What size generator?

[Marvintinyhomes]

I’m looking for a propane generator for our line of tiny homes. These tiny homes are built on trailers, and power is distributed thru an RV style 30amp 120v panel, this gives our clients the option to plug into shore power at RV parks. For our off grid models, we’re planning to use a Victron 5000W 120V system with a generator to power the house and help charge the batteries (we’re looking at 48V, 200Ah).
For the generator, I talked to someone from signature solar awhile ago and he told me that I would need at least twice the inverter power. So for a 5000W system, I would need at least a 10kW generator.
Can someone please confirm this?

Thanks

 

[Samsonite801]

Well, a lot of these inverter types must run in 'inverter bypass' mode in order to charge the batteries, which means the generator is supplying the household AC loads in addition to powering the battery charging load. Many of them allow the user to turn down the amperage on the battery charging to a lower than max level, but cannot limit what the household users will draw. So the generator needs to be sized to handle the sum of both respective loads.

There are some other inverter models which can charge only the batteries, while the AC household loads are still being run off the inverter. You have to check on the Victron specs, as I am not familiar.

The difference has something to do with how the mosfets for the inverter circuit are used for inverting, and for converting grid/gen AC into battery charging (can't be simultaneously used on certain topologies)...

My inverters for my permanent house system are MPP Solar LV6548, they are the first type which do not allow inverting and AC battery charging to occur at same time, so I purchased two standalone AIMS 48v battery chargers so I can charge on the battery bank using a smaller sized generator, where the house loads can still use the inverter power only.

 

[Checkthisout]

I’m looking for a propane generator for our line of tiny homes. These tiny homes are built on trailers, and power is distributed thru an RV style 30amp 120v panel, this gives our clients the option to plug into shore power at RV parks. For our off grid models, we’re planning to use a Victron 5000W 120V system with a generator to power the house and help charge the batteries (we’re looking at 48V, 800Ah).
For the generator, I talked to someone from signature solar awhile ago and he told me that I would need at least twice the inverter power. So for a 5000W system, I would need at least a 10kW generator.
Can someone please confirm this?

Thanks

No. Not even close to correct. The victron allows you to use a generator that's smaller than your needed load by sucking juice from the batteries when needed to make up the difference.

When charging the batteries off the Gen, it seamlessly blends generator power to house power will then pull from the batteries....

You need to talk to someone who can explain it to you.

A 3kw generator would be perfect.

Are your tiny homes 100% electric?

 

[sunshine_eggo]

No. Not even close to correct. The victron allows you to use a generator that's smaller than your needed load by sucking juice from the batteries when needed to make up the difference.

An excellent feature that I use regularly, but Victron still recommends a generator about 60% larger than the inverter power:

MultiPlus Generator FAQ [Victron Energy]

www.victronenergy.com

Essentially, size the generator for max charger power and max inverter rated power. This allows full pass through support of all rated loads and maximum charging.

When charging the batteries off the Gen, it seamlessly blends generator power to house power will then pull from the batteries....

You need to talk to someone who can explain it to you.

A 3kw generator would be perfect.

Useful, yes. Perfect? Far from it.

Marvin is selling to folks. Folks aren't DIYing it for themselves. In the latter case, I would agree with you, but for customers, you basically want to provide a near seamless experience, maximize charging to minimize generator run time and not interfere with normal living.

The answer to this question is tied to the desired customer experience:

1) seamless operation without altering behavior and minimized generator run time.
2) customer has to pay attention and may have to run generator for extended periods.

#1 is an 8kW generator.
#2 is 3-4kW.

 

[Vigo]

Unfortunately there is a bit of a gray area in terms of how small a generator you can use, even IF you can set a limit on how much the inverter will pull from it. What you can't control is how quickly it applies that load to the generator. If the inverter ramps up the load slowly even a tiny generator could be used, with proportionately small limit on the inverter draw. My Growatts do ramp up slowly in terms of battery charging from the ac input, but NOT in terms of the transfer switch. I would expect most inverters with transfer switches are the same.

A generator has a certain amount of momentum in its rotating assembly, and takes a certain amount of time for its governor/throttle mechanisms to adjust to load. Part of the reason why a generator needs to be 'oversized' when hooked to something with a transfer switch, is because the transfer switch can apply load to the generator so quickly that if its momentum can't bridge the gap, it will stall before the governor/throttle can compensate. It would be like taking a car with a manual transmission and giving one person the throttle and another person the clutch, and they're not allowed to talk to each other, and the one with the clutch has never driven a manual before. Putting a big honking engine in front of that clutch is a pretty inelegant solution, but that's basically what is recommended in this situation for a 'seamless user experience'.

Which, for the record, i agree with Sunshine that as the seller you should stupid-proof the system to the extent possible, which means putting the big honking engine in there.

 

[sunshine_eggo]

We're speaking specifically about Victrons, and they interface with generators very well with gradual load application. There's even a setting to base it on historical load characteristics. Furthermore, the PowerAssist feature is OVERLY aggressive by default in that it will initially assume a large portion of the overload and gradually back off shifting the load to the generator.

 

[Checkthisout]

An excellent feature that I use regularly, but Victron still recommends a generator about 60% larger than the inverter power:

MultiPlus Generator FAQ [Victron Energy]

www.victronenergy.com

Essentially, size the generator for max charger power and max inverter rated power. This allows full pass through support of all rated loads and maximum charging.



Useful, yes. Perfect? Far from it.

Marvin is selling to folks. Folks aren't DIYing it for themselves. In the latter case, I would agree with you, but for customers, you basically want to provide a near seamless experience, maximize charging to minimize generator run time and not interfere with normal living.

The answer to this question is tied to the desired customer experience:

1) seamless operation without altering behavior and minimized generator run time.
2) customer has to pay attention and may have to run generator for extended periods.

#1 is an 8kW generator.
#2 is 3-4kW.

Yeah, I see your point.

I'm curious what electrical items his tinys have that requires 5kw.

I disagree a but on the stupid proof. There is no way to stupid proof a portable house that you have to plug in, start a generator and have solar to power along with all the equipment it takes to manage that stuff.

If I was the OP I would omit supplying all that stuff and just say the thing is "solar ready" or some such nonsense.

Of course some neighbors at my recreational property just bought a cabin kit where everything is designed and supplied including batteries and solar so its probably me who is clueless about the market.

 

[Vigo]

We're speaking specifically about Victrons, and they interface with generators very well with gradual load application. There's even a setting to base it on historical load characteristics. Furthermore, the PowerAssist feature is OVERLY aggressive by default in that it will initially assume a large portion of the overload and gradually back off shifting the load to the generator.

I love it, and maybe someday I'll own one, but I was just explaining the 'general reason' why someone might recommend such a large generator. Signature Solar doesn't sell Victron, and they're saying double the generator size because what they do sell will dump the load onto the generator in a split second.

I'm curious to know why Victron says +60% when they have the ability in their equipment to gradually transfer the load. I can see making some accommodation for altitude and poor engine performance on the generator... maybe that is where the 60% comes from. Not entirely out of line, i guess.

 

[Checkthisout]

I love it, and maybe someday I'll own one, but I was just explaining the 'general reason' why someone might recommend such a large generator. Signature Solar doesn't sell Victron, and they're saying double the generator size because what they do sell will dump the load onto the generator in a split second.

I'm curious to know why Victron says +60% when they have the ability in their equipment to gradually transfer the load. I can see making some accommodation for altitude and poor engine performance on the generator... maybe that is where the 60% comes from. Not entirely out of line, i guess.

We are diy people.

Eggo has it right I think. I forget that people use victron in enterprises and things like yachts where your customer is not going to actively manage power intake and consumption. Everything just better work perfect and be as seamless and passive as grid power.

 

[Checkthisout]

I love it, and maybe someday I'll own one, but I was just explaining the 'general reason' why someone might recommend such a large generator. Signature Solar doesn't sell Victron, and they're saying double the generator size because what they do sell will dump the load onto the generator in a split second.

I'm curious to know why Victron says +60% when they have the ability in their equipment to gradually transfer the load. I can see making some accommodation for altitude and poor engine performance on the generator... maybe that is where the 60% comes from. Not entirely out of line, i guess.

Like Eggo said they expect the customer is a yacht owner who wants to sacrifice nothing so that means full power to charge batteries and run most of the normal load plus a little extra.

In other words it assumes recharging fully depleted batteries while simultaneously supplying the normal house loads.

 

[Mattb4]

If you are a US based firm the probably best option is Onan RV propane fueled generators. They are designed for the rigors of being mobile and providing a continuous power. Most of your portable generators ratings are more marketing than actuality. The other thing is you probably should go to a separate battery charger rather than use the AIO to do that duty. You would need to install a transfer switch after the AC out.

Size the generator sufficient to start your heaviest load and power the battery charger.

 

[Marvintinyhomes]

Thanks so much everyone for your replies. Our tiny homes are electric and propane. The stove is propane and 12VDC to run the lights and controls, biggest draw is the combi boiler unit (202Wh). We’re located in Alberta so I’m expecting this to run 16-20 hours in the wintertime. Other draws include LED lights, 900W microwave which will probably run 15 minutes a day max, washer/dryer combo (296Wh) and a 39W fridge. The tiny home is calculated to run off a 30 amp 120VAC RV panel, we’re looking at a 5000W 48V victron design and hoping to get 10kW battery bank. I will start a new thread on batteries for your recommendations.
The generator that we’ve used previously is a Westinghouse iGen4500. We’re looking for something that can run on propane, since the tiny homes will have propane available. Any generator and/or battery recommendations will be greatly appreciated.
Thanks again everyone and have a great day.

 

[ElectricIslander]

You need to think about energy storage (KW-hrs) not just the power providing capability / loads that you've outlined (KWatts).

There are lots of online calculators that help figure out household power system storage and PV panel requirements based on a list of loads.

A 10 KW (hr!) battery seems small for such an extensive load list (ie don't forget to factor in several days without sunshine which will multiply that by 2 or three times).

 

[photomankc]

Funny, I just got smacked in the face by this too. I've got a 650sq foot cabin we built as a getaway location. Not for extended stays, just a place to go and enjoy the outdoors and country for a few extended weekends a year. As such I didn't want to sink a huge fortune into the system. I went with a single EG4 6500ex with a 3 slot rack and starting with a single 5KWh battery. It's a little louder than I expected but once the walls are all up I think it will be manageable as my average load would run less that 10% capacity.

However, my assumptions on the generator to buy last year were all based on the notion that I could charge while the loads were running, similar to some of the all-in-one portables but once I installed the EG4 6500EX I realized I had a problem. The 3000W generator is not going to cut it for charging AND running everything else in bypass mode. It basically cuts my legs out because the surge capacity was what I was interested in and anytime I'm charging I don't have enough to even run the microwave. Cutting the charge current back to 20A (~9A at the generator) means the charge times on anything more than 5KY get ugly. Going to 30A charge (~14A at the generator) leaves just 1300W for everything else but makes charge time a lot more realistic.

I have a couple options I can see.

1.) Upgrade to a generator that can deliver say up to 50A @ 120V through the outside inlet on the cabin. That inlet was rated to 50A so as long as I could find a generator and cable that can deliver that I could be good. There are some 6000-7000W generators that look appealing though I hate to foot that expense. Main issue is most I see with only 30A breaker for the twist-lock or RV outlets. I'd need to be able to bring in at least 40A to keep things viable while charging and in bypass so I could run a microwave or kettle at the same time.

2.) Could I simply disconnect the generator from the 6500 and get a separate charger so that the generator is not ever suppling the loads in the cabin? It seems like it could create a problem though as how would the inverter not be drawing from the charger as though it were another battery? So say its charging at its max at 55V and 40A but the inverter demands 55V at 60A. Would that overload the charger and cause smoke?

I'm planning next year to start work on a ground solar array of 6 to 8 440W panels and to expand to 10KW or 15KW battery but for now its 5KW and using the generator to recharge. I just was not expecting that limitation of charging only while in bypass mode. I went with the 6500 mainly to make sure that my average load left lots of headroom for surge for a water pump and maybe a small A/C unit for use on a really hot day.

 

[sunshine_eggo]

2.) Could I simply disconnect the generator from the 6500 and get a separate charger so that the generator is not ever suppling the loads in the cabin?

This is what Signature solar recommends as an alternative to a 9kW generator in your case. They have a 18A charger for about $200.

This is also why the portables can charge from AC while in operation... they are being charged with an EXTERNAL charger that's supplying DC.

It seems like it could create a problem though as how would the inverter not be drawing from the charger as though it were another battery?

Nope. It will draw from the 48V system as a load. The charger is a source to the 48V system. It will supply up to 18A throughout its working voltage range. It can't supply more than that regardless of any load on the 48V system. Battery voltage will fall as a result of the load and the charger will simply keep pumping 18A into the system.

So say its charging at its max at 55V and 40A but the inverter demands 55V at 60A. Would that overload the charger and cause smoke?

It's not an overload. The battery is there to supply the load. Net result of your 40A charge, 60A load scenario is: 20A is coming out of the battery.

I'm planning next year to start work on a ground solar array of 6 to 8 440W panels and to expand to 10KW or 15KW battery but for now its 5KW and using the generator to recharge. I just was not expecting that limitation of charging only while in bypass mode.

The clue was "bypass". You're bypassing the inverter with the source. As such, you are limited by your source.

ALL inverter/chargers operate this way. They can either invert or charge - not both. The same circuitry performs both functions and can only do one or the other at any given time.

 

[John Frum]

For our off grid models, we’re planning to use a Victron 5000W 120V system with a generator to power the house and help charge the batteries (we’re looking at 48V, 200Ah).

I think you mean 5000VA which is ~= 4000 ac watts continuous.

 

[Checkthisout]

ALL inverter/chargers operate this way. They can either invert or charge - not both. The same circuitry performs both functions and can only do one or the other at any given time.

I thought the victron multiplus did exactly that?

 

[photomankc]

This is what Signature solar recommends as an alternative to a 9kW generator in your case. They have a 18A charger for about $200.

This is also why the portables can charge from AC while in operation... they are being charged with an EXTERNAL charger that's supplying DC.

Awesome! I'd much prefer to keep the investment in my current generator. It will complicate wiring I imagine but saves me $1200 on a new generator. I'll look for a seperate charger and just connect solar to the 6500 at a later date.

Nope. It will draw from the 48V system as a load. The charger is a source to the 48V system. It will supply up to 18A throughout its working voltage range. It can't supply more than that regardless of any load on the 48V system. Battery voltage will fall as a result of the load and the charger will simply keep pumping 18A into the system.



It's not an overload. The battery is there to supply the load. Net result of your 40A charge, 60A load scenario is: 20A is coming out of the battery.

Cool. I'm supposing the the electronics in the charger are making sure that they don't exceed that, which makes sense now that I think on it since they have to control the current to the batteries in the process of charging them. Thanks for confirming.

The clue was "bypass". You're bypassing the inverter with the source. As such, you are limited by your source.

ALL inverter/chargers operate this way. They can either invert or charge - not both. The same circuitry performs both functions and can only do one or the other at any given time.

I was not clear on that prior but I got the picture after reading up more. That bypass was the mode it operated in while charging from utility just was not known to me before hand. As soon as I started it though and noticed it refused to charge while in BSU priority mode but in USB it did and stayed in bypass I knew my previous assumptions were probably in error. Makes sense if the pathway through the electronics is shared.

 

[John Frum]

I'm guessing the generator has to be portable, confirm?
How many service hours should the generator last?
This one is made to last https://www.norwall.com/products/Cummins-Onan-QG-6-5-Propane-RV-Generator-6.5HGJAB-904/
Not so sure about this one https://www.norwall.com/products/Cu...el-Electric-Start-Portable-Generator-P9500df/

Do you need auto-start?

 

[photomankc]

I thought the victron multiplus did exactly that?

It advertises it does. Will 'boost' from battery if the load sags the 'shore power' and then when it drops will go back to charging. I suspect it's internals are different for that. It's also as expensive as buying another generator though and I would still need an external solar charge controller as well from what I gather. I think it's more a limitation of the cheaper Chinese 'everything in the box' unit we mostly use.