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I want to build a backup system that will get me through the winter in case of emergency

D0T-C0M

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Hi everyone. This is my first post so please bare with me. We had an ice storm here in eastern Canada about 3 yrs ago. Some people went without power for almost 3 weeks. Fortunately I was lucky living near a substation I was out for only 4 days. This woke me up and I promised myself when I got in a better financial situation I would install a system to make my home self sufficient through the winter. My home is 2000sqFT 2 levels (basement and upper floor) I just recently installed a 18k BTU minisplit in the basement which will be the workhorse for heating my home this winter. I have baseboard heaters upstairs for backup for now until I installed a 9k-12k minisplit next year. I just installed a watthour meter on my 18k minisplit to monitor my daily KWh daily usage vs outside ambient temperature which should give me some good real world data. The unit draws 2250W max on the nameplate. This morning I raised the temperature to get it working hard and it was drawing 1920W and outside ambient temperatures were around 0c/32f.

I am a Power Systems technician by trade and as such well versed in maintaining 125V DC battery lead acid battery banks and also work on AC systems from 120V/240V residential service to 12.47KV distribution to 345KV transmissions systems. I am seriously looking at building my own LiFePO4 battery bank as you can get raw cells really cheap.

During an emergency situation I will want to be able to run at a minimum, a deep well pump, Fridge, deep freeze, microwave, 10 lights, router/modem and hopefully a dedicated 9k-12k(1600W max) minisplit for heat for the whole winter if needed. I will most likely use propane for cooking.

I live on a 5 acre wooded lot so I am considering putting in a wood stove as a backup emergency heat source although I will never use it except for emergency. I would still like to do a cost analysis if its possible to drive the minisplit off a battery bank backup and one without.

I live in Canada on the 47degrees N so I know off the bat I'm gonna need a lot more solar for the winter to drive my emergency backup needs. Also want to grid tie my system and try to achieve net zero. Here the utility company will take all my surplus power and I can take it back on a 1 to 1 system meaning every KWh I give them I can take back. The only caveat is I can accumulate a reserve with the utility company throughout the year but on april1st any accumlated KWhs are zeroed. So if I give them 100KWh more than I used on April1st I forfeit those KWh and start banking from zero.

I would appreciate any input in cost analysis and system designs.
 
Since winter is almost upon you, I would suggest starting with a generator. You will want one to charge your batteries if the solar isn't cutting it or in case of charge controller failure. So get a genny that will suit those needs and then get to work on the rest of the system. You will have a backup in the shortest possible time and won't be rushed designing and building the rest of what you need.
 
Thanks for the respose. I have a standby generator 6500w continuous for backup right now but its a gas generator and while its fine for localized outages where fuel is still available its no good for extended durations during major outages like we had here a couple years back. Also I might add this project will likely take 6-8 months to complete so its more likely will only be put in service next winter. I'm in the design phase right now.
 
Excellent! That leaves you plenty of time to do it right. So, the next step would be to do a thorough energy audit. For example, you give the wattage of your mini split but we need to know the duty cycle. You need to add up just about every little thing you are going to run in watt-hours. Figure out max average power draw and surge draw. Then we can get to the easy part of calculating battery bank and array size and selecting equipment. The real work in design is figuring out the system requirements, the rest is some simple math and shopping.
 
Anything heating related is going to make your system get very big, very quick. I highly recommend being able to offload that to other sources of energy such as your propane for cooking, and your wood stove idea (I highly recommend looking into a masonry fireplace if you have the space/means to put it). Once you do this, your solar and battery combination should be very reasonable.
 
A few design challenges:

Deep well pump - Typically a big surge current, 5X higher than run. Often a struggle for inverters to handle.
Baseboard heaters - Likely completely impractical to try to run on an off-grid backup system.

Heat order of source efficiency:
  1. Fire
  2. Heat pump
  3. Direct heat (about 5X as much as a heat pump)
Minisplit average consumption:
BTU per hr/SEER rating

So a 18k BTU/h 27 SEER unit: 18000/27 = 667W

It may have a higher draw at times, but that should be your typical average consumption, BUT that gets much worse as you try to increase the delta between ambient and interior.
 
Almost no sun in the winter; you could keep your cell phone charged and the like, but for heat your wood lot is the way to go. It has a slower discharge rate than even lithium batteries, and will warm you twice.

Since net metering is available, grid-tie inverters are the way to go. Well pump? What is the nameplate rating for current? Need an inverter which can deliver 5x that for about a second. Sunny Island (my picture) will do that and interact with grid-tie inverters to run the system during power failures. But, if you have a large enough holding tank, you can just run the well pump when grid is up and draw water from the tank during power failures. Suitable size generator could also kick over the pump, if you want to go with a lightweight (inexpensive) battery inverter.

I sized my batteries for one night only. I run my large loads (A/C) only while the sun is up, in the case of grid failure. For your case, a fossil fuel generator would work to recharge batteries during cloudy days.

My math says PV + GT inverters costs $0.05/kWh amortized over 10 years. $0.03/kWh over 20 years assuming one inverter replacement.
Batteries cost $0.20 to $1.00/kWh by the time they wear out, therefore, I want to get by with minimum battery.
In your case, if you expect to be off grid and without sunshine many days per year, then larger batteries may be worth the convenience. However, and automatic start generator is likely a more cost-effective option.
 
Be sure to consider the 280 Ah cells that are mentioned a lot around here. EVE/Xuba/etc seems like 150 usd per kilowatt hour is common.
 
Be sure to consider the 280 Ah cells that are mentioned a lot around here. EVE/Xuba/etc seems like 150 usd per kilowatt hour is common.
Ten years of nightly cycling 100% DoD would be 3650 cycles, gets the cost down to $0.041/kWh.
What does a complete battery with BMS end up costing?

Those prices are good enough to compete with utility rates, and for shifting time of use from peak to off-peak.
The commercial battery assemblies I ran my math on were not.
 
Thanks for all the great comments. Just want to clarify I will not be using baseboard heating during times of emergency. Also I only have a 40' well but I'm using a 3/4HP submersive pump and you are right the inrush can be anywhere from 6 to 10 times the rated current draw. I could tee off the water inlet to my storage tank or run a separate dedicated line down my well and use a smaller above ground pump to use during emergencies. I could probably go through the whole winter using the generator just for the water pump. If I only start it once or twice a day for minutes at a time I shouldn't need a lot of gas to go through the winter.

The 18k minisplit I have has a SEER of 26 and HSPF of 12.5 which is very good for a unit that size. The 9k-12k units have a much better SEER of 33-35 and HSPF of 13.5 so I probably would consider using that if I choose to really go with a hige system but realistically I will most likely be installing a wood stove for emergency backup and it can be used for cooking and heating water too.

So roughly if I want a 1/3HP shallow well pump, Fridge, deep freeze, microwave, 10 LED lights, router/modem, LED TV How big a LiFePO4 battery bank are talking about if I want at least 2 days battery backup? Roughly how many watts in solar panels would I need to keep the batteries topped up while at the same time carry the load during the day for my area?

As far as battery cycling goes, keep in mind the batteries would only be cycled during an outage. 99% of the time I will be on the grid and feeding it KWhs 16hrs days during the summer and using them back during the winter with my batteries on float.
 
One full size refrigerator/freezer uses about 1500 Wh/day according to consumer use labels. That one load would need 3000 Wh of battery for two days.

For PV, you'll have to look up solar insolation tables for your location. Depending on tilt of panels, perhaps you get 4 hours effective sun in summer, 1 hour during non-cloudy times in winter. PV panel PTC specs may be around 85% of STC specs, although during cold winter more likely to approach 100%. Inverter, charge controller, battery round-trip efficiency may be 85%.

Maybe 2000W of PV panel would run the full-size refrigerator during the winter, given that it is inside your heated house. (outside, less power needed of course.)

Maybe 400W of PV panel would run the same refrigerator during summer.

Similar math to size for the rest of your loads.
Fridge will likely need at least 1 kW of sine-wave inverter to kick it over. 1/3 HP pump with induction motor may be rated 5A, needs 25A or more to start so at least a 3 kW inverter. Something like my 6 kW low frequency Sunny Island (11 kW surge rating) or similar quality units of other brands would do nicely.

Sizing battery, include no-load consumption of inverter. Mine is 25W, so 600 Wh/day or 1200 Wh for 2 days without sun.

"I will most likely use propane for cooking."

An electric start propane generator triggered by inverter based on battery state of charge would make the system pretty much hands-off.
Disable operation of the well pump, maybe also refrigeration, at night. That lets more loads run directly off PV rather than cycling battery, and avoids generator noise at night.
 
Ah. Well... "deep" in my off-grid property's area is 700-1100 feet with 3hp pumps. 3/4 is a triviality relatively speaking. My pressure tank jet pump is 3/4 hp, and my 4kW Victron runs it fine.

Best way to get a good number is use link #1 in my signature to download the spreadsheet and conduct an energy audit. Define everything you want to run, its power and the run time - battery and solar very accurately derive from those needs.
 
Maybe 2000W of PV panel would run the full-size refrigerator during the winter, given that it is inside your heated house. (outside, less power needed of course.)
Thanks you bring up a good point about the deep freeze and fridge. I could get a second fridge and put it and the deep freeze in my unheated garage. It will never use electricity in the winter except for those rare days above freezing. That would reduce my load a lot on my system.
 
Quick battery question, i checked out those 280Ah cells mentioned above and wow they look to be a great deal. I was thinking at that price I can get 16 - 280Ah cells and make a 560Ah 24v bank for the same price I could have gotten 16 - 100ah Fortune cells to make a 200Ah (24v) bank. I've read conflicting advice in other posts, is it better to parallel 2 cells together and then series all 8 pairs of cells in series or series 2 groups of 8 cells together and parallel them together? Also how fast can a bank this size be charged assuming you can provide full charging current?
 
It's a preference thing.

2 8S batteries will each need their own BMS, but you have a level of redundancy. If you have a problem with one battery, the other can provide limited function.

1 2P8S battery only needs 1 BMS, but if you have problems with any one cell, the whole battery will need to be broken down to address it.

A 0.5C charge takes about 2-3 hours to charge., so a 560Ah battery would take 2-3h with a starting current of 280A.
 
The nice thing about having a decent size battery in combination with a generator back-up is that you can efficiently run the generator for a short time to charge the batteries. This means you don't have to run it constantly and you can capture more energy out of your fuel.
 
It's a preference thing.

2 8S batteries will each need their own BMS, but you have a level of redundancy. If you have a problem with one battery, the other can provide limited function.

1 2P8S battery only needs 1 BMS, but if you have problems with any one cell, the whole battery will need to be broken down to address it.

A 0.5C charge takes about 2-3 hours to charge., so a 560Ah battery would take 2-3h with a starting current of 280A.
I really appreciate your input. Thanks. 280A is quite the charge current :). I don't plan ever using no where near the capacity of the bank in a 24 hr period but I've got the room for the panels on my 5acre lot so my goal would be to be able to fully charge my batteries in a 24 hr period using the solar panels with 4 sunhours I have available in the winter. Besides all this excess power will be put to good use in the summertime when we get 12-13hr usable sun hours. I should be pretty close to net zero by backfeeding my power meter giving it to the utility and taking it back in the winter on a 1KWh to 1KWh basis.

Are my calculations right that I'm going to need at least 16-18, 375W panels if I only have 4 sun hours to work with in the for the 4-6 weeks around dec21st. with some extra to account for cloud cover and efficiency losses? I should still have some extra left over for loads.

I was also looking at the all-in-one MPP Solar V/V2 hybrid 5K inverter. Its a 48vdc system so I could rearrange my battery bank in a 16S 48v arrangement at 280Ah instead of 8S2P 24v system at 560Ah. One nice thing is it can handle 450Vdc from the PV panels which will reduce cabling costs/losses but it can only handle 6000W so I would have to limit the number of panels to 16. The charging current is limited to 120amps but if I arrange the bank in a 8S 280Ah arrangement this should be sufficient to charge at ~0.4C. Is this a good choice? pros and cons with going with an all-in-one unit?

Edit: The above all in one only gives me 120v output, I might need to reconsider maybe getting 2- 3KW units instead and to achieve a split leg 240v system for my water pump. SO many options to think about. So glad I found this forum.


FYI I'm also planning on installing a 1000lbs propane tank and converting my 6500w gas generator to burn propane for short term outages and battery topping up if ever needed.

Just a side note on these Canadian Solar panels I'm looking at, they can produce up to 30% more power from the reflected light hitting the underside of the panel. This is great in the winter due to the snow on the ground is an excellent reflector.
 
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You've mentioned 12 hours sun in summer, 4 hours winter. Some of that is off-angle, also lower intensity due to longer path through atmosphere. One way this is adjusted is as equivalent hours of full sun. For instance, my San Jose location is 5.5 hours average over the year, that is, a fixed orientation panel produces as much as 5.5 hours pointed straight at the sun.

Here's a site with kWh per month, per kW of PV panels, for various locations in Canada


Winnipeg, 62 kWh in January from one kW of panel (2 hours effective per day), 136 kWh in April (4.5 hours effective per day)
probably that is based on 1 kW PTC (realistic conditions of tempreature and wind) rather than STC (standard conditions or 25 degree C panel)
Summer with higher temperatures, panels produce less but winter might approach STC.

Assuming 85% due to inefficiency of charge controller, inverter, PTC of panel, 375W x 18 panels x 0.85 x 2 hours = 11.5 kWh/day
Did you say 10 kWh of battery, 560 Ah at 24V? Then yes, this just about charges it in one day (assuming no loads)

Great if backside illumination adds power; obviously depends on mounting conditions and whether sun can illuminate snow under them. That would be greater for individual panels spread apart, zero for an array mounted right on the roof.
 
Thanks yes the panels will not be roof mounted, They will run in a string behind my house. Thanks for the input :)
 
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