Sizing Inverter Based on Home Consumption

[jeff88]

I'm researching parts for my system and was wondering what size in watts inverter I should get for my batteries. I'm planning to backup the whole house (all my loads are in one panel) so I was wondering if I could use my utility data to determine inverter size. For example, if I use 15kWh in a day, can I extrapolate my inverter wattage from that?

 

[Brucey]

I'm researching parts for my system and was wondering what size in watts inverter I should get for my batteries. I'm planning to backup the whole house (all my loads are in one panel) so I was wondering if I could use my utility data to determine inverter size. For example, if I use 15kWh in a day, can I extrapolate my inverter wattage from that?

No because that only covers your average consumption. You need to have enough inverter capacity to handle your peaks.

 

[Brucey]

Your historic consumption could be used to help calculate your battery sizing though.

 

[jeff88]

No because that only covers your average consumption. You need to have enough inverter capacity to handle your peaks.

My utility provides 15 minute intervals of kWh usage. The most I’ve used in any 15 minute interval is about 2 kWh. Would that not be enough to calculate out to wattage output?

 

[jeff88]

Your historic consumption could be used to help calculate your battery sizing though.

That part is done, now to get that DC power into my loads!

 

[Brucey]

My utility provides 15 minute intervals of kWh usage. The most I’ve used in any 15 minute interval is about 2 kWh. Would that not be enough to calculate out to wattage output?

Are most of your loads on propane/natural gas? Do you have A/C? Well? 15 minute interval may not pick up things like a freezer starting a defrost cycle, well pump cycle etc

 

[Quattrohead]

Very approximately, these are your heavy hitters.
AC - around 100A to start, then 15A running.
HWT - 18A
Dryer - 22A
Stove - very random upto 40A
It seems like you are a light user, but knowledge is power and if you like data, get an Emporia monitoring system.

 

[wpns]

My utility provides 15 minute intervals of kWh usage. The most I’ve used in any 15 minute interval is about 2 kWh. Would that not be enough to calculate out to wattage output?

Well, you still need to know (or be able to extrapolate) your peaks, but 8KWHR/HR means you need at least 8KW. As a first approximation you could use your daily usage (really only 15KWHR/day?) to size your batteries.
Measurements from something like the Emporia will tell you a lot if you run it for a year, as your loads may be seasonal, but given what we know from the above, one EG4 18KPV and a couple of EG4 PowerPro batteries might be overkill, but would almost certainly work. Solar panel details will be location-dependent but also easy to figure out with online calculators.
[Yes, I chose the EG4 ESS, because that's my hammer, with better details and a real load profile you could certainly optimize.]

 

[jeff88]

My two large loads are a 32A stoven and a 24A dryer. Everything else is lights and plugs. No A/C, no electric WH, etc. I was hoping based on my usage history, even accounting for my "worst" day I could calculate my wattage. I don't want to log for a year and I don't want to assume wattage for all of my lights and plugs.

 

[wpns]

My two large loads are a 32A stoven and a 24A dryer. Everything else is lights and plugs. No A/C, no electric WH, etc. I was hoping based on my usage history, even accounting for my "worst" day I could calculate my wattage. I don't want to log for a year and I don't want to assume wattage for all of my lights and plugs.

The 18Kpv should handle those no problem, though maybe not simultaneously.
Hair dryers, microwaves, kettles, anything that makes heat? Most 120V oulets will only supply about 1.5 KW, so you should be fine.

 

[42OhmsPA]

Not sure if it will be helpful, I attached my usage for the past 12 months, my usage prior to solar was ~11KWh / day. (The grid usage in April was the generator getting exercise)

I was running a 6KW TP6048 until last September and it powered my entire house except the electric dryer and stoven (could have handled it with limited burners / oven only). In September I upgraded to a 10KW ASF and added the dryer and the stoven.
Other loads are deep well pump I clamped ~40A/leg, 1HP air compressor I never clamped, 12k BTU mini split, fridge, deep freeze, convection oven, induction cooktop, microwave, shop vacs, chop saw, miter saw, hair dryer, etc (standard house stuff).

Battery bank is ~33KWh and arrays total ~11KW.


Edit to clarify the well pump clamp readings were inrush current on the AC side.

 

[jeff88]

Well, you still need to know (or be able to extrapolate) your peaks, but 8KWHR/HR means you need at least 8KW.

So an 8,000 watt inverter would be my worse case scenario.

 

[OffGridForGood]

My two large loads are a 32A stoven and a 24A dryer. Everything else is lights and plugs. No A/C, no electric WH, etc.

These could be on at the same time, along wiith some lights and plugs. Say you design for 60A peak
60A 240 = 14,400 W peak
the EG418kpv is a 12kW output inverter - may require you to be sure the stove and dryer are not on together.
A pair of SRNE 8kW inverters = 16kW output.

 

[chrisski]

IME, the only way to do it is to measure loads as they turn on and then add the ones that will be together. Since all the loads you really want to measure are hard wired or wired with plugs too big for a kilawatt meter, this is not easy. Next best data is a neighbor keeps track of data.

The timing in the utility got me in the ball park, this got me as using almost 7 kW per hour, mostly two Air conditioners:
-one hour increments available to me, I did not know the duty cycle of the air conditioner so A 25% duty cycle meant 28 kW load, and a 50% duty cycle meant 14 kw, 100% duty cycle meant 7 kW.
-I did the calculation for the Running Load Amps, and it came out to 3kW. I totally forgot about the other circuit for the fan. That was power hungry.
-When I got my system installed, I could measure the one AC on the critical loads panel and it was 4.5 kW. I still have no idea how much AC 2 uses, but its bigger than AC1.

My utility provides 15 minute intervals of kWh usage. The most I’ve used in any 15 minute interval is about 2 kWh. Would that not be enough to calculate out to wattage output?

I would use that to start thinking A 10 kW inverter is needed if 2 kWh are expended in 15 minutes. There's more to it like moving all loads to run off battery. Also, what about the one time a year electric events if those will be covered like cooking two turkeys while the air fyer is going.

I really recommend you leave the oven and dryer off the critical loads panel. The extra electricity you generate should be used by those prior to the power company charging you for it, if needed those can be not used during an outage, and this will make the deign quite a bit easier. Turning both those items on could cost you 13.5 kW which ups the inverter requirement.

 

[Warpspeed]

As others have already said.
You need two figures, average daily total consumption for battery sizing.
Peak power consumption for inverter sizing.
No way to guess at this, you need some actual recorded figures over at least a couple of months.

Don't cheap out on the inverter.
Batteries and solar panels can be added to and upgraded bit by bit.
The inverter is a big investment, and your power usage is more likely to increase than decrease in the future.

 

[wpns]

So an 8,000 watt inverter would be my worse case scenario.

Umm, no, an 8KW inverter would be the minimum you should get based on your PoCo numbers above. If you want to run your “32A stoven and a 24A dryer” full blast simultaneously then you should have at least (32+24)*240=13.4KW

 

[wpns]

.

 

[Warpspeed]

Just get your load monitoring/data logging system going and then forget its there.
Just live your normal life and come back to it much later and see what the recorded data is telling you.
Its usually a pretty big big surprise for most people.

 

[Bananaman321]

The 18Kpv is a hybrid inverter it has an excellent serge capacity to it and it's expandable.
Being that it's a hybrid you can back it up with utility until you're sure it's enough to cover you by matching the phase and blending the power seamlessly and has the ability to sell back to your utility.
It also has a function were you can connect a cheaper inverter 3k, 4k, 5k 8k you get the idea up to 50a through the generator port and activated via the dry contact at a pre set load
So say you set it to activate at 9kw and shift 4kw load to the slave inverter until demand drops below 8kw total then 18kpv takes over and puts slave into standby.
You can of course stack more 18kpv inverters in parallel

I've started a 4ton AC unit with over 8000 W of load with no problem multiple times with my 18kpv

 

[Quattrohead]

Umm, no, an 8KW inverter would be the minimum you should get based on your PoCo numbers above. If you want to run your “32A stoven and a 24A dryer” full blast simultaneously then you should have at least (32+24)*240=13.4KW

The chances of the dryer heating element being on along with four to six stove top burners plus the oven plus the grill are as near to 0 as being hit by lightning. The 18K can do 16 KW output for 30 seconds and I think it's 13 or 14 KW for 5 minutes.

 

[Q-Dog]

My two large loads are a 32A stoven and a 24A dryer. Everything else is lights and plugs. No A/C, no electric WH, etc. I was hoping based on my usage history, even accounting for my "worst" day I could calculate my wattage. I don't want to log for a year and I don't want to assume wattage for all of my lights and plugs.

That's a bit like saying you want to build a house but you don't want to use blueprints.

 

[ksmithaz1]

The chances of the dryer heating element being on along with four to six stove top burners plus the oven plus the grill are as near to 0 as being hit by lightning..

Until the fam-damily is there over thanksgiving, and you found out the table cloth for the big DR table has a stain on it and cousin Julia needs a hot shower, and someone needs to make the gravy, the Turkey is in the oven, and ....

The problem with demand is it's unpredictable. That is why most homes nowadays have at least 200A service. I have hit over 100A on my panel, not for very long, but I hit it. From a sustained high load standpoint (> 80% for more than 2-3 hours) and the cut-overs I had in place worked. I've tweaked a few things since. Get a good handle on your normal highest demands: Get *real* numbers on your HVAC,HWH, etc, actual usage / amp clamp-whatever when running not breaker sizes, not plate numbers, and add them up for a true peak potential. The problem with the likely-hood of everything being on at the same time is as unlikely as it is, ... That it will occur at the most in-opportune time. Just understand the ramifications in your design. If that is you kick over to grid, that's fine, just plan for it.

 

[ksmithaz1]

My two large loads are a 32A stoven and a 24A dryer. Everything else is lights and plugs. No A/C, no electric WH, etc. I was hoping based on my usage history, even accounting for my "worst" day I could calculate my wattage. I don't want to log for a year and I don't want to assume wattage for all of my lights and plugs.

My two large loads _today_ are ...

The dryer is likely close to 18A running. Most oven/range are on a 40A breaker, probably ~30A with *everything* on full which it will not be very long, oven will cycle, burners will turn down.

You have other items on your 'lighting' that will cause demand. Microwave Oven, Blow dryer, Toaster, Toaster oven, disposal .... Oddly your fridge/freezer is not really going to be severe.

Just build it to handle an overload condition gracefully if you want to roll light. I'd look at 10-12KW of output, and/or I'd put together something you can scale up if you are not happy with your results. On the battery front, scribbling on a napkin, you need battery to last for 18 hours, you will have 6 hours of sunshine to put electricity in them. You will need to store power for 3/4's of your daily usage. This will be a perfect world scenario, usage is generally not flat thru the day. If you build it to scale up if needed you should be happy.

 

[Crowz]

Its VERY easy to under estimate max load until its check via real world usage like others have mentioned. When I moved from my old house to the current one I brought my stuff from there to here thinking that I knew what I was using power wise at the house with its solar setup so I knew what I would using at the new house. No brainer I thought...

First power bill was triple the old houses bill. Thinking the meter had to be messed up at the new house I got an emporia for it and started monitoring.

The highest I calculated I would see was 12kw with 9kw being a normal high point with the stove going and such.

What I found was 25kw max. Turns out the houses 80 gallon hot water heater and another 30 gallon hot water heater plus the stove and all of the outlets the family had plugged stuff into and left going or at least plugged in went way over what I even thought possible.

While you may be against using this for a long term test since your wanting it now it will still be useful for a quick inspection of real world usage before choosing what to go with equipment wise.

Every single time I have crunched the numbers on what I think the max load will be it has been way short of real world data from the emporia or other testers.

 

[Warpspeed]

All we can do is offer advice.
You are not obliged to follow it.

I did some extensive monitoring of every electrical load in the house. There were quite a few real surprises, and I managed to make a few simple changes that HALVED my total power consumption without making any changes to life style or usage pattern.