Re-thinking Inverter Sizing...

[Shimmy]

So I have two years of great metering data that shows me that the right size inverter is a 6.8kW unit with 10-12kW surge capacity, running in parallel to my existing 7kW enphase system. Then the in-laws came during Christmas time... and we managed to have a grid outage at the same time (along with near-zero solar). Our daily consumption went up from 36kWh to 66kWh, along with seeing 10kW sustained for over an hour with 15kW surge for 10 minutes.

I have four big loads-- EV charger, dryer, hot water heater, and oven. Everything else is under 3-4kW in normal operation. Normally, hot water is more than adequately provided by a solar hot water system-- we used 8kWh of electricity for water heating in 2019. With the 6.8kW inverter I could manage the dryer and EV charger easily enough. It is a lot harder to manage with the oven and water heater added into the mix though.

So, the questions: 1) what do you size the inverter for? Simplicity-- let it handle the worst-case scenario at the cost of efficiency, expense, and space, and 2) If I do need a 1-minute surge of 15-18kW, what would you recommend for an inverter?

Part of me thinks I should stick with my plan for a 6.8kW inverter (XW-Pro), and parallel a second grid-tied inverter for when solar is down, but the practical logistics of doing that are a little beyond me right now.

 

[sirslayer]

As in having one 12kW inverter vs 2 x 6kw inverters would both have its advantages which one is better than the other setup. You can always add more 6kw inverters as the demand for electricity goes up . But it can lead to more cabling and protection to add more to the over all cost. Reliable goes down a bit with more wiring to worry about but if it’s in code and you can afford the expensive. Then having two inverters ir more is not a problem because you can estimate the usage of electricity as you need by adding another inverter because you’ll never know in the future unless you planned for example you buy all electric truck which consumes more then a electric car but you design your inverter setup for the car. Having planned to run multiple inverters just in case of needing more electricity would be ideal because you can never plan what would change to electricity and how much would it take ?!

 

[rodrick]

Sol-ark 15k for a tier 1 or a couple of 6548 mpp or 6500 eg4 which would give some redundancy and easily expanded if needed

 

[400bird]

Put the big EVSE in the main panel, so it automatically drops out when the grid goes down. Run the little level 1 EVSE from the back up loads panel if you need to charge.

For the hot water heater/oven situation: set up a current sensor abd relay where if the oven is pulling current, open the relay for the hot water heater.

I'd put the dryer in the main (not backed up) panel with the EVSE. Or go current sensor relay with the oven and water heater, if the dryer is needed when the grid is down.

From the info provided, I think you'd manage on the XW, with room to spare, if you only ran one of the big loads at a time

Part of me thinks I should stick with my plan for a 6.8kW inverter (XW-Pro), and parallel a second grid-tied inverter for when solar is down, but the practical logistics of doing that are a little beyond me right now.

Can you explain this?
The XW is a grid forming inverter, which means the Enphase micros will sync to it and produce power as usual.

 

[rodrick]

One thing I should of ask is what if any do you have for battery storage using the mpp or eg4 units would be no grid tie only grid assist

 

[Mattb4]

So I have two years of great metering data that shows me that the right size inverter is a 6.8kW unit with 10-12kW surge capacity, running in parallel to my existing 7kW enphase system. Then the in-laws came during Christmas time... and we managed to have a grid outage at the same time (along with near-zero solar). Our daily consumption went up from 36kWh to 66kWh, along with seeing 10kW sustained for over an hour with 15kW surge for 10 minutes.

...

While it may be nice to have every possible worse case scenario planned for it costs a lot to implement. In your situation it sounds like no one decided to limit themselves on using electricity. Sometimes you have to establish rules.

 

[timselectric]

I'm building my system to not be limited.
It's half the benefit of being my own power source.
I've spent my life turning off light switches, when others forget to. I will enjoy the rest of my life, not worrying about it.

 

[Mattb4]

I'm building my system to not be limited.
It's half the benefit of being my own power source.
I've spent my life turning off light switches, when others forget to. I will enjoy the rest of my life, not worrying about it.

Understand the feeling but (not being among the wealthy) I have practical limits to what is possible.

ETA: After a lifetime of being frugal it would be hard for me to change habits. I conserve resources wherever possible.

 

[timselectric]

Understand the feeling but (not being among the wealthy) I have practical limits to what is possible.

ETA: After a lifetime of being frugal it would be hard for me to change habits. I conserve resources wherever possible.

Not wealthy here, either. But, the economics are working for me.

 

[Shimmy]

Put the big EVSE in the main panel, so it automatically drops out when the grid goes down. Run the little level 1 EVSE from the back up loads panel if you need to charge.

For the hot water heater/oven situation: set up a current sensor abd relay where if the oven is pulling current, open the relay for the hot water heater.

I'd put the dryer in the main (not backed up) panel with the EVSE. Or go current sensor relay with the oven and water heater, if the dryer is needed when the grid is down.

From the info provided, I think you'd manage on the XW, with room to spare, if you only ran one of the big loads at a time

That was my initial thinking and planning.

The oven has issues with power interruptions on several levels though and I am a little unsure of how to functionally address it. I guess a dedicated transfer contactor normally on Utility is easy enough. The "recovery" period of the power outage would need to prioritize loads.

Can you explain this?
The XW is a grid forming inverter, which means the Enphase micros will sync to it and produce power as usual.

I was referring to a battery-sourced secondary inverter when I don't have sufficient solar power. My thinking was to maybe have a second unit, cheaper than an XW-Pro, that could be turned on manually if needed.

 

[Shimmy]

I'm building my system to not be limited.
It's half the benefit of being my own power source.
I've spent my life turning off light switches, when others forget to. I will enjoy the rest of my life, not worrying about it.

This is my new curiosity, in two ways. First, resale value of the home. A limited supply might impact that.

The second way is that something that "just works" has a lot of value.

This particular event we actually needed 12kW of power; three people needed to take showers, we were finishing up some cooking, and I still needed to add 40 miles of range to the car before we could head into the city. Since we were leaving we needed the clothes to be dry. It was very much a cat-hearding moment, but it was a kind of situation I hadn't planned for before.

Going from 6.8kW to 12-15kW ends up at about a $6-8k premium; adding the flexibility to do it later adds $2k. Eliminating the load-shed might save me $2k. The lost space in the garage is a minor concern as well with the extra equipment.

 

[400bird]

Going from 6.8kW to 12-15kW ends up at about a $6-8k premium; adding the flexibility to do it later adds $2k. Eliminating the load-shed might save me $2k. The lost space in the garage is a minor concern as well with the extra equipment.

I'm not following the math. If your looking at the Schneider XW, the incremental cost of going from 6.8kw to 13.6 kw would be roughly the cost of a second XW plus the conduit box, right? So $3500-4000?
That's not quite $6-8k

Also, I assume you've seen the YouTube's where they run a single XW at 11kw for an extended period.

 

[Shimmy]

I'm not following the math. If your looking at the Schneider XW, the incremental cost of going from 6.8kw to 13.6 kw would be roughly the cost of a second XW plus the conduit box, right? So $3500-4000?
That's not quite $6-8k

Also, I assume you've seen the YouTube's where they run a single XW at 11kw for an extended period.

There is a bit more electrician time needed, and I switch from the small PDP to the big one. I also end up going to 80A wiring instead of 60A, but that is minor. It might only be $5,500 premium.

I'm starting to think going to a SolArk 15k would be almost cost-neutral, since I eliminate the RSD-MPPT, the MPPT80-600, and the PDP and get the 200A transfer capability integrated. Space-wise it also ends up being significantly more compact. I do lose some functionality I was looking forward to via Modbus, but things become much more simplified.