Clarification on charger in 6kW inverter

[Baz]

A couple of questions on the built in charger.

I understand the charger works on 120VAC even though the inverter produces split 240VAC.  Does the inverter continue to function while the charger is connected, drawing from the battery while the charger keeps it topped up, or like some UPS's does it switch the load directly to the 120VAC input?  If the latter is the case does it use the transformer to create the split 240VAC?  That seems unlikely as it would require at least 50 amps input at 120VAC to provide the full 6000W...

I assume the charger can be set at a lower battery bank voltage than any connected solar so that the MPPT controllers can provide the primary charge if available and the charger only would activate if enough solar isn't available?

It also isn't clear to me how many watts the charger is capable of providing.  I would assume it isn't the full 6000W, but can it handle the 3600W available in 30 amp circuit?  If so, can that be limited so that it wouldn't trip the breaker if connected to a 15 amp circuit?

My use case is somewhat of a learning thing for me as part of it is very much overkill...  what I want to do is put it in my popup during the summer and cover the roof with solar panels (as I said, 6000W is overkill in this case).  While I'm at home (which is most of the time) it will be connected to my generator input (30 amp 240VAC) and a 30 amp 120VAC charging circuit from the main supply.  The generator panel provides the critical loads to my house (kitchen but not counter outlets, microwave, fridge, freezer, kitchen heat pump, living room, internet and network, master bedroom, and possibly the living room heat pump but I need to measure the loads first).  The intent is to use the solar panels to decrease the cost of the critical loads when I'm home.

In the winter, I intend to remove it from the trailer plug it into the generator inlet permanently, but not to run it full time.  It will be kept charged by the main supply, and in the event of a power outage I'll switch the generator panel over to the inverter (manually).  If the outage lasts more than a few minutes I have a Firman 3600W generator (120VAC only) that I can use to keep the batteries charged.

Finally, and I'm not sure about this part, is I want to use my Highlander Hybrid to charge it either as required while camping or during an outage, and sell the generator.  I've installed a 1500W pure sine inverter under the dash, but in reality it's only 1200W max because in the Highlander there is a DC-DC converter for 12VDC loads which is rated at 100 amps output, getting it's power directly from the 288VDC batteries.  I'd like to build a pure sine wave inverter rated at 30 amps (3600W) directly off the main battery bank.  It will be a high frequency design to make it smaller because the loads will be constant.  Need to be careful working with that battery bank as it is rated at 288VDC and can get as high as 320 fully charged.  They make a good generator as the engine will start and stop as required to keep the battery bank charged, they can be used while driving, and they are very efficient including in effect braking energy will be used to charge the trailer.

I haven't settled on the size of the battery bank, as it will be a balance of weight and capacity, but I think I'm going to use 24VDC sized appropriately.

 

[The Blind Wolf]

Hello.  The GS inverter can charge at 240v or 120 v, but this has to be specify during ordering.  In my case I have a Gen Invert that puts out only 120v 30A so I have my GS invert setup to do just 120 input on chargeing.  The charging is set to what ever your setting is in the config of the charging area on the gs invert, but normally it will go by your battery bank voltage, so if you got a 48v system and you want it to charge up to 56v it can do that and then cut off the generator.

 

Please note, you might want to talk with Sid or Sean to make sure you got everything you need to have your generator start and stop by the gs inverter.  

 

In my case I have to send in a remote to have wired up to the gs invert to remotely start my gen up since its wireless start and stop.

 

Some generators have a wire to do such things.

 

But, if you charge at 240v, you will get a higher charging rate then 120, in my case I can only push 125a at 120v, where as if I was able to do 240v I could do 250a to my bank at 24v battery bank.

[The Blind Wolf]

Also please note, pulling power from your battery pack from a EV is going to void your warrenty if you do so, I know on Tesla that is a big no, no and will void your warrenty, the converter is not design to perform such task as supplying a inverter, let alone jump starting a lawn mower.  I've seen folks do such a thing with a tesla Y and they kept on wondering what all the clicking sound was, and don't understand that that converter is not design to do what tehy was doing.  The most that I know you can saftely pull from those converts to power 12v is only 20a, any higher and it causes problems. simply because most everything in the car is power off of a 12v battery and most items in the ev is running at less then 20a, the most power hog is the power windows and head lights.  Even the Tesla computer only uses 100watts to run and thats at 12v, which is not even 10a in power.

[Baz]

My Hybrid is old... it's been off warranty a long time... I do most of the work on it myself.

Understand the issues with the converter, that's why I'm going to pull it off the main 288VDC battery (maybe).  The DC-DC converter on a highlander is rated at 100 amps at 12VDC, so 1200W.  Like I said, I do have a 1500W inverter on the start battery (there is a separate small battery under the hood that runs some stuff when it is off and engages the main safety contactor when it is started, and is charged by the DC-DC converter), but that is because I had it.  It only runs small loads inside the car (mostly phone chargers, TVs, XBox, etc).

I'm not doing an auto start on the generator.  Mine isn't even electric start, it's pull start.  It's fine because I just use it for back up power in the winter.  That's the nice thing about the Highlander, when it's on it will start and stop the engine to keep it's battery charged correctly, regardless of what loads are being drawn.

If I understand you correctly, you can set both the charge points and the charge current through the interface?

[The Blind Wolf]

40 minutes ago, Baz said:

thing about the Highlander, when it's on it will start and stop the engine to keep it's battery charged correctly, regardless of what loads are being drawn.

If I understand you correctly, you can set both the charge points and the charge current through the interface?

 

 

From what Sid said about the settings on the GS inverter, yes you can set the low and high charge VDC level and I think, and he can confirm it, you can even change the level of charge rate such as only 50A instead of 125A charge rate.

 

Well, if the hylander is useing a standard battery to start and stop the motor and not pulling from the main power pack then that shouldn't be a isue, I just know the 12v system is only rated for so many amps on those ev cars, and jump starting isn't ment for them to do.

[Cali Carlos]

2 hours ago, Baz said:

Does the inverter continue to function while the charger is connected, drawing from the battery while the charger keeps it topped up, or like some UPS's does it switch the load directly to the 120VAC input?

From my understanding you can set it to either option through the inverter.

 

2 hours ago, Baz said:

I assume the charger can be set at a lower battery bank voltage than any connected solar so that the MPPT controllers can provide the primary charge if available and the charger only would activate if enough solar isn't available?

 

3 hours ago, Baz said:

It also isn't clear to me how many watts the charger is capable of providing.  I would assume it isn't the full 6000W, but can it handle the 3600W available in 30 amp circuit?  If so, can that be limited so that it wouldn't trip the breaker if connected to a 15 amp circuit?

Yes charge voltage and current can be set. I don't think there's really a hard limit on max charge current, I believe it depends more so on dc amperage and inverter temperature.

I'm sure sid will chime in with some more info

[Cali Carlos]

3 hours ago, Baz said:

I'd like to build a pure sine wave inverter rated at 30 amps (3600W) directly off the main battery bank.  It will be a high frequency design to make it smaller because the loads will be constant.  Need to be careful working with that battery bank as it is rated at 288VDC and can get as high as 320 fully charged.  They make a good generator as the engine will start and stop as required to keep the battery bank charged, they can be used while driving, and they are very efficient including in effect braking energy will be used to charge the trailer.

I've heard that the universal ac appliances that have the input range of 120-240 can be plugged directly into an ev battery without an inverter. Supposedly the first thing those appliances do is convert the power to dc anyways. Do some research first but might be worth looking into.

If you don't like that route look up vehicle to home/grid inverters. V2H V2G. There's not much out yet that I've seen though.

Edited March 16, 2021 by Cali Carlos

[Baz]

1 hour ago, The Blind Wolf said:

Well, if the hylander is useing a standard battery to start and stop the motor and not pulling from the main power pack then that shouldn't be a isue, I just know the 12v system is only rated for so many amps on those ev cars, and jump starting isn't ment for them to do.

Yes, but it's a little more complex than that with the Toyota system (not sure about the others).

If the 12VDC battery has gone flat (which is what will happen if you, say, leave the lights on) it can be jumped started; it's on page 541 of the manual.  It's done just like any other car.  However, when you start it you aren't turning the gas engine motor over, you're connecting the main contactor which will allow the main battery pack to provide 12VDC through the DC-DC converter.

If the main battery bank is discharged, which it won't do unless there is a malfunction, then you are out of luck; it's being towed.  The gas engine can't be started because there is no starter motor.  The gas engine is turned over by one of the motor generators by the main battery pack through the synergy drive system.

The reason it is impossible to discharge the main battery pack when the car is off is because it is disconnected by a contactor when off; there is absolutely nothing connected to the battery when it isn't running (running has a different meaning, in that the car can be running but the gas engine isn't necessarily doing so).

Therefore, the best place to get power from one of these, which is exactly what the new Ford F-150 Hybrid is doing, is after the main contactor:

[Baz]

8 minutes ago, Baz said:

Yes, but it's a little more complex than that with the Toyota system (not sure about the others).

If the 12VDC battery has gone flat (which is what will happen if you, say, leave the lights on) it can be jumped started; it's on page 541 of the manual.  It's done just like any other car.  However, when you start it you aren't turning the gas engine motor over, you're connecting the main contactor which will allow the main battery pack to provide 12VDC through the DC-DC converter.

If the main battery bank is discharged, which it won't do unless there is a malfunction, then you are out of luck; it's being towed.  The gas engine can't be started because there is no starter motor.  The gas engine is turned over by one of the motor generators by the main battery pack through the synergy drive system.

The reason it is impossible to discharge the main battery pack when the car is off is because it is disconnected by a contactor when off; there is absolutely nothing connected to the battery when it isn't running (running has a different meaning, in that the car can be running but the gas engine isn't necessarily doing so).

Therefore, the best place to get power from one of these, which is exactly what the new Ford F-150 Hybrid is doing, is after the main contactor:

Sorry, submitted it by mistake, continuing:

- it is safe to work there if the car is off as it isn't connected to anything

- there is beaucoup amps available (more than 50kW at a nominal 288VDC)

However, that means you need an inverter running off of 288VDC nominal, or a DC-DC converter down to something another inverter would take.  Given that 3600W at 288VDC is only 12.5 amps I'm going to see if I can make that work.

You are completely correct that it is inadvisable to draw heavily off the 12VDC.  There is less power available there than a regular car (even the start battery is smaller) and even the fuse for the 100 amp converter is expensive.

I'd really like to get a F-150 Hybrid, which has 7200W at 240VAC at the tailgate, but it's not in the cards right now.

[Baz]

1 hour ago, Cali Carlos said:

I've heard that the universal ac appliances that have the input range of 120-240 can be plugged directly into an ev battery without an inverter. Supposedly the first thing those appliances do is convert the power to dc anyways. Do some research first but might be worth looking into.

I haven't seen it done but theoretically that makes sense.  A standard PC power supply first rectifies the AC to DC and then uses a high frequency (greater than 20kHz) inverter to drive a small ferrite core transformer, which is tapped and rectified to give the different supply rails.  This is one of those times a high frequency inverter is the better design choice.

Unfortunately that doesn't fit into my design goals.  I'm looking at a drop in replacement connected to my current generator panel to provide backup power in the winter and solar supplement in the summer.  I'm not even thinking about going fully off grid at this house.

[The Blind Wolf]

Hmm, you said that is the highlander?  Not the Rad 4?  cause the Rad 4 that was fully EV with no motor was a Tesla under the hood drive train.  Not sure what they are useing under the highlanders,  I know the motor is connected to the tranny and the eletric motor is as well on a Prime, kind of forgot about it, I was thinking the old, old hybrid that still used a normal starter, so yes you would have one heck of a time trying to charge a dead battery even at 12 v 😛

 

if I remeber from my small research that battery pack is split in the middle, I think its a 144vdc. but, I could be wrong.  I watched a guy forogt his name that does a you tube vid of taking ev apart and he is in a wheel chair, he is a cool guy.

[Baz]

16 minutes ago, The Blind Wolf said:

Hmm, you said that is the highlander?  Not the Rad 4?  cause the Rad 4 that was fully EV with no motor was a Tesla under the hood drive train.  Not sure what they are useing under the highlanders,  I know the motor is connected to the tranny and the eletric motor is as well on a Prime, kind of forgot about it, I was thinking the old, old hybrid that still used a normal starter, so yes you would have one heck of a time trying to charge a dead battery even at 12 v 😛

 

if I remeber from my small research that battery pack is split in the middle, I think its a 144vdc. but, I could be wrong.  I watched a guy forogt his name that does a you tube vid of taking ev apart and he is in a wheel chair, he is a cool guy.

The RAV 4 EV was built by Tesla and Toyota between 2012-14; ie no longer in production.  The new Rav 4 Hybrid went on sale in the US in March 2019 and uses a similar hybrid synergy drive as the Highlander and Prius.  The Rav 4 PHEV uses a new drive is only currently available in Japan since June 2020.

The highlander (and RX400) pack is one of the biggest hybrid battery packs historically (but I'd bet the F-150 is larger).  It is 240 cells providing 288VDC under the back seat.  There's a good explanation of how it works and pictures here.  I have monitoring apps in my Android in the dash so I can lack at the state of all the modules (temperature, voltage, charge state, health, etc)  The guy you mention is a professor named John Kelly at Weber State University, whose You Tube channel is here.  He is probably the best available source of information on how hybrids work.

[The Blind Wolf]

Yep, thats the guy, he rocks!!!.  I can sit for hours watching him, and I learn a bit about batts in ev.

 

I want a EV so bad, but being disabled and unable to drive one and they cost so dang much lol.

 

I thought about finding a Leaf for the heck of it, just for local drive,.

 

Though I do have a Cyber Truck on order 😛

[Cali Carlos]

You'd think there'd be more inverters for EVs out there but what few I've seen are very vehicle specific. I would assume there'd be a huge market for a universal EV inverter. Most EVs have enough battery capacity to run an entire house for at least a couple days and if you figure in solar or in your case a hybrid you'd be able to go much longer and maybe even indefinitely. I think there'd be a market for backup power and for people trying to lower their electricity bill through a TOU grid tie setup and pumping electricity back into the grid during peak hours. Maybe @Sid Genetry Solar can invent one once he has less on his plate 😄

[Baz]

6 minutes ago, Cali Carlos said:

You'd think there'd be more inverters for EVs out there but what few I've seen are very vehicle specific...

I think, if the early reactions to the one on the F-150 Hybrid being any indication, that inverters will be available soon on all the larger (ie truck and SUV) hybrids. 7200W of 240VAC is useful in many areas, not the least of which is contractors, RVers, and home power backup.

[The Blind Wolf]

Frankly if Sid came out with a inverter that could handle a ev power pack, I would gladly snag several power packs and the inverter, instead of building batts.  Would make my system a bit stream line.

 

My setup is kind of a mix up Pup that don't know where his bone is, and well, thats what I get for starting in a hobby that I had no idea about, and buying things that really was just dumb in the first place.  Either way, now I know what I'm doing and understand a lot more on eletrical and all the crazy things about it, I can some what stream line it and not waist money on stuff that are just dumb.  If the GS inverter didn't came around I would have already bought two Victron 3k inverts by now.  But, when Sean announce the GS line and demo one, I jumpped in cause the fact is, the GS inverter from all I've seen blows even victron out of the water and its all thanks to Sid.

[Sid Genetry Solar]

9 hours ago, Baz said:

I understand the charger works on 120VAC even though the inverter produces split 240VAC.

If you request 120vAC input, yes.

 

9 hours ago, Baz said:

Does the inverter continue to function while the charger is connected, drawing from the battery while the charger keeps it topped up, or like some UPS's does it switch the load directly to the 120VAC input?  If the latter is the case does it use the transformer to create the split 240VAC?

No, in charge mode, "inverter" is off.  Load is switched directly to the AC input; the transformer creates split 240vAC via the autoformer principle if configured for 120vAC.  Yes, this does limit the maximum input power...it is safe to say to 1/2 (i.e. 3kw), though for all practical purposes 4.5kw may be possible without overheating the transformer.

 

9 hours ago, Baz said:

I assume the charger can be set at a lower battery bank voltage than any connected solar so that the MPPT controllers can provide the primary charge if available and the charger only would activate if enough solar isn't available?

Absolutely.  Fully configurable.  (Gosh I need to write that manual...!)  Charge voltage (float and "bulk") are completely configurable, as is the charge amperage (based on AC input amperage FYI.)

 

9 hours ago, Baz said:

It also isn't clear to me how many watts the charger is capable of providing.  I would assume it isn't the full 6000W, but can it handle the 3600W available in 30 amp circuit?  If so, can that be limited so that it wouldn't trip the breaker if connected to a 15 amp circuit?

Yes, charge current can be limited.

I'm not sure how many watts it can provide either 😉.  The limit will be the temperature of the FETs, as they are considerably more lossy in charge mode (due to the use of the backsplash diodes in the FETs, which drops up to 1.2v per quadrant).  I've run 33A back into a battery during tests (at least until my oscilloscope started to emit a terrible smell due to the noisy AC input!)...33 amps @ 55v is 1.8kw...really nothing to write home about--but nothing was getting warm/hot, so that's good.  It should handle 3.6kw without any issues.

I should note that the intrisic boost charge method (very common in LF inverters--it doesn't require any external parts) isn't particularly efficient...I was measuring 50% efficiency...which is pretty pathetic if I say so myself.  (Worth noting: I've never seen "charge efficiency" ratings on LF inverters, probably because of this!)  However, there's a "PFC" software tweak I can implement that reportedly will increase charge efficiency by 30%...which is quite significant.  Something to add to my list...

 

9 hours ago, Baz said:

I'd like to build a pure sine wave inverter rated at 30 amps (3600W) directly off the main battery bank.  It will be a high frequency design to make it smaller because the loads will be constant.

Why stop there?  https://theinverterstore.com/product/30kw-pure-sine-power-inverter-charger-300-vdc-240-vac-split-phase/

Haha.  Dangerous stuff, but...!

3 hours ago, Cali Carlos said:

You'd think there'd be more inverters for EVs out there but what few I've seen are very vehicle specific. I would assume there'd be a huge market for a universal EV inverter. Most EVs have enough battery capacity to run an entire house for at least a couple days and if you figure in solar or in your case a hybrid you'd be able to go much longer and maybe even indefinitely.

There's a good reason that the designs are very vehicle-specific: the battery systems are completely nonstandardized.  From a design standpoint, it becomes extremely difficult to try to support every little difference in manufacturer designs--not to mention battery voltages, connectors, interfacing, BMS systems, etc., etc.

 

4 hours ago, Baz said:

I haven't seen it done but theoretically that makes sense.  A standard PC power supply first rectifies the AC to DC and then uses a high frequency (greater than 20kHz) inverter to drive a small ferrite core transformer, which is tapped and rectified to give the different supply rails.  This is one of those times a high frequency inverter is the better design choice.

You'll be perfectly dandy with most older switching-power supply appliances (yup, bridge rectifier -> filter cap -> switching circuit), however, some designs with PFC correction might not be too thrilled with DC input.

Something you MIGHT consider, would be to "hack" an existing high-frequency inverter setup.  As they're all different in their own ways, there may be a few "gotchas" to keep in mind...BUT most of them have a boost converter to generate 180-200vDC (for 120vAC output) or 360-400vDC (for 240vAC single-phase output).  Technically you might be able to use a 240v inverter, sneak power to the high voltage filter caps, figure out how to power the control module (most often on the cheaper Chinese units, it's an EG8010 / EGS002 board...horrible with surge loads by design, but not too shabby otherwise)...and have a solution of sorts at your own risk.

Of course, the 280-300v range is just simply difficult to work with--but you might figure something out.

[The Blind Wolf]

Hey Sid, so when my Gen kicks in, the invert will be relying on my gen to provide the house the power?  Or can I have it just go into charge mode and have the slave take the load for the few mintues?  Dang, I wish my gen was a 240v now, I do got a honda with a turn switch to crank on it, and its got 240v, but I don't think its pure sign as its one of those construction type that frankly I can not even move it, I have to use a dolly to even move it and it takes two people just to even pick it all up. lol.  Its a 8500 watt model. I wish I could use it, as its got a massive tank and can run for nearly 24 hours on a full load, problem is its franken loud *grin*.  I'll stick with the 120v

[Cali Carlos]

1 hour ago, Sid Genetry Solar said:

No, in charge mode, "inverter" is off.  Load is switched directly to the AC input; the transformer creates split 240vAC via the autoformer principle if configured for 120vAC.  Yes, this does limit the maximum input power...it is safe to say to 1/2 (i.e. 3kw), though for all practical purposes 4.5kw may be possible without overheating the transformer.

Is there a mode that allows charge and inverter at the same time with an ac input or is that not possible due to the FETs not being able to do both at once?

[Baz]

Sid,

Thanks.  That answers all my questions.

So when charging from the mains the load needs to be 3600W or less, due the ability of the transformer to form 120VAC into 240VAC, with a surge to 4500 or so, but that would depend on having that much available at the plug.

I could use a more efficient separate 30 amp 120 VDC (ie 3600W or nominally 150 amps into a 24VDC bank) to maintain the battery charge from the mains as my solar will be initially undersized for my generator panel loads.

I could also recharge the batteries from the mains when the loads are smaller, like possibly at night.

And it may make more sense to use a DC-DC converter/charger directly into the 24VDC battery bank when away from home.

... or maybe just come up with the $60k Cdn for an F-150 Hybrid :-!

I think my next step is to start monitoring the loads on at least the generator panel to see what my real world requirements are before I go any further.

Thanks again, that helped a lot.

[Sid Genetry Solar]

13 minutes ago, Baz said:

So when charging from the mains the load needs to be 3600W or less, due the ability of the transformer to form 120VAC into 240VAC, with a surge to 4500 or so, but that would depend on having that much available at the plug.

Yes and no.  Continuous 3kw for sure.  But it might be able to sustain 4.5kw total input draw, depending on the heat load of the tranny (this hasn't been tested).  I specced the output windings for 25A...not saying they can't do more, but they'll generate more heat at higher amperage...which limits the total power.

 

33 minutes ago, Cali Carlos said:

Is there a mode that allows charge and inverter at the same time with an ac input or is that not possible due to the FETs not being able to do both at once?

Sorta yes and quite a bit no 😉.  In the "charge" mode, it is not possible to produce any AC output (all AC output is a direct connection from the AC input).  In the "inverter" mode, it can't regulate battery charge.  Basically, it's kinda impossible to try to regulate 2 things with one throttle (i.e. AC output voltage AND backcharge rate.)

One half-exception: technically the grid-tie master functionality on a GS inverter allows for battery charge during inverter mode--but the crucial point to keep in mind here is that the inverter is the master device.  It throttles down the slave grid-tie system via frequency shift (either linear throttling if supported, otherwise it's an effective on/off) based on the charge settings and loads.  If the "slave" device does not accept/handle a frequency shift throttle, then it is not possible to regulate battery charge voltage/current.

But obviously, this cannot function with either AC Mains (grid) or a generator--neither accept a frequency shift, nor can either be throttled down.

[Sid Genetry Solar]

1 hour ago, The Blind Wolf said:

Hey Sid, so when my Gen kicks in, the invert will be relying on my gen to provide the house the power?

Yes.  Generator has to take 100% of the load, otherwise it's not possible to charge the battery.  If the inverter is drawing power from the batteries, obviously, that's classified as "charge" in at least all of the books I've read 🤪.

1 hour ago, The Blind Wolf said:

Dang, I wish my gen was a 240v now

The autoformer principle of the transformers' secondary will provide 240v loads from the 120v input--but obviously, all the power has to get funneled into the inverter through that single 120v phase winding--there's a lot to keep in mind here.

 

1 hour ago, The Blind Wolf said:

Or can I have it just go into charge mode and have the slave take the load for the few mintues?

Technically yes, the slave will continue running (which might be a problem of its own)...BUT if the load on the battery is not offset by the charge current, you'll still be draining your batteries, even with the generator running.  Maybe less of a drain, but definitely not a charge.

Not to mention that battery charge mode is less efficient than inverter mode (due to system losses).  Kinda a catch-22.

[Cali Carlos]

36 minutes ago, Sid Genetry Solar said:

If the "slave" device does not accept/handle a frequency shift throttle, then it is not possible to regulate battery charge voltage/current.

At which point I assume the inverter would error out and shut down? 

Even if that wasn't an issue, and say a 10kwh bank was nearly depleted and you wanted to hook up a 1800w generator to the output and set the inverter to grid tie master mode for a few hours just to top up the bank a little but nowhere near long enough to fully charge the bank to where it wouldn't need the frequency shift. Could that work? And if so would it need to be a 240v generator or could a 120v work?

[The Blind Wolf]

Well, I've decided to run the Master on the LI batts, and run the slave on Lead  So one can charge while the other is running off batts on its own, then I can have the lead batts charge in the morning on its own charger.

[The Blind Wolf]

The slave won't get used that much to effect the batts as much as the master, as I don't hit over 6k the offten, matter of fact during the summer, only the dryer and the hot water heat is the only two devices that would push me to useing the slave, and that would only last a hour on the dryer water heater I think only kicks on for like 10 mins at the most unless after I take a show then I think it takes 30 mins to heat back up, never really timed it. going to change it anyhow to a heat pump heater.