Generator calculation validation

[MaikaiLifeDIY]

As the title of the thread says I’m looking for community validation/clarification.

Background: I’ve got a potential project to inspect multiple off-grid homes and their solar systems. For purposes of scope I’m simply interested in evaluating if these home owners have over spent on large generators where they’re unable to utilize the full output potential of their generator investment.

Things to keep in mind I’m not considering brands of generators, their reliability, outdoor rating, fuel type or anything else other than their continuous output and purchase price.

Scenario:

The home has a generator capable of a continuous output of 22kW at 240v. The generator is attached to the gen input on a Sol-Ark 12k. According to the spec sheet for the Sol-Ark 12k the maximum amperage in/out DC is 185 amps.

Spec sheet: https://www.sol-ark.com/wp-content/uploads/12K-2P_SpecSheet.pdf

If the generator was producing 22kW, and if the inverter was able to convert 100% of this energy to dc at 51.2v, my rough math shows 429 amps dc. So if the Sol-Ark 12k can only output 185 to the batteries then there is an excess of approximately 244 dc amps of capacity unutilized from the generator.

In this scenario I could potentially conclude that the generator is over double the size needed if the goal was to match the generator closer to an inverter that could make use of the generators entire rated continuous output.

I realize that the generator might also have some utilization for loads while charging, but for purposes of charging only I’m omitting this consideration.

In the scenario above if the conclusion is accurate (ish) what would be the ideal generator size for the stated configuration. I’m looking for any figures that include a potential for a little bit of extra capacity based on any sorta rules of thumb.


Thank you in advance for any suggestions/corrections/guidance. ?

 

[DIYrich]

Max generator would be max load of house during the worst part of the year, plus max charge rate of batteries. In other words: Run the house and charge the batteries. On top of that, add 40-100% to avoid overloading the generator. But if it is a really high quality diesel low rpm generator, then only 20%, as they are designed to run at 80% of max for long periods of time.

Imho: I would size a generator at average daily load, run time of 8 hours to generate a full days power, running at 70%. So. A 30kWh daily load is about 4kW to recharge batteries in 8 hours, so about 6kw generator to run at 70% to produce 4kw.

 

[MaikaiLifeDIY]

Max generator would be max load of house during the worst part of the year, plus max charge rate of batteries. In other words: Run the house and charge the batteries. On top of that, add 40-100% to avoid overloading the generator. But if it is a really high quality diesel low rpm generator, then only 20%, as they are designed to run at 80% of max for long periods of time.

Imho: I would size a generator at average daily load, run time of 8 hours to generate a full days power, running at 70%. So. A 30kWh daily load is about 4kW to recharge batteries in 8 hours, so about 6kw generator to run at 70% to produce 4kw.

I like the 70% of load estimate for ideal generator load.

What I was trying to get at is, over simplified for illustration; a large generator for a small load isn’t needed, efficient or cost effective. If the equipment you’re powering from the generator can’t even consume 30% of its output, you probably bought too large of a generator, no?

 

[TimE]

I like the 70% of load estimate for ideal generator load.

What I was trying to get at is, over simplified for illustration; a large generator for a small load isn’t needed, efficient or cost effective. If the equipment you’re powering from the generator can’t even consume 30% of its output, you probably bought too large of a generator, no?

I routinely run my diesel generator at approximately 70% loaded. Its good for the generator

 

[timselectric]

I like the 70% of load estimate for ideal generator load.

What I was trying to get at is, over simplified for illustration; a large generator for a small load isn’t needed, efficient or cost effective. If the equipment you’re powering from the generator can’t even consume 30% of its output, you probably bought too large of a generator, no?

You're only factoring battery charging.
You also have to account for the loads of the home.
If the Sol-Ark 12k is running at full output. And charging at full output. That's over 24k.
One could argue that the 22k generator is undersized.
The Sol-Ark would have to be limited to 15k, to not exceed 70% of the generator output.
In this scenario, I would say that the generator is probably the right size. As most systems are not designed to be pushed that hard.

 

[GuyG]

The Gen input on the 12k has a 50 amp AC breaker and supposedly supports only 10k of gen input, according to its manual. The larger Solark 15k supports up to 19kw of gen input. To utilize generators at higher current, you’d prolly want to wire them into the inverter’s AC input via a transfer switch.

 

[GuyG]

You're only factoring battery charging.
You also have to account for the loads of the home.

This too.

 

[DIYrich]

The Gen input on the 12k has a 50 amp AC breaker and supposedly supports only 10k of gen input, according to its manual. The larger Solark 15k supports up to 19kw of gen input. To utilize generators at higher current, you’d prolly want to wire them into the inverter’s AC input via a transfer switch.

Larger generators go on the grid input.

 

[GuyG]

Larger generators go on the grid input.

That’s what I was saying. Instead of using the gen input, you’d use the AC-in lugs, aka Grid.

 

[MaikaiLifeDIY]

All good input so far, another good point connecting that 22kW generator to the generator input limits its output to 50 amps or 12kW, so again another miss configuration choice for some installs with that setup.

What about the amp conversion assuming 100% conversion 22kW AC at 240v is 429 amps at 51.2v dc. Does this sound accurate?

 

[timselectric]

The math is correct. But I don't know of a single unit that can charge at that level. It would take multiple units in parallel. You only need to convert the actual charging amount to DC amps.

 

[MaikaiLifeDIY]

The math is correct. But I don't know of a single unit that can charge at that level. It would take multiple units in parallel. You only need to convert the actual charging amount to DC amps.

Correct which if there is no load then it’s again more energy than can be consumed.

I have another scenario with a 6k (ish) Schneider inverter that has a 24kW generator connected, which based on the conversation in this thread is even more overpowered than what can actually be consumed.

 

[timselectric]

Each system is designed for a different purpose.
Many things contribute to the calculations.
It's never just a simple charging calculation.

 

[Skypower]

If you have too much generator, you can always throw a heavy duty extension cord to a nice neighbor to plug their necessary appliances into.

 

[MaikaiLifeDIY]

Each system is designed for a different purpose.
Many things contribute to the calculations.
It's never just a simple charging calculation.

I get that, I'm just trying to keep the scope of this thread to a single focus so we don't go too far out into many directions.

 

[timselectric]

It's possible in some cases that the inverter is only for critical loads. And the generator is for the entire house and charging.

 

[timselectric]

I get that, I'm just trying to keep the scope of this thread to a single focus so we don't go too far out into many directions.

My point is that you can't keep it simple.

 

[MaikaiLifeDIY]

My point is that you can't keep it simple.

Understood and I agree, I'm not resistant to the fact in any way that there are other considerations in choosing the size. I know I'll have to look at the wire sizes, how it's hooked up, what the house loads are, what the inverter can handle for charging, how big the generators are etc. Just trying to understand a small piece of the puzzle one piece at a time. Thanks again so much for your contribution to the thread!