Backup Generator Size

[NakeDiesel]

We have been planning on moving towards solar in the future, but it's a multi step process for us, we have other items we need to get done before we get to that point. I just had all our windows replaced in the house (22k worth), and I am getting ready to start residing our house this fall with adding double bubble insulation and steel ag panels and trim now that the windows are on to further seal the house up.

The first step towards solar is to add a backup generator and ATS. We have been having a lot of power issues from PSO here lately, power going on and off with storms, several times a week for hours at a time and with me working from home, puts a damper on the work day. I've installed an Emporia 16 node monitor on our house panel and been monitoring it, with both HVAC systems being hammered in our 100+ days it looks like we have been hitting peaks of 15.7 kw max over the last few days I've had it installed.

Presently the house has 2 HVAC systems with 2.5 ton AC systems and propane 60k BTU furnaces, both of which are 20 years old, those are in the plans to get replaced in the next year as well and we are planning that out with my buddy that's an HVAC dude. Most likely going with the Mistsubishi Intelli Heat pump system (haven't decided on hyper heat or regular yet) with a goodman 60k 97% propane furnace for backup heat.

I'd initially thought of going with a 20kw backup genset on propane, but looking at the consumption we have on our current peak high usage days with high heat, I'm wondering if we can't get away with an 18kw generator.

I won't be running my well on this, I'll be installing a plug for my trailer mounted 10kw welder/gen to run it if needed if we are out for a long term situation.

Any thoughts?

 

[50ShadesOfDirt]

Do these older hvac units already have "soft-start" devices on them? In an outage, can you live with just one older HVAC unit running? These things might help you limp by w/ a smaller gen unit until the old HVAC units are upgraded.

Unless really worried about the costs of the gen's themselves, I'd get the most KW size you can afford. A 22kw generac isn't much more than a smaller-sized generac, and it's always better to have more power than not enough, especially if the need is for somewhat infrequent outages. If you are worried about fuel/hr costs, then keep in mind that the 2nd step (after gen + ats) is inverter+charger+battery-bank, which will be a buffer between running the gen continuously during an outage (up to 24 hours/day), or just long enough to refill the battery-bank (perhaps 4 hours or thereabouts).

Depending on how the ATS is wired up, perhaps the trailer welder/gen can stand in for a test of gen capacity in your home; throw some breakers, and see how well the trailer gen can handle a reduced set of loads.

Finally, you could have the "backup genset" folks (generac, kohler, etc.) or any auth'd dealer do the "sizing" quote thing for you, and perhaps that would help to zero in on an appropriate size for your home & loads. One quote for max load, and perhaps one showing the reduced load of new/planned HVAC unit(s).

Hope this helps ...

 

[sunshine_eggo]

If moving towards solar in the future and expecting to use backup generator as needed when solar lets you down, the generator needs to be 30-50% greater than the inverter output power, i.e., a 10kW inverter (for up to 10kW of loads) needs a 13000-15000W generator.

An inverter/charger can't be both at the same time. Either it's using the transformer for inverting DC to AC, or for AC to DC for charging. When acting as a charger, it passes through the source AC to the loads, i.e., when charging with generator, the generator is also powering loads. If your generator is not powerful enough to handle both charging and loads, you may need to compromise your utilization when charging.

 

[NakeDiesel]

Thanks, both. Not going anywhere near a Generac generator, don't think they are very reliable and haven't liked the dealers I've talked to in the past. Most of the sizing tools I've looked at and gotten quotes for are in the 24kw range. Our long outages that we would have to deal with are winter time outages an could be up to a week like we have had in the past. Spring through fall, usually a few to 8 hours at most on an outage. We won't get completely off poco power, my shop has it's own 200 amp feed to it and don't have any plans to move it to solar anytime in the future, it has it's own 5 ton heat pump hvac system set at 80 in the summer and 50 in the winter unless I'm out there, plus my big air compressor, chiller, welders, etc...

No, I don't have soft starts on the AC's, I've thought about that, but with me planning on replacing them in the next year I'd rather just turn one off and run it till we can get them both converted over to the new ones.

Most like going with a 20kw Cummins propane generator and tying it into my current 500 gallon tank and a 200 amp ATS with the extreme cold package as we have has weeks of -19 at a time here on the farm. Kohler would be my backup brand for propane powered generators. Still want to check with my Son and see if he can get me a swinging deal on a Used older Cat generator at the Cat dealer where he's a mechanic at. I just can't justify the cost of a new 24kw diesel generator. I'd drop a 200 gallon diesel tank next to it and also use it to fill up my equipment as well and get it refilled periodically with dyed diesel.

 

[pvdude]

We had a generator decades before solar, so the solar "inherited" whatever the generator specs are.
The generator is a 21kw Perkins Diesel.
87A of 240vac.
87A allowed us to be less careful about what is simultaneously powered up in the house.
However, starting the 4T heat pump compressor was a big hit for the generator.

Later, I learned how to measure the LRA (Locked Rotor Amps), found the compressor wanted 132A to start the rotor turning.
Solar inverters would not start it.
Installed a Micro-air "Easy start" on the compressor, LRA is now 38A, both generator and solar start it easily.
Discovered that the maximum input the solar inverters can accept from the generator is 60A (14,400 watts). So far, no problems, as we are careful to manage loads when there is no grid.
I connected the generator to the inverters, use it to charge the batteries in extreme conditions, and to support the load when the batteries / PV are not sufficient.

The generator is connected via a Hubbell "pin & sleeve" 100A connector.
This way, I can either:
- connect the generator directly to the house and use the full 87A
- Connect the generator to the solar inverters and use up to 60A

 

[AHTrimble]

Depends on how you want to use your “back-up” generator in the bigger scheme of things. If the genset is to provide assistive power to your inverter/charger due to low battery SOC, then I suggest that a person consider increasing their PV production and/or battery capacity.

We are 100% off-grid, what we do is use our genset for two options:

1) Providing assistive power through the inverter/charger to the system to provide additional power to charge the batteries. Since the PVs are generally still producing some power, the genset is adding to that power to charge batteries. This has never failed us.

2) When working on the solar system I use a transfer switch to provide all the household power via the genset. This allows me to completely shutdown the solar system while working on it.

When it comes to the generator we use a dual fuel Champion 4250 inverter style unit. We have 2 x Victron 5kW inverter/charger units (10kW total – 220vAC, split-phase). However, we seldom consume over 16kWh per day and I have never seen us over 6kW at any one time. So our 4kW generator is more than sufficient for our needs. I run it at 22amps limited input to the a single inverter/charger unit, about 2/3 of genset capacity. That provides about 45amps charging to the batteries (if I did the math right) if the system is completely down.

If I have the solar system completely down (i.e. while I am working on it), that is only about 2.6kW powering the house. And that is fine because we go into conservation mode if needed until I have the solar system back up and running...which is easy for us to do. 22amps AC gives us plenty of power for our household needs when needed.

A couple notes on the generator:

1. Only use an inverter style generator for power...that is “clean” energy and is much easier on all your household equipment, especially electronics.
2. You should only run your generator at about 50 – 66% of rated capacity. This will extend the life of your generator and be much kinder on fuel usage. Think of it this way...you don’t drive your car at full speed all the time do you?
3. I love Champion generators!! They are incredible machines, 1 pull to start, run forever, and I have owned, do own, 3 and love each one...especially the dual-fuel version. Just as reliable as a Honda and FAR more economical.

I hope this helps a little bit.

AH Trimble

 

[DIYrich]

My thoughts are a minimal generator. If you have solar and battery adequate for 24 hours, then your generator only needs to be about: 24 hrs kWh / 8 hours × 1.3. That will provide your daily power in 8 hours of run time. The 1.3 is to run the generator at 75%.

 

[NakeDiesel]

It will be quite some time before I get the solar started. Trying to hopefully get the first phase (generator) done before winter this year, don't know if I'll get there or not yet. Partly depends on how my back does and if/when I end up needing my other shoulder replaced. I need to build the pad where the generator/solar house will be located. I need to still verify that I can use polaris type connectors inside a junction box under the house or if I'm going to have to run a whole new copper line from the pole -> under the house -> to the solar house/generator pad where the equipment will be located.

The point of this is to have it automated so it's there for my wife if I'm gone, I have to travel to London, UK, 3 to 4 times a year for a week or more a trip. I can control our HVAC from anywhere if the sat link is up at the house as well as monitor the cameras and my array of temperature sensors around the house, shop, under the house, in the 3 freezers and the well house.

Here are the phases I have planned out:

Phase 1

Phase 2

Phase 3

I've designed it this way, so that if the solar part ever needs to be taken down for maintenance or repair, I can just flip the manual transfer switch and the house will be on poco/generator power and the house is still going while I work on it or send off the inverter for repair, or whatever.

 

[packnrat]

i just have a simple honda eu 3000. semi hard wired to the house main panel. WITH a interlock. so no power can be back fed to the grid.
to keep the law and tax man off my back, the "semi" part is i need to manually plug into the honda, before any power can feed anywhere.
i have a small home, only 1000sq feet. built in 1950, so lacking in outlets anyhow. main use is keep the fridge working so as to not lose food. a couple lights. more?
just i have to be home to start it all up. but i am retired living on ss.
solar would only increase my taxes. not save any money. pluse $50 grand in solar/ batteries, wire, controllers, permits, taxes, etc. i would never break even. not counting interest over 21 years to break even. (i am 63 now).