Sizing generator to start and operate central air conditioner and electric motors

[csheath]

This is a good read on generator sizing for starting AC units.

http://www.generators.smps.us/start.html

 

[jamawieb]

Very good article.

 

[Chainbreaker]

Great info! That link should be added to a STICKY.

 

[Guyfang]

Well, don't just talk about it. You might also consider looking for a little more info to go along with it.

 

[NATCAD]

Well, don't just talk about it. You might also consider looking for a little more info to go along with it.

Who has authority to make sticky

 

[Chainbreaker]

Yes, a "What size generator do I need?" sticky would help a lot of people choose the right generator and probably save a lot of folks from making a costly mistake.

I'm sure there are members here that have a lot of tips and know-how in determining how to go about sizing a genset. How do we start a Sticky, or is this the beginning of one?

 

[Chainbreaker]

My first set of questions to someone asking "What type and size generator do I need?" would be:

Intended Use:

- Home Backup
- Workshop use
- Offgrid House or Cabin
- RV/Campsite/Tailgating
- Contractor/Portable/Events
- Commercial/Industrial
- Other ??

Basic Electrical Requirements:

- 120V 1 Phase?
- 120/240V 1 Phase?
- 120/208V 3 Phase?
- Max kW Requirement? * To be verified per "Electrical Load Requirements" audit
- Hook up method? Interlock, Transfer Switch?
- Load shedding necessary?
- Auto-start & switchover?

Typical Use Operation & Duration:

- Intermittent outages - hours to a few days?
- Planned routine use - daily, weekly or 24/7?

Special Requirements:

- Medical needs? (reliability factor MTBF)
- Budgetary? - Initial purchase & operational costs

Fuel Type - Diesel/Gas/Propane:

- Do you have a fuel preference?
- Do you have existing fuel storage that you can utilize?
- Anticipated run time hours between fuel delivery/replenishment?

Servicing & Repair Expectations:

- Owner's level of electro-mechanical aptitude & skill?
- Who will service/repair it? You, a Mechanic or Dealer?

Location Requirements/Issues:

- Environmental - Location altitude & climate?
- Sound sensitive issues?
- Security Issues?
- HO Covenants or other local restrictions/requirements?
- Portability requirement or stationary - Trailer, pad or ?

Then, with those questions answered I would begin to focus on GENERATOR SIZING as it pertains to electrical load to accommodate the intended application. I would recommend that the person conduct an "energy audit" for their intended application. In addition to gathering information from various sources (electric bill, smart meter, etc.) I would use a form or spreadsheet and identify all electrical needs and their power consumption/loading. Then I would identify critical needs that must be met while on generator power as well as "nice to have" capability. I would then try to determine peak loads and % of how long those loads must be carried. That's where the link that "csheath" provided in the first post, becomes useful for calculating motor driven loads in addition to any other tips and tools that might be out there.

That's just my approach. I'm sure other members have additional suggestions.

 

[devilphrog]

Guys, check this calculator out.

http://www.hardydiesel.com/calculators/generator-sizing-new.html

 

[Farmitall]

My first set of questions to someone asking "What size generator do I need?" would be:

What is it intended to be used for:

- Home Backup
- Workshop use
- Offgrid House or Cabin
- RV/Campsite/Tailgating
- Contractor/Portable/Events
- Commercial/Industrial
- ??

Special requirements:

- Medical needs (reliability factor)
- Auto-start & switchover
- Budgetary - Initial purchase & operational costs

Fuel type, Diesel/Gas/Propane:

- Do you have a fuel preference?
- Do you have existing fuel storage that you can utilize?

Location requirements/issues:

- Environmental - Location altitude & climate?
- Sound sensitive issues?
- Security Issues?
- HO Covenants or other local restrictions/requirements?
- Portability requirement or stationary - Trailer, pad or ?

Then, with those questions answered I would begin to focus on GENERATOR SIZING as it pertains to electrical load to accommodate the intended application. I would recommend that the person conduct an "energy audit" for their intended application. In addition to gathering information from various sources (electric bill, smart meter, etc.) I would use a form or spreadsheet and identify all electrical needs and their power consumption/loading. Then I would identify critical needs that must be met while on generator power as well as "nice to have" capability. I would then try to determine peak loads and % of how long those loads must be carried. That's where the link that "csheath" provided in the first post, becomes useful for calculating motor driven loads in addition to any other tips and tools that might be out there.

That's just my approach. I'm sure other members have additional suggestions.

That is a great list of questions and that post should be included in any sticky that is made.

 

[Chainbreaker]

I edited my list of questions above that I would ask someone before making any recommendations. Also, I would expand a Sticky title to "What type and size generator do I need?" Its possible to determine a perfectly sized generator from an electrical standpoint, but it may not be the right "type of generator" for one's needs.

 

[jaxbill]

The problem I hope to figure out in the next 60 days is dealing with sizing but factoring in load shedding and running multiple motors.

If you follow traditional generator sizing logic and have an all electric house, you'll need a generator with a ton of extra capacity simply to start motors simultaneously or when other infrequent extremely high demand loads are active. My "always on" load is less than 1kW and that includes a lot of electronics that don't add up to much. I need a total of 6.5kW when the water heater runs a few times a day or a total of 3-4kW when the heat pump is running for it's typical 10-15 minute cycle but only maybe twice an hour.

Clearly, starting an AC or heat pump when the water heater is already on might be a problem for a 10kW MEP-803A. (Running it at the same time is no big deal)

I have a load shedding device with CTs on the generator feeder that will do real time calculations and drop loads via relays. However, I don't know if it will react fast enough to drop my 5,550W water heater load so that the heat pump can start. I'm afraid the compressor will try to kick on and with half of my generator capacity consumed by the water heater, it won't start.

What I really need to do is drive a relay that opens the water heater circuit when the thermostat's compressor control wire is activated. I need to drop the water heater before the compressor even tries to kick on. That's probably easy enough to accomplish for those two devices at least.

The other problem is multiple motors. The last thing you want is your refrigerator motor starting at the exact same time as the heat pump compressor. I also have a sewage pump that could start simultaneously because someone flushed a toilet. Many more folks have well pumps that pose a similar problem. I don't have any ideas on this yet except to hope my load shedding solution using the CTs will react fast enough.

It's a shame because none of these high demand loads really needs to operate simultaneously. I don't see a clear cut way to proactively (and automatically) handle the scheduling of when these different loads kick in. It would be nice if a solution were smart enough to know that the AC or water heater just turned off and now it's time to give the sewage pump a chance to empty its tank. It would be nice to know that the fridge has been off for an hour and it needs to run for 20 minutes even though the AC might want to run. All of the solutions I've seen are reactive by opening relays based on real time CT measurements.

 

[Chainbreaker]

Here is another good link with downloadable worksheet for calculating loads: https://offgridsurvival.com/emergencygenerators-selectingtherightgenerator/