Sizing your backup: how to judge risk vs cost tradeoffs

[danielh]

I just have to go back to this again! Since th OP is asking about balancing the cost and the risk for what is admittedly "luxury" during outages here is my take. $1000 of Eu 2000 genny, 10 gallons of gas (plus what you might have in your car!) would give you pleanty of run time for 24/7 luxury for most items, including at least furnace heat, single room air, radio/tv etc.

Show me the price for a battery bank with enough capacity to run room air, or furnace and fridge for the duration, and I'm betting it is close to the cost of the genny.

I have to chuckle (and I am not without sympathy to those in this situation) at some of the commentary a out the recent heqt wave in the States. People are commenting that you simply can't live without air conditioning, when the fact is people have been doing so forever. Arabs have lived on the desert for yers, and have learned how to cope. People in the US have only had regular A\C for the last generation or so, and field workers till routinely work outside of A/C. It is all a matter of what we are used to and how we cope.

I for one couldn't live in Florida with temps in the 90s regularly and RH high, but folks mow lawns, build houses, work on roofs ect because they adapt. Just like many of you wouldn't like to live in my environment where -40 is not uncommon. We work outside routinely, but we have adapted.

Sorry for he digression. I just think investing in a bunch of life cycle batteries for a POSSIBLE multiple day outages every NW and again is a good bet. I would buy a genny. (I know the OP already has one, but in general)

Tony

OP?

Well, although one should not be overly swayed by recent events; the recent outage did help clarify the benefits.

I do wonder how well a 1600kw genset would cover a family's needs; such as cooking with the wave, while the fridge andni tv is one. That isn't an extravagant luxury!

More importantly, the UPS protection is just as big a benefit (if not bigger).

And if one thinks of this as an appendix to a much larger project, and this project may or may not be be a solid investment, then spending <10% on batteryization doesn't seem too frivolous.

At least that is what I am telling myself these days. Perhaps I have become irrationally enamored of the idea.

[BB.]

As for AGM vs FLA -- the better surge capability of the AGMS may be a real benefit -- one could buy a smaller size set but still ride out occasional large draws (say, a +5 kw draw if the boiler and fridge start up while the microwave and toaster oven are going, and I am on the dual monitor computer). Also, I will probably be keeping these batteries in a basement utility area, that has ng hotwater heater and boiler 6 or so feet away. The area is opened along one end to the rest of the basement, so it has air circulation; but for safety sake it seems that AGMs would be better (or should I plan on building some kind of vented-to-the-outdoors compartment for them).

AGMs are nice and "clean". And they don't vent unless they are over charged and/or failing from age... So, I would still suggest making sure they are well vented.

I am not sure what code says about open flames and battery banks--But it is something to consider. If you build a system that can take either AGM or flooded cell--then you will error on the "safe" side.

There is also "non-outage" sizing concerns. In normal times, the setup will provide UPS backup to both critical loads (that I want available during outage) AND "electronics to protect". That is, stuff that during an outage I will just turn off and do without. Since I don't want to think about power during non-outages, the system has to be able to provide more than enough power to both kinds of loads for extended periods: say, 6kw for over an hour, perhaps more for short periods. Just to clarify: this is non-outage demand supplied from the grid, as supplemented by my full PV system (which includes panels not connected to the hybrid inverter).

Now to be clear... Are you talking about 6kW of peak load, or 6kWH of outage protection (say 1kW * 6 hours = 6kWH)?

6kW load is A LOT OF POWER. That is 50 amps of 120 VAC current or ~4x 1,500 watt electric heaters...

The "cheap guy" in me says "...do you really need that kind of power?"

Thus, do I need to oversize the inverter so that its pass through capability is a lot larger than its outage requirements?
More precisely, to size it to UPS protect against transient outages that coincide with periods of high demand (say 6kw)/
Note that if a transient event lasts longer than a few minutes, we will reduce loads -- we will switch to "outage" mode.

It is not unusual to install a relay to prevent two loads from cycling on at the same time. It will cut the maximum power requirement and can save you a lot of money.

Remember a 6kW inverter depending brand/model can source upward of 12kW of surge.

Or am I misunderstanding the power rating of these inverters. Perhaps under normal cases, when they are NOT inverting, these ratings are irrelevant -- they can pass through as much power as it is being fed to it from the main cb panel (say, 12kw). Hmm, given that these inverters are also feeding PV power to the "protected" loads (or back to the main panel), somehow I think these ratings ARE relevant.

Yes, many inverters can pass higher peak current than they can invert.

BTW: an(other) ignorant questions: I assume that inverters will NOT overdraw a battery, that it will stop pulling power and shut down when it detects the batteries approaching a target DOD (say, 50%). If so, under what circumstances would you end up with "a lot of scrap lead"?

There are lots of ways to kill batteries (run plates exposed, over/under charge, let set partially discharged for days/weeks/months will sulftate). Running occasionally below 50% state of charge will not kill a battery, but it can reduce its cycle life (from several thousand cycles to less than a thousand if always cycled below 50%).

The killer is to cycle a battery bank below ~20% state of charge--Below that point is is possible for a "weak" cell to actually go to zero volts and begin to reverse charge. Reverse charging will pretty much kill a cell/battery.

Cutoff voltage of ~11.5 volts (~46 vdc for a 48 volt bank) will help reduce the chance of a below 20% state of charge damage--But a Battery Monitor is really the only accurate way to "estimate" the SOC of an operational lead acid battery bank. And with sealed/AGM batteries, you cannot even use a hydrometer to measure the specific gravity.

BTW2: the above "normal times provision" makes me think the Xantrex 6048, with its 6k continuous and 12kw surge, is a better choice than the smaller Outback 3648.

Look at the battery bank as the "heart" of your system. Oversizing inverters/loads and/or undersizing charging is not doing it any favors. Large inverters are (relatively) cheap--Large battery banks, not so much:

Deep Cycle Battery FAQ
www.batteryfaq.org

-Bill

[mikeo]

I have to chuckle (and I am not without sympathy to those in this situation) at some of the commentary a out the recent heqt wave in the States. People are commenting that you simply can't live without air conditioning, when the fact is people have been doing so forever. Arabs have lived on the desert for yers, and have learned how to cope. People in the US have only had regular A\C for the last generation or so, and field workers till routinely work outside of A/C. It is all a matter of what we are used to and how we cope.

I would like to point out that in the last 2 months in Arkansas we have had 13 heat related deaths due to some cities having 15 days of temps exceeding 100F in a row with the highest temps at 115F. This has happened before but it's uncommon in our climate. Older people just can not adapt in time for an emergency like this. Fortunately our power has held up due mostly to our two nuclear plants that generate about 30% of our energy locally. If there had been a power outage, the death toll would have been much higher. Unless you design your house with survival in mind like I have, it would be very hard to live in these climate extremes with out power for a few weeks.

[danielh]

I am not sure what code says about open flames and battery banks-

That one can't be ignored!

Now to be clear... Are you talking about 6kW of peak load, or 6kWH of outage protection (say 1kW * 6 hours = 6kWH)?
6kW load is A LOT OF POWER. That is 50 amps of 120 VAC current or ~4x 1,500 watt electric heaters...
The "cheap guy" in me says "...do you really need that kind of power?"

Peak load -- for those moments where a lot of the "critical loads" happen to be on (or turning on).
I figure that during outages an average of 1kw an hour is more than enough, and if we want to avoid running the genset (or if it is cloudy), that could be halfed.

That said: in normal times there may be some periods where demand from these "critical loads" is high -- say 5 computers, the microwave, the big screen tv.
Well, that's probably about 3-4kw, but the point is that during normal times I just don't want to think about balancing and offsetting loads.

It is not unusual to install a relay to prevent two loads from cycling on at the same time. It will cut the maximum power requirement and can save you a lot of money.
Remember a 6kW inverter depending brand/model can source upward of 12kW of surge.

Yes, many inverters can pass higher peak current than they can invert.

If the oversized portion of the inverter would be used almost always during normal times (when power is from the grids or non-battery-connected panels), is that a problem?

To repeat, my concern is that the cb subpanel that contains the critical loads (the loads that the inveter powers during outages and are UPS protected) gets all the power it needs the 99% of the time the grid is working fine.

And that is why the idea of a Xantrex 6400 is appealing. Now, if the Outback 3648 could "passthrough" +6 kw of grid power for several minutes at a time, it might be more appealing (its a little cheaper, etc.). I currently assume that is not the case -- that the rated output (continuous and surge) always apply.

There are lots of ways to kill batteries (run plates exposed, over/under charge, let set partially discharged for days/weeks/months will sulftate). Running occasionally below 50% state of charge will not kill a battery, but it can reduce its cycle life (from several thousand cycles to less than a thousand if always cycled below 50%).

The killer is to cycle a battery bank below ~20% state of charge--Below that point is is possible for a "weak" cell to actually go to zero volts and begin to reverse charge. Reverse charging will pretty much kill a cell/battery.

Cutoff voltage of ~11.5 volts (~46 vdc for a 48 volt bank) will help reduce the chance of a below 20% state of charge damage--But a Battery Monitor is really the only accurate way to "estimate" the SOC of an operational lead acid battery bank. And with sealed/AGM batteries, you cannot even use a hydrometer to measure the specific gravity.


Look at the battery bank as the "heart" of your system. Oversizing inverters/loads and/or undersizing charging is not doing it any favors. Large inverters are (relatively) cheap--Large battery banks, not so much:

Deep Cycle Battery FAQ
www.batteryfaq.org

I read that people like their battery monitors. But that leaves the question of what to do if you leave the house unattended, say everyone goes to work for 8 hours. If the batteries get too low,
I would much rather the house be a little cold when I get home (or the fridge a little warm), then to ruin my batteries. Would these battery monitors help with that -- can they inform the inverter to stop drawing power. Or do they just provide accurate information to the human eye.

BTW: That's one advantage of a battery backup over just genset: they aren't as likely to walk away. Or perhaps you guys don't worry about loud gensets attracting unwanted attention?

[Cariboocoot]

OP?

Original Poster. A forum acronym.

I do wonder how well a 1600kw genset would cover a family's needs; such as cooking with the wave, while the fridge andni tv is one. That isn't an extravagant luxury!

I think you mean 1600 Watt genset. 1600 kilowatts would power a small town.

It might surprise you to know that a fairly large segment of the world's population lives without refrigerators, televisions, and microwave ovens. More likely it would surprise your children to know this.

The cabin here is far out in the woods. Before the solar install there was no electricity save running a big generator. Refrigeration was by propane, as were lights. No microwave, no television (still none), none of the "must haves" so many people think they can't live without. Before we bought it a family lived here year-'round, accessed by lake only (frozen in Winter). The improvements I've made certainly make life easier and more enjoyable, but they are hardly necessary.

There is just a small chance that the recent wide-spread blackouts will clue a few people in to just how much they take for granted. Maybe some of them will prepare for it. I don't mean the "doomsday scenario" which is highly unlikely to happen, but the inevitable next time the lights go out that absolutely will happen.

But too many people wander through the day blissfully unaware or purposefully ignoring the actuality that things do go wrong from time to time.

One thing we off-gridders have learned: when you have this much power, you tend to use that much more.

[icarus]

Wow,

Critical loads of 5 computers, wide screen and Micro?

Cooking can be accomplished in a variety of ways in an emergency situation. Barbeque, camp stove (I think ever gone should have a camp stove and a bunch of fuel stockpiled for just such emergencies!) the big issue is to be able to store water for a long term if city water goes out, or well power is not available. That said most houses have a pretty good stock pile of water if you are clever about getting it. hot water tank for example holds 40-60 gallons, toilet tanks, pressure tanks etc all hold water even if there is no pressure to get it out.

I guess it comes down to a matter of what one considers a "critical" load.

As a side note to mikeo, I am not m asking light of the situation in the efctd parts of the continent. What I am saying is that A/C has allowed explosive growth in climates where people generally didn't live in previous generations in large numbers. Those that did live there had specific adaptations to the climate, like building design, clothing design, and indeed work specific designs for labor etc.

Nowadays, we have generic buildings and people who leave in a hermetically sealed environment,, and when the lights go out,, they are not inured to the heat and humidity.

Tony

[ggunn]

Wow,

Critical loads of 5 computers, wide screen and Micro?

Cooking can be accomplished in a variety of ways in an emergency situation. Barbeque, camp stove (I think ever gone should have a camp stove and a bunch of fuel stockpiled for just such emergencies!) the big issue is to be able to store water for a long term if city water goes out, or well power is not available. That said most houses have a pretty good stock pile of water if you are clever about getting it. hot water tank for example holds 40-60 gallons, toilet tanks, pressure tanks etc all hold water even if there is no pressure to get it out.

I am from south Louisiana; a standard part of preparing for a hurricane strike is to fill all the bathtubs in the house with water. Of course, hurricanes often spawn tornadoes, and they say that if a tornado comes, a bathtub is a good place to take refuge. It would be a bummer to drown in your bathtub.

[BB.]

For me, 5x computers would be laptop which use around 20-30 watts--So a 150 watts of computer, another few tens of watts for cable modem and router. Only turn on the laser printer when printing (even do that when we have power).

It is a tough set of questions to answer. Yes the XW6048 is a really nice unit. And you need 600+ AH worth of 48 volt battery bank to feed it (assuming 6kW of load and 6kW of solar panels).

Around here, we have had entire multi-kWatt arrays walk away from the schools/buildings they were mounted on. Solar arrays are not exactly "stealth".

Two things to "protect the batteries". Say the low voltage limit to 46 volts (plus a couple of minutes delay to allow for starting surges) and there are battery monitors (Xantrex/Schneider and Victron) that do have a programmable external alarm contact which you may be able to connect to your inverter/load relay if things go south (battery monitors are typically more accurate than a voltage trip alarm).

I wonder if the Outback Flexmate battery monitor subsystem can be programmed to do similar?

-Bill

[danielh]

Wow,

Critical loads of 5 computers, wide screen and Micro?

Cooking can be accomplished in a variety of ways in an emergency situation. Barbeque, camp stove (I think ever gone should have a camp stove and a bunch of fuel stockpiled for just such emergencies!) the big issue is to be able to store water for a long term if city water goes out, or well power is not available. That said most houses have a pretty good stock pile of water if you are clever about getting it. hot water tank for example holds 40-60 gallons, toilet tanks, pressure tanks etc all hold water even if there is no pressure to get it out.

I guess it comes down to a matter of what one considers a "critical" load.

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 :)

[danielh]

For me, 5x computers would be laptop which use around 20-30 watts--So a 150 watts of computer, another few tens of watts for cable modem and router. Only turn on the laser printer when printing (even do that when we have power).

It is a tough set of questions to answer. Yes the XW6048 is a really nice unit. And you need 600+ AH worth of 48 volt battery bank to feed it (assuming 6kW of load and 6kW of solar panels).

Around here, we have had entire multi-kWatt arrays walk away from the schools/buildings they were mounted on. Solar arrays are not exactly "stealth".

Two things to "protect the batteries". Say the low voltage limit to 46 volts (plus a couple of minutes delay to allow for starting surges) and there are battery monitors (Xantrex/Schneider and Victron) that do have a programmable external alarm contact which you may be able to connect to your inverter/load relay if things go south (battery monitors are typically more accurate than a voltage trip alarm).

I wonder if the Outback Flexmate battery monitor subsystem can be programmed to do similar?

-Bill

Sigh, is it true that matching a 6kw xantrex with 200AH of battery and 2kw of panels is not so good -- that it will lead to unnecessary cycling of the battery.

That it won't be straightforward to achieve the goal of keep the batteries ready for use as a UPS and for use during an outage backup (but to otherwise not use them), to
use available PV and the grid for local loads (when the grid is up), and sell excess PV to the grid?

Is this a setup that the Outback does a better job at providing?