Emergency Backup that can grow

[tobascodagama]

Hi, all! First post here, figured I would describe the system I've been planning for a couple of months and see if anybody had opinions or ideas about it.

As an intro, I live in a somewhat rural part of Maine. The town I'm in has its own backup generator, but I want to have a plan in place in case a snow storm knocks out our power (and, thus, heat) for more than a couple of hours. Eventually, though, I want to build up a full hybrid roof-based solar setup where hopefully most of our power comes from the sun and we just use the grid for heavy-duty appliances, so I'm picking out components with an eye to stuff I can reuse for a whole-house setup. For now, I'm just planning to use a portable ground rack for the solar that I only hook up as needed.

Components I've got right now:
- 2x 200W Renogy panels (should have done 320W, since it seems 60-cell is a more standard size for roofs, but live and learn)
- Renogy Rover Li 40A charge controller
- 2x 100Ah Battleborn batteries
- brackets and connectors

Load I want to run:
- 121W (continuous) propane heater; manual says it requires a 10A outlet
- internet router (~76VA)
- low-wattage string light?

Obviously, the inverter is the most important part I'm still missing. The load sizing is pretty clear, ~363W for the heater after the sizing factor plus the other loads, round up to say 600W. BUT, sizing factor recommendations aside, I'm strongly inclined to take the 10A requirement in the appliance manual at face value. So now I'm looking at 1500W and up.

I was originally looking at a 12V system, but Will's video on 12V vs. 24V sold me on the latter. Unfortunately, most of the action in 24V seems to be more in the 3kW and higher range as inverter-chargers. I was originally planning to go for a sub-$1k standalone inverter rather than an inverter charger, but since the idea is to be flexible and grow this is really starting to seem like the right approach?

Currently, I'm considering a Victron MultiPlus 24/2000. I like how much documentation Victron puts out, and I like that their equipment is heavily programmable. But most of all, I like that their equipment is designed with interoperability in mind. Like, eventually I want to also power a well pump that uses 240V, and when I get there I could add a second MultiPlus to the system to get that. So that seems like a smart way to grow the system as I add more panels and batteries.

My only concern with Victron is that parallel/split-phase operations require identical models, so the logical thing to do would be to get the MultiPlus-II series so I can be certain of get a matching model when I add more inverter-chargers to my system in the future... But I can't seem to find them on sale anywhere? AltE only seems to have the older MultiPlus in stock; they do have the Quattro as an alternative, but they start with the 24/5000 model which is extreme overkill for what my starting point is. (And I certainly couldn't run it safely off my current battery bank.) So I guess, after all that rambling, my question is: What's up with the MultiPlus-II? Is there a good place to get them?

EDIT: Did some more reading around the forum, and apparently I had missed that the MultiPlus-II series is only for 48V. So I'm guessing that means the other MultiPlus models are going to be around for a good long time?

 

[svetz]

Welcome to the forums!

Bad news I'm afraid. In winter in Maine, two 200W panels will get you about 400 watt-hours per day, not enough power to run your heater for 3.5 hours.

A propane heater that consumes 121 watts continuously is nearly 3 kWh of power per day (in Maine in winter that's like 10 300W solar panels and batteries just to run the heater). That's more than some refrigerators. Most likely it only consumes that when running, and the consumption is blower motors. I'd say dump it as an idea for emergency heat. I'd also get a router that wasn't so power hungry.

Ideally power is only needed to provide the spark and make sure the valve is off it's not lighting. Some don't even require electricity (e.g., Mr. Heater MH18B). So, a few of those per room and you're done.

For emergency heat you're probably better off with wood stove, esp if you have a lot of trees.

But here's a look at the math. First you want to do an energy audit (see the signature below for details). It'll look something like the table below but alter for the correct duty and watts:

Item	watts	duty (# hrs/24)	Wh/d
Heater	121	30%	900
router	76	100%	1824
lights	100	20%	480
	297		3204

The inverter only needs to supply 300 watts (sum of column 2), but add on 20% for safefty. If it is to power blower motors, the inrush can be 10x the continuous, so it might need 1200W of inrush capability.

In winter in Maine the insolation is probably around 1 (check an insolation table). You need to supply 3204 W/per day (sum of column 4). Let's say the inverter is 85% efficient and the battery round trip is 90%. So, you need to generate at least 3240/.85/.9 = 4200 W/day

With an insolation of 1 that's an array size of at least 4200 (watts-needed/insolation) (on cloudy days you won't have enough power, so more is better). That's 13 330W panels. For battery, you'll probably want a couple of days of reserve, let's say 3. So 3x4200 = 12,600 Wh. If you're using 12V 100 Ah batteries, that's 12 batteries in 3 banks (4 in series for 48V).

 

[tobascodagama]

TBH, it's pretty hard based on the documentation to figure out what the consumption actually is. 121W continuous is my assumption. What the manual actually says is "Electrical Connection AC 120V, 60 Hz, 121 watts", verbatim. And then under installation instructions it says, "Rinnai suggests that a dedicated electrical circuit with a 120VAC, 60 hz, 10 amp power source be used." So is that 1200VA continuous, or is the 10A just for surge consumption? Who knows?

I'm assuming that 121W is actually the blower fan, which runs while the furnace is lit, but it's thermostat-controlled, so obviously it's not lit 100% of the time. I probably should just slap a Kill-a-Watt on the outlet and remove all doubt.

At any rate, wood heat is definitely our long-term plan. Powering the furnace off a solar rig is a step stone. We're going to install wood heat for backup next year, and I'll expand the solar to run our 240V well pump off it at that point.

EDIT: I see you've added a bunch more info to your reply, thanks for that! I'll take some time to digest it all.

 

[SolarQueen]

Definitely throw a Kill A Watt on that heater. The answer is critical to if you can make it work. It is surprising how much power those blowers do take, including for pellet stoves.

I have not heard anything about the MultiPlus going away. I think you are OK there.

Also take a look at the Schneider SW4024. It can do 120/240 on its own, 4000W gives you plenty of room for growth, and the price is right.

 

[tobascodagama]

Thanks again for the answers, both of you! I just ordered a Kill-a-Watt, definitely should have done that a long time ago.

You need to supply 3204 W/per day (sum of column 4). Let's say the inverter is 85% efficient and the battery round trip is 90%. So, you need to generate at least 3240/.85/.9 = 4200 W/day

Let's take the solar out of the system for a sec and just treat the inverter/charger plus battery as a UPS instead. So assuming that the low voltage cut off trips at ~20% charge or so, my 2400Wh bank could keep my loads running for ~9-10 hours. Did I do that calculation right?

If so, that should cover most of the outages we experience. Like I said, the town has its own backup generator, and they usually switch it on within a couple of hours. Those little propane space heaters are so cheap that I'll probably pick up one of those and a small tank for longer outages anyway.

It's disappointing to find out that I undersized the solar so badly, though. Even more so because 40-cell panels don't seem to be a common size for non-mobile installations, so it's not clear I'll be able to integrate them into further expansion of the system.

 

[svetz]

...EDIT: I see you've added a bunch more info to your reply, thanks for that! I'll take some time to digest it all....

Sorry! It takes me like 20 edits until I'm happy with a post ;-)
I'm also complete rubbish with a calculator, never trust any of my addition/subtraction/multiplication/division.

Let's take the solar out of the system for a sec and just treat the inverter/charger plus battery as a UPS instead. So assuming that the low voltage cut off trips at ~20% charge or so, my 2400Wh bank could keep my loads running for ~9-10 hours. Did I do that calculation right?

OOPS... I forgot about adding depth of discharge in there....

I didn't see your calculations.... but let's say you have a 2400 Wh bank and step through it...

Let's say you want to hypermile those batteries so you're charging to 80% and discharging to 20% (see SoC degradation of LiFePO4 for more).
You can pick whatever numbers best match your use case, so change the values to what you think. In this example, that's 80-20 = 60% usable capacity, so 2400 x 60% = 1440 Wh.

Let's say the inverter is 85% efficient (although Victron or Schneider are quality, probably somewhere in the 90s), we don't have to worry about the round trip efficiency since you asked to take the charging out. We'll assume cozy temperatures so no losses from low temperatures and ignore the peukert effect for LiFePO4 (see the Battery FAQ for more on those). So, 1440 Wh useable in the battery x .85 conversion = 1224 Wh available as AC power.

If you're consuming power at 300 wh per hour, then the batteries would last 1224 wh / 300 Wh/h = ~4 hours
There's actually one other calculation to run, which is that you don't exceed the C rate of the battery chemistry (rare with anything solar). Drawing 300 Wh/h from a 1224 Wh battery gives a discharge C-Rate of 300 / 1224 = .25, which is well below the max for LFP.

Hope that helps!

Hah! Only 15 edits on this post!

 

[tobascodagama]

That does help dramatically, thanks again!

The router is a real wildcard factor here. The only rating I have on it is that the sticker on the back says "0.6A max". That's how I got the 76VA rating. I'd be surprised if it consumes anywhere near that, actually. One more reason to use the Kill-a-Watt ASAP (should be here over the weekend).

Let's say that I can live without internet (unrealistic assumption ) and have a separate light source. So I'm looking at just the furnace, which gives a W-hr number of 968 for daily consumption. Based on the Energy Audit spreadsheet in your signature, that would require 1076 W-hr battery capacity for a full day's operation, factoring in 90% efficiency for a Victron inverter. Further factoring in the 90% for battery round trip yields ~1196 W-hr. Using 60% DoD (80->20% SoC) for maximum lifespan -- though we usually have enough warning of a bad wind storm to top off the battery --, that requires a ~2215 W-hr battery bank for 24 hours.

Now here's where I butt up against "stupid question" territory: Two 12V 100Ah batteries in parallel have a capacity of 200Ah. But the same two 12V 100Ah batteries in series driving a 24V load has a capacity of... it's 100Ah, right? So to drive a 24V inverter with a minimum battery capacity of 200Ah would require four 12V 100Ah batteries in a series-parallel bank, right?

But on the bright side that brings the bank up to a total power capacity of 4800 W-hr based on the label, which translates to a practical power capacity of ~2880 W-hr if I'm running them with a "surprise outage" 60% DoD or ~3840 W-hr with an "advance warning" 80% DoD (100->20% SoC). Based on a 1196 W-hr daily consumption, that yields ~2.41 days or ~3.21 days of operation, respectively. Did I do that right?

Thanks again for your responses, it really helps to bounce numbers off someone more knowledgeable.

 

[svetz]

Let's say that I can live without internet (unrealistic assumption )

It probably doesn't consume that much power, and you can minimize the power usage by only turning it on when you need it...after all, it's an emergency situation so a little inconvenience is probably worth it.

... though we usually have enough warning of a bad wind storm to top off the battery...

You can probably go with 100% SoC then. There's nothing wrong with having the battery at 100% SoC, it's just storing it forever at that charge that causes premature aging. If you have seasonal bad weather, just leave the SoC at 100% for that one month out of the year. If you're handy with code you could probably scan the NWS report and anything that looked potentially bad (my Enphase battery does that for me with a feature called storm guard, it's a little on the over-cautious side but that's good if you ask me). Someone may have already written such code for your inverter (or it might bias your inverter selection). Worth a look on GitHub, less us know what you find.

... two 12V 100Ah batteries in series driving a 24V load has a capacity of... it's 100Ah, right? So to drive a 24V inverter with a minimum battery capacity of 200Ah would require four 12V 100Ah batteries in a series-parallel bank, right?

Yup. The Battery FAQ is well worth the read (or skimming at least) just to familiarize yourself with any gotchas you might encounter.
As I recall, some manufacturers don't want you to put their drop-in batteries in series, so be sure the brand you get is okay with that (something to do with their built in BMS I think, if you go DIY you'll be fine).

But on the bright side that brings the bank up to a total power capacity of 4800 W-hr based on the label, which translates to a practical power capacity of ~2880 W-hr

So, four 100 Ah 12V batteries is 4 x 100 Ah x 12V = 4800 Wh. At 60% capacity (charging to 80% and cutoff at 20%), 4800 x .6 = 2880 Wh
But if you go 100 to 20%, then it's 3840 Wh.

if I'm running them with a "surprise outage" 60% DoD or ~3840 W-hr with an "advance warning" 80% DoD (100->20% SoC). Based on a 1196 W-hr daily consumption, that yields ~2.41 days or ~3.21 days of operation, respectively. Did I do that right?

Easier with the math... but let's walk through it...

I suspect your math left out this step:
At 90% inverter efficiency the 2880 Wh of battery is 2880 x .9 = 2592 Wh of AC power. Similarly, 3840 Wh DC @ 90% = 3456 Wh AC

1196 Wh/day, 24 hours per day... so 1196 / 24 = 50 Wh/h (which sounds low, in fact the inverter itself might be pulling 10 to 20 W, so check the datasheet and add that in).

If your AC gear is pulling a total of 50W at 100% duty that will last 2592/50= 51.8 hours. Or utilizing 80% of the battery it's 3456 / 50 = 69 hrs

Let's say the max watts is 300 W. What's the max amps on your wires?
On the battery side the amps would be 300W / 24V = 12.5 amps. On the AC side at 120V, the amps would be 300 / 120 = 2.5 amps. That can be used to pick the wire gauge (see the FAQ) and from that the fuse sizes.

Hope that helps!

 

[tobascodagama]

That does help a ton, I really appreciate the detailed response.

Oh, and for the record I bought BattleBorn batteries, which are advertised as being capable of series connections.