/or VFD type controllers).

In the end, you will probably do several designs and compare the cost/benefits for your needs. And they probably will not be black and white type answers.

I figure 2-3 year life for a UPS, at $100 a pop. Which is one of the "economic" reasons for seriously considering some kind of "batteryization". In fact, up until our friendly neighborhood dereecho provided a refresher course in coping with extended outages, the bigger reason.

And how much planning/equipping do you do for a once in 100 year event? Our last 1 week power outage was in the Mid-1950's. The rest have been for a few hours every couple years.

We have "the big one" (earth quakes) every 100-150 years.... Last "big" one was in 1907. Earthquakes tend to have very scattered heavy damage among almost no damage. "Earthquake" proofing an older home is a much better idea here vs adding a entire off grid solar capable system.

I figure I am going to need a genset -- and I got one already (though I might ditch it for a honda eu2000).

Two is always better. Spare and a backup. Plus you can always "barter" fuel/unneeded genset/electricity if ever needed. Plus the larger genset can be useful for power tools/AC etc. when needed and convenient (fuel availablility, etc.).

Definitely have net metering up here. And, quite importantly, SRECs. So value of electricity sold to the grid is > retail cost. Achieving optimality may not be possible for both functions, but reasonable efficiency should be achievable. I posit that will be cheaper & easier than two seperate optimal systems.

Assuming install a large GT system (or Hybrid GT system)--Yes, Net Metering and SRECs will help reduce overall system cost (plus tax breaks for "whole" solar system vs smaller UPS plus smaller solar system which is not 100% tax break compatible).

The suggestion by stevendv (http://www.wind-sun.com/ForumVB/show...056#post123056) to have a transfer switch to select GT > main_cb_panel or GT -> inverter/charger_subpanel is interesting; it might remove this concern (99% of the time the battery is being properly charged (if charged at all) by the inverter/charger.

True, the amount of time you would spend on GT/OG "bang-bang" charging will be low... So battery life will be unaffected.

I am a big believer in looking at maintenance (costs, time, debugging, locating old/obsolete parts vs forced upgrades, etc.).

A whole home UPS--replace batteries every 7 years or so, repair/replace electronics after 10+ years (for the most part, 10+ year old "high-tec" electronics are not repairable as vendor has cut parts/service support and electronic compenents are not available on the open market, etc.

So--Figure on saving cash for new MPPT controller, new GT controller, new Off Grid inverter, every 10+ years... New battery bank every 6-10 years. And look at the energy costs (loses, battery charging, conversion losses for UPS's, etc.).

Not saying it is not worth it for you... Just that you will be replacing 2/3rds of your system every 10 years or so.

VS me--use laptops (have own battery), small $100 UPS every two years for one desktop system. And buy a new Honda eu2000i for $1,000 every ten years (if it still runs--I now have spare/something I can loan out for non-emergency use).

Buy 20 gallons of fuel and preservative once a year, recycle old fuel back into car (has been making me money lately with the rocketing price of fuel--better than my savings account).

Look for propane/natural gas version of gensets if you have natural gas/propane available.

-Bill

[danielh]

It seems to me that the rules for sizing PV/AH that are so crtical for a strictly off grid system become less so with a Grid-tied with battery back up type system:

If the system is primarily to provide UPS like functionality for short grid outages then the PV is almost irrelevant since the grid can be relied on to keep the batteries charged as long as the banks AHs are enough to last through the typical outage. (the caveat is the too much PV /AH - AC ripple when selling issue) Genset/charger can obviously extend this. Incorporating PV would enable a tax credit...

If the system is needed to supply power for extendeded grid outages (days to weeks) then PV/AH becomes more like an off grid scenario. This is my situation and why I will be doubling my PV soon.

The OP's situation stands all this on it''s head since he has plenty of PV but is trying to decide how much of it to devote to strictly grid tie and how much (and is it worth the cost) to devote to a seperate grid interactive system. It's further complicated since he has not precisely defined his "critical" loads or back up needs and because he wants to do this system for $6K or less (excluding PV)) which may not be possible in any high quality way.

Despite this - I'll give it my best shot anyways..

One proposal for a "balanced" system:

3X3 array of his 260 watt panels = 2340 x 0.77 derate = 1800 watts PV

Outback GVFX3648 with CC in Outback FP1 or Midnite Solar Epanel prewired system - about $4000
4 x Universal battery 12 volt 200 AH AGMs (http://www.solar-electric.com/unba200amagm.html) = $1372
(not as high quality as SunXtenders or others but would probably meet the needs of this system)
Meanwell PB1000-48 battery charger to run with existing genset when the sun don't shine = $270 (http://www.powergatellc.com/mean-wel...er-supply.html)
Balance of system components $500-1000
Shipping - guestimate $500
Electrician install (excluding PV) - guestimate $1000

Total = $7500-8000 ( could possibly get it lower with some bargain hunting)

At 10 KWH of battery bank it would provide up to 5 Kwh of "no sun, no generator" back up to 50% DOD.
How long that lasts of course depends on the "critical loads". Sun hours and fuel supply would dictate length of extended back up capacity.

Does not meet his budget but I think it would serve the intended purpose of extended back up of critical loads for an "average" household" .

OK that's all I got - good luck

Thanks MT, that is useful.

Note that some fraction of the costs replicate costs that would of occurred anyways, so that $8000 could be a less; maybe a lot less (i.e.;
the outback replaces $1200 of microinverters). But can it get costs down to $4500 or so (0.70 x 4500 = close enough to my budget cap). We shall see.

[danielh]

Another reasons for grid interactive?

http://www.washingtonpost.com/blogs/...vJdW_blog.html

[BB.]

Another reasons for grid interactive?

http://www.washingtonpost.com/blogs/...vJdW_blog.html

Are you saying another reason to have Off Grid capable solar? (panels+charge controller+batteries+inverter)?

For those new to the forum, pure Grid Tied inverters (solar panels + GT Inverter) can only run when there is AC power present... If the grid goes down, so will your GT Solar System (there are ways around the issue--but that involves more inverters, battery banks, etc.).

-Bill

[vtmaps]

Was it you who said (in some other forum) about a generational difference in off-gridders. The old notion was "batteries cheap, panels expensive" , but now its the exact opposite

I think you are referring to something I wrote to you on this forum a long, long time ago:
http://www.wind-sun.com/ForumVB/show...675#post122675

BTW, you might find it instructive to reread that old thread... it seems to me that this thread is recapitulating a lot of that old info.

--vtMaps

[danielh]

Are you saying another reason to have Off Grid capable solar? (panels+charge controller+batteries+inverter)?

For those new to the forum, pure Grid Tied inverters (solar panels + GT Inverter) can only run when there is AC power present... If the grid goes down, so will your GT Solar System (there are ways around the issue--but that involves more inverters, battery banks, etc.).

-Bill

Yes, that is what I meant. It was sort of a for-grins post, or at least I hope it is.

The one question is whether pv panels etc might be disabled by such a solar storm. Probably not (not massive enough)?

[BB.]

In general, the size of the electrical network must be on the order of a mile or more to "receive" the magnetic flux from the sun... A "solar power system" is way to small to be affected (my humble opinion--if I am wrong, you can post back once the world is on-line again).

However, a GT and Hybrid GT system is directly connected to the utility grid, so it is possible a surge from the utility could take out anything connected.

-Bill

[danielh]

In general, the size of the electrical network must be on the order of a mile or more to "receive" the magnetic flux from the sun... A "solar power system" is way to small to be affected (my humble opinion--if I am wrong, you can post back once the world is on-line again).

However, a GT and Hybrid GT system is directly connected to the utility grid, so it is possible a surge from the utility could take out anything connected.

-Bill

Oh no, now I have to get a 3rd inverter that I only connect for use during systemic emergencies !!!