Help shopping for emergency home battery
March 6, 2023 9:03 AM Subscribe
I'd like to have a large-ish battery that I can use to provide power to my heater in the event of a power outage, but I'm not sure what kind is appropriate.
I have one of these Navien condensing boilers, and I'd like to be able to keep it running for some time in the event of a power outage during cold weather. It plugs into 120V 60Hz AC, and is listed as "maximum power consumption less than 10A." I can't find typical draw and I have no idea how often it draws that much.
Given this, what size of battery (in Watt hours?) should I get if I want to keep the heat on for 24 hours? What type of battery technology is best for this kind of thing?
I'm not interested in something to power high draw stuff like a fridge or air conditioning etc., though I'd want to be able to do some charging of phones/flashlights/laptops etc in the event of a longer power outage too. For the purpose of this question assume budget is not a big issue.
Any general advice as well as specific brand/model recommendations are welcome, thanks!
I have one of these Navien condensing boilers, and I'd like to be able to keep it running for some time in the event of a power outage during cold weather. It plugs into 120V 60Hz AC, and is listed as "maximum power consumption less than 10A." I can't find typical draw and I have no idea how often it draws that much.
Given this, what size of battery (in Watt hours?) should I get if I want to keep the heat on for 24 hours? What type of battery technology is best for this kind of thing?
I'm not interested in something to power high draw stuff like a fridge or air conditioning etc., though I'd want to be able to do some charging of phones/flashlights/laptops etc in the event of a longer power outage too. For the purpose of this question assume budget is not a big issue.
Any general advice as well as specific brand/model recommendations are welcome, thanks!
I think it would be worth looking at your actual power consumption with a Kill-a-watt or similar - I got mine at a heavy discount from the power company, and some libraries check them out. I think they're like $30 if you buy them new - would pay for itself compared to buying the wrong size battery.
posted by mskyle at 9:20 AM on March 6, 2023 [3 favorites]
posted by mskyle at 9:20 AM on March 6, 2023 [3 favorites]
Just in case it's not clear-- those boilers have an inlet for a gas line. I'd assume the heat generated is from burning the gas, and the electricity is just to power the control electronics, and probably an electronic igniter. So I'd expect your power draw to be more in line with a cell phone than a space heater.
posted by hovey at 10:00 AM on March 6, 2023 [1 favorite]
posted by hovey at 10:00 AM on March 6, 2023 [1 favorite]
Best answer: Installation manual says:
posted by achrise at 10:08 AM on March 6, 2023 [1 favorite]
The boiler’s current rating is 5 AThat's in the same paragraph that contains the warning:
A battery back-up (available at most computer retailers) may be used to supply hot water during power outages
If there is a power failure in cold weather areas, the freeze prevention system in the boiler will not operate and may result in the heat exchanger freezing. In cold areas where power failures are common, you must completely drain the boiler to prevent damage if power cuts last for extended periods.I'd check it with a Kill-a-watt meter during operation. I doubt it pulls much at all when you're not using hot water. It does have a fan so there will likely be a current peaks when that starts up.
posted by achrise at 10:08 AM on March 6, 2023 [1 favorite]
Response by poster: Thanks all. That's a really good point about kill-a-watt to get real data. I can check one out from my library but I'm not sure how much heating I'll be doing this spring. Still, I can get a feel of what it actually uses while running. The unit works at best efficiency (in terms of gas used) when running at low temps and long duty cycles, but I suppose in an emergency I could crank it to high temp and short cycles, that should be more efficient in terms of electricity used.
I guess I was hoping someone might have some real-world estimates for this kind of boiler, bc I really don't believe it's using anywhere close to 1.2kWh per hour. And yes hovey it is primarily using gas to heat water and I agree it shouldn't use much power, I figure this thing is more in line with a small computer but I'm not sure why they list such a high max draw, and under what conditions it would happen. Even a fridge uses only around 1-2kWh per day, and intuitively I feel that's got to be more than this little box. One variable is that is uses two electric pumps. Maybe they are included in the 10A max, but I can turn those off in an emergency too, they aren't really required they just vastly improve efficiency.
posted by SaltySalticid at 10:10 AM on March 6, 2023
I guess I was hoping someone might have some real-world estimates for this kind of boiler, bc I really don't believe it's using anywhere close to 1.2kWh per hour. And yes hovey it is primarily using gas to heat water and I agree it shouldn't use much power, I figure this thing is more in line with a small computer but I'm not sure why they list such a high max draw, and under what conditions it would happen. Even a fridge uses only around 1-2kWh per day, and intuitively I feel that's got to be more than this little box. One variable is that is uses two electric pumps. Maybe they are included in the 10A max, but I can turn those off in an emergency too, they aren't really required they just vastly improve efficiency.
posted by SaltySalticid at 10:10 AM on March 6, 2023
Any chance your power company might be able to give you an hourly (or at least daily) energy-use graph?
posted by humbug at 10:15 AM on March 6, 2023 [1 favorite]
posted by humbug at 10:15 AM on March 6, 2023 [1 favorite]
Excluding the pumps, the power used is basically a very small amount to run the electronics, a little bit for the ignition, but only for a very brief period, and a slightly larger amount for the ventilation. I'd be surprised if it used more than 100W in normal operation, probably a lot less. I think you could get by with a UPS of the type used for computers, though probably you'll want one on the larger size. I agree that measuring the power draw would be best, both to see the max draw on ignition and the power consumption over time.
posted by ssg at 10:18 AM on March 6, 2023 [1 favorite]
posted by ssg at 10:18 AM on March 6, 2023 [1 favorite]
I did some playing with battery sizing when building out a very rudimentary system for a camper van. Agree on the kill-a-watt to find actual usage. Most models let you track kWh over whatever time period you want. Just remember that it’ll be higher as it gets colder outside.
Other points to consider: Batteries are DC, so unless your boiler is also DC with an internal power supply, you’ll also need an inverter to make AC. There is a parasitic loss in this conversion.
Batteries are most commonly sized in Amp-hours. A cheap lead acid battery should always be kept above 50% capacity to avoid damage (and just one really deep discharge can ruin it). A lithium battery can be drained to zero, but cannot be charged at all if the battery temp is below 32F.
If you want the good stuff for chargers and inverters, Victron is top notch. Batteries are going to depend on which chemistry you’re interested in, but definitely check out van builds for tips. I had a Lifeline 8DL lead-acid, since the van was expected to see sub freezing temps. That’s the gold standard for leads.
posted by hwyengr at 10:50 AM on March 6, 2023 [1 favorite]
Other points to consider: Batteries are DC, so unless your boiler is also DC with an internal power supply, you’ll also need an inverter to make AC. There is a parasitic loss in this conversion.
Batteries are most commonly sized in Amp-hours. A cheap lead acid battery should always be kept above 50% capacity to avoid damage (and just one really deep discharge can ruin it). A lithium battery can be drained to zero, but cannot be charged at all if the battery temp is below 32F.
If you want the good stuff for chargers and inverters, Victron is top notch. Batteries are going to depend on which chemistry you’re interested in, but definitely check out van builds for tips. I had a Lifeline 8DL lead-acid, since the van was expected to see sub freezing temps. That’s the gold standard for leads.
posted by hwyengr at 10:50 AM on March 6, 2023 [1 favorite]
I'm not sure why they list such a high max draw, and under what conditions it would happen.
As achrise mentioned, this is probably due to the momentary high current draw -- the inrush current-- when the fan motor kicks on. It is very brief, but can be much greater than the normal operating current of the motor. They've probably listed 10A to ensure that electrical installers leave enough room on the circuit breaker that it won't be at risk of tripping when the fan kicks on.
Definitely get a Kill-A-Watt. You'll probably end up using it on your other appliances too.
posted by hovey at 11:04 AM on March 6, 2023 [5 favorites]
As achrise mentioned, this is probably due to the momentary high current draw -- the inrush current-- when the fan motor kicks on. It is very brief, but can be much greater than the normal operating current of the motor. They've probably listed 10A to ensure that electrical installers leave enough room on the circuit breaker that it won't be at risk of tripping when the fan kicks on.
Definitely get a Kill-A-Watt. You'll probably end up using it on your other appliances too.
posted by hovey at 11:04 AM on March 6, 2023 [5 favorites]
If you are a hopeless geek like me, many - not all, but many - smart plugs have power meters in. Offhand I know that the Emporia one does and at least the Kasa TP115 does. They have the advantage that you get usage over time as well as in the moment.
And yes, that 10A circuit is all about 'don't go pop when the fan starts'. I don't know how well a battery/inverter (I would probably look at Jackery and similar models) will deal with that, so you might want to get one from a place with a returns policy and test it out.
It draws the most power when the motors start (I imagine it has a fan and a pump), which will happen when there is demand. You can fake that by turning up your thermostat. I doubt any power meter of any sort will give you a true sense of that momentary peak demand, so that is probably a 'try it and see' situation.
posted by How much is that froggie in the window at 11:29 AM on March 6, 2023
And yes, that 10A circuit is all about 'don't go pop when the fan starts'. I don't know how well a battery/inverter (I would probably look at Jackery and similar models) will deal with that, so you might want to get one from a place with a returns policy and test it out.
It draws the most power when the motors start (I imagine it has a fan and a pump), which will happen when there is demand. You can fake that by turning up your thermostat. I doubt any power meter of any sort will give you a true sense of that momentary peak demand, so that is probably a 'try it and see' situation.
posted by How much is that froggie in the window at 11:29 AM on March 6, 2023
Also, fridges usually have similar demands like 'put me on a 15A circuit'. Same reason: they have a pump and getting the pump started takes a peak current. That said, I have had a fridge on a smart plug and you don't actually see that initial current spike; it's probably there but it's very brief.
posted by How much is that froggie in the window at 11:32 AM on March 6, 2023 [1 favorite]
posted by How much is that froggie in the window at 11:32 AM on March 6, 2023 [1 favorite]
You're going to find that:
- You need two things: a battery and an inverter, but your constraints on the two are quite different.
- What inverter you need will be determined by its ability to produce enough inrush current to start up the boiler successfully. Most good inverters have a peak power rating that is about twice their continuous power rating. You want to pick the lowest rating you can get away with, both because it will be cheaper, and because a smaller one will typically be more efficient when running at lower power, which it will be doing most of the time.
- The battery on the other hand, will have no problem supplying the peak power regardless of size. What matters for the battery will be its capacity, which you get from the average power consumption of the boiler, times the duration you want to run it for, multiplied by the inefficiency of the inverter, and multiplied again by the excess capacity you need to have - e.g. for lead acid batteries, you don't really want to discharge them below about 50% full or they degrade quickly.
- Measuring average power consumption is easy with a plug-in meter, but it may be hard to work out what peak current you need, because the actual peak current is only drawn very briefly - a plug-in meter may not indicate it accurately. And even if you can measure it accurately when starting from the mains, that doesn't really tell you what you can get away with, i.e. how much voltage drop the boiler can deal with while that inrush happens. Ideally, you want to find someone who can tell you "yes I have that same boiler and I needed inverter X, inverter Y wasn't beefy enough".
posted by automatronic at 11:35 AM on March 6, 2023 [5 favorites]
- You need two things: a battery and an inverter, but your constraints on the two are quite different.
- What inverter you need will be determined by its ability to produce enough inrush current to start up the boiler successfully. Most good inverters have a peak power rating that is about twice their continuous power rating. You want to pick the lowest rating you can get away with, both because it will be cheaper, and because a smaller one will typically be more efficient when running at lower power, which it will be doing most of the time.
- The battery on the other hand, will have no problem supplying the peak power regardless of size. What matters for the battery will be its capacity, which you get from the average power consumption of the boiler, times the duration you want to run it for, multiplied by the inefficiency of the inverter, and multiplied again by the excess capacity you need to have - e.g. for lead acid batteries, you don't really want to discharge them below about 50% full or they degrade quickly.
- Measuring average power consumption is easy with a plug-in meter, but it may be hard to work out what peak current you need, because the actual peak current is only drawn very briefly - a plug-in meter may not indicate it accurately. And even if you can measure it accurately when starting from the mains, that doesn't really tell you what you can get away with, i.e. how much voltage drop the boiler can deal with while that inrush happens. Ideally, you want to find someone who can tell you "yes I have that same boiler and I needed inverter X, inverter Y wasn't beefy enough".
posted by automatronic at 11:35 AM on March 6, 2023 [5 favorites]
If backup power for your boiler is the only thing you'd be using this for, I'd like to make the case for investigating whether your need can be met with a generator. If you use natural gas, it's even possible that it could power the generator as well. Possible arguments in favor:
1. Generators are cheap and stable technology compared to batteries, barring maybe only lead acid. It's possible even a high-quality gas generator could be a fraction the cost of the batteries-plus-inverter you're speccing here--a Honda EU2200i is something like $1200, and it's not inconceivable that you could get a permanently installed natural gas powered generator (like a Generac) for less than the cost of what you need to cover just your boiler for few days, though I have to admit I don't have a great sense of how small you could go if you're running it with the pumps turned off.
2. If you're concerned about power outages lasting for longer than a couple of days you'll be out of luck with batteries unless you can recharge (e.g. you already have solar); a generator will cover you as long as you can get fuel.
3. You can get a generator that handles the inverter thing for you--it's not an extra expense or piece of equipment.
4. No matter what type, batteries have a finite lifespan and require maintenance even if they're not in active use. In fact, I think you'd be much better served by a bank of batteries if they ARE being used every day and can also incidentally cover your boiler in an emergency. If this is an I-might-need-it-once-or-twice-in-five-years kind of thing, a generator only requires that you fire it up and make sure it's working once every four to six months (and that you have/can get fuel).
5. I realize I'm going out on a limb here, but I think there might even be an argument to be made that a generator plus the limited amount of fuel you're going to consume during emergencies and periodic testing would have a smaller environmental impact/cradle-to-grave carbon footprint than equivalent batteries.
posted by pullayup at 12:23 PM on March 6, 2023 [1 favorite]
1. Generators are cheap and stable technology compared to batteries, barring maybe only lead acid. It's possible even a high-quality gas generator could be a fraction the cost of the batteries-plus-inverter you're speccing here--a Honda EU2200i is something like $1200, and it's not inconceivable that you could get a permanently installed natural gas powered generator (like a Generac) for less than the cost of what you need to cover just your boiler for few days, though I have to admit I don't have a great sense of how small you could go if you're running it with the pumps turned off.
2. If you're concerned about power outages lasting for longer than a couple of days you'll be out of luck with batteries unless you can recharge (e.g. you already have solar); a generator will cover you as long as you can get fuel.
3. You can get a generator that handles the inverter thing for you--it's not an extra expense or piece of equipment.
4. No matter what type, batteries have a finite lifespan and require maintenance even if they're not in active use. In fact, I think you'd be much better served by a bank of batteries if they ARE being used every day and can also incidentally cover your boiler in an emergency. If this is an I-might-need-it-once-or-twice-in-five-years kind of thing, a generator only requires that you fire it up and make sure it's working once every four to six months (and that you have/can get fuel).
5. I realize I'm going out on a limb here, but I think there might even be an argument to be made that a generator plus the limited amount of fuel you're going to consume during emergencies and periodic testing would have a smaller environmental impact/cradle-to-grave carbon footprint than equivalent batteries.
posted by pullayup at 12:23 PM on March 6, 2023 [1 favorite]
I don't know how well a battery/inverter (I would probably look at Jackery and similar models) will deal with that
That's the thing, they (often) don't. I have a 10kWh battery as part of my solar system, and besides the fact I'd need another battery to make it even worth it to have my A/C be part of the my backup loads (I'd rather my refrigerators keep running overnight and be hot, rather then getting an extra few hours of A/C and then no refrigerators), I'd need to get a brand-new compressor that is designed to keep the in rush current under the threshold my hypothetical battery system can handle.
posted by Back At It Again At Krispy Kreme at 2:24 PM on March 6, 2023
That's the thing, they (often) don't. I have a 10kWh battery as part of my solar system, and besides the fact I'd need another battery to make it even worth it to have my A/C be part of the my backup loads (I'd rather my refrigerators keep running overnight and be hot, rather then getting an extra few hours of A/C and then no refrigerators), I'd need to get a brand-new compressor that is designed to keep the in rush current under the threshold my hypothetical battery system can handle.
posted by Back At It Again At Krispy Kreme at 2:24 PM on March 6, 2023
Best answer: Looking around at power ratings that people are talking about for condensing boilers connected to [hydronic] radiator systems in the UK, people are saying 50W-100W continuous power while the heating's on, so assuming a similar power draw for what looks like a very similar boiler (I don't know what heating you're running off it), then that would need 2kWh battery and inverter system for 24 hours continuous running, including the radiator pump, which is the majority of the power.
That's $2,000 from Jackery or Ecoflow. There are problems with inrush current and harmonics if you're running very big motors (e.g. circular saws) and sensitive electronics simultaneously, but I think the inrush current off a boiler pump is much gentler, so the inverter should be fine.
Assuming the boiler and connected heating system aren't running flat out, you'll get much more than 24 hours out of a battery that size though, so it's moot whether it makes sense to get a smaller battery or to feel safer for longer. If you go for this solution, then you have a nice compact but heavy 20kg/45lb box you can also use to get electricity other places as well, and everything works out quite neatly with no fumes or fuel. You do need the boiler to be connected to electricity via a plug, not hardwired, so that might be something you need to change in your current setup.
posted by ambrosen at 2:50 PM on March 6, 2023 [1 favorite]
That's $2,000 from Jackery or Ecoflow. There are problems with inrush current and harmonics if you're running very big motors (e.g. circular saws) and sensitive electronics simultaneously, but I think the inrush current off a boiler pump is much gentler, so the inverter should be fine.
Assuming the boiler and connected heating system aren't running flat out, you'll get much more than 24 hours out of a battery that size though, so it's moot whether it makes sense to get a smaller battery or to feel safer for longer. If you go for this solution, then you have a nice compact but heavy 20kg/45lb box you can also use to get electricity other places as well, and everything works out quite neatly with no fumes or fuel. You do need the boiler to be connected to electricity via a plug, not hardwired, so that might be something you need to change in your current setup.
posted by ambrosen at 2:50 PM on March 6, 2023 [1 favorite]
You will need to power the water pump to circulate the hot water. Mine draws an amp, and does have significant inrush as it’s not particularly sophisticated device.
posted by zenon at 3:06 PM on March 6, 2023
posted by zenon at 3:06 PM on March 6, 2023
I bought a large car battery and a 1200w inverter to do this exact thing. I had to wire an auxiliary and plug so I could plug into the inverter. It works BUT, the inverter shuts off if the battery goes below 9 volts. So, it runs the furnace long enough to get some heat in the building, but doesn't last very long. Check the specs before you spend a lot of money and time on this.
I guess you could power the inverter by running your car if you have long jumper cables.
A dual fuel generator (gasoline/propane) might make more sense and be more general purpose.
posted by H21 at 5:42 PM on March 6, 2023
I guess you could power the inverter by running your car if you have long jumper cables.
A dual fuel generator (gasoline/propane) might make more sense and be more general purpose.
posted by H21 at 5:42 PM on March 6, 2023
Response by poster: So many great angles on this! A little more detail: the pumps are optional because up until I replaced the boiler in 2019, the house was heated with the original 1929 boiler system using the same radiator network, with no pumps. My old boiler guy said it was the oldest boiler he'd ever seen in a residence.
Anyway, I did forget about the fan. It can really get going but here's the catch: it only runs fast if it's on short duty cycle and the water is cool! When I run it on long (near continuous) duty cycles as recommended to me by the installer, the fan is either off, or so low it can't be heard under the gentle simmer or the burner. It's like the difference between a blast furnace and keeping a gentle simmer of a kettle that's already simmering.
I'm inclined to take the information from the manufacturer found by achrise at face value, and pursue UPCs made for computer systems, since it seems like it is a roughly similar use case. There are a lot of price points and capacities out there, happy to hear any further thoughts on this approach.
posted by SaltySalticid at 6:05 PM on March 6, 2023
Anyway, I did forget about the fan. It can really get going but here's the catch: it only runs fast if it's on short duty cycle and the water is cool! When I run it on long (near continuous) duty cycles as recommended to me by the installer, the fan is either off, or so low it can't be heard under the gentle simmer or the burner. It's like the difference between a blast furnace and keeping a gentle simmer of a kettle that's already simmering.
I'm inclined to take the information from the manufacturer found by achrise at face value, and pursue UPCs made for computer systems, since it seems like it is a roughly similar use case. There are a lot of price points and capacities out there, happy to hear any further thoughts on this approach.
posted by SaltySalticid at 6:05 PM on March 6, 2023
My circulation pump was just rebuilt because large parts of the house weren't getting heat. My radiators date from before WWII, and so part of the issue was internal resistance. But for water to move it needs the lower section of the system to be cold, and I have insulated most of the lower pipes. I did that because when the pump is working this part of the system was just wasting heat on the basement.
When the circulation pump was failing the boiler was running constantly and regularly hit its thermal limit and shut off. Sometimes it even dumped water out of the blow off valve. Perhaps the original boiler would be able to heat this house without a pump, but my more modern one couldn't with 1/2 a pump.
posted by zenon at 7:43 AM on March 8, 2023 [1 favorite]
When the circulation pump was failing the boiler was running constantly and regularly hit its thermal limit and shut off. Sometimes it even dumped water out of the blow off valve. Perhaps the original boiler would be able to heat this house without a pump, but my more modern one couldn't with 1/2 a pump.
posted by zenon at 7:43 AM on March 8, 2023 [1 favorite]
The speed of the fan is, oddly, unlikely to make much difference. It's only moving combustion air, and there's not a huge quantity of it needed even at full power, so there's not a lot of power required to move what is needed.
Back of the envelope calculations says it moves 10-100 cubic feet of air a minute at full whack. For comparison, the extraction hood on your range is probably moving 400cfm, and still uses barely any electricity.
posted by How much is that froggie in the window at 3:48 PM on March 8, 2023
Back of the envelope calculations says it moves 10-100 cubic feet of air a minute at full whack. For comparison, the extraction hood on your range is probably moving 400cfm, and still uses barely any electricity.
posted by How much is that froggie in the window at 3:48 PM on March 8, 2023
Response by poster: Update, in case it's useful to anyone in the future. I got a kill-a-watt and left it plugged in for 314 hours. It registers 24.08 kWh total for that period, which is 0.076 kWh/h (i.e. 76 watts average), or 1.84 kWh per day, right in the range ambrosen gave. This has been during a cool spring, and the heater has been running a lot (majority of the day). As discussed above, the electric consumption is mostly a matter of on-time, and not heat generated, so I think I'll be ok with a larger computer UPS or a 1-2kWh battery thing like Jackery etc. In case of emergency, assuming the gas is fine, I can definitely run it at high heat for shorter duty cycles to save electricity, rather than at low heat and very long duty cycles, to maximize gas-use efficiency.
I'm leaning toward a Jackery (or comparable), so that it's useful for other stuff too, and I can get a solar panel charger for it as well.
posted by SaltySalticid at 8:17 AM on March 29, 2023 [1 favorite]
I'm leaning toward a Jackery (or comparable), so that it's useful for other stuff too, and I can get a solar panel charger for it as well.
posted by SaltySalticid at 8:17 AM on March 29, 2023 [1 favorite]
I'm leaning toward a Jackery (or comparable), so that it's useful for other stuff too, and I can get a solar panel charger for it as well.
FWIW, my neighbor took delivery of a large Jackery and an associated portable solar panel set three weeks ago, and appears to be sufficiently satisfied that they just bought another one for even more storage.
posted by aramaic at 9:08 AM on March 29, 2023 [1 favorite]
FWIW, my neighbor took delivery of a large Jackery and an associated portable solar panel set three weeks ago, and appears to be sufficiently satisfied that they just bought another one for even more storage.
posted by aramaic at 9:08 AM on March 29, 2023 [1 favorite]
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...that starts you down the road of some fairly huge-ass (and expensive) batteries. I've been looking at a 13kWh battery for circa $15K installed.
Do you know how much the device runs, in a given 24h period? It's probably not running constantly, which can massively reduce your electrical demands. Given the cost/hassle of that 13kWh battery I mentioned above, I've also been looking at smaller more portable units.
posted by aramaic at 9:18 AM on March 6, 2023