Lost on some basic math.

[TomW]

Ok, folks, I seem to have completely lost my math skills.

Been trying to sort out our % of power production since we commissioned the SMA Sunny Boy.

I was trying to use the "Net" KWH from the smart meter and the production total KWH from the SMA.

I just can't sort out how to do it with these numbers, or if it can even be done with this info.

Net= 658 KWH
SMA=444 KWH

I normally would use what you have divided by what you should have times 100 for % of total. This worked for grading tests when I was a teachers aid back when dinosaurs roamed the Earth.

Friends and neighbors constantly ask what % of our use the system produces and I can't seem to sort out how to accurately tell. I am pretty satisfied myself with the performance so far this close to Winter Solstice. I calculated the system size before we replaced the tank type gas water heater with  a whole house tankless demand electric water heater.

So does anyone have a hint?

Do I need different data? Do I need a 10 year old to help or what?

Thanks.

Tom

[Westbranch]

doing 'head math' I see ~ 65% from the SMA    sma/net

OR if it really is Net sma / Net + sma  about 39%

now to use the calculator   67.4%  and 40.29%     

[TomW]

doing 'head math' I see ~ 65% from the SMA    sma/net

OR if it really is Net sma / Net + sma  about 39%

now to use the calculator   67.4%  and 40.29%     

westbranch;

I was telling folks "about 3/4" in rough numbers.

I got confused by overthinking what factors I needed. Still kind of confused but the fog is clearing slowly.

Considering this point in the year we are seeing the least solar hours by June we should be at or about parity on the power unless the AC uses way more than we have in the past.

Even on overcast days we produce 3 or 4 KWH. I wasn't really expecting that. 

Appreciate the feedback.

Tom

[TomW]

Golly, I think I just figured it out.

Need a total KWH to work with Net + SMA (1102) and the 444 KWH produced by the SMA.

444/1102=.40 or 40%. So I need to adjust my numbers from about 3/4 to closer to 3/8. 

I think, maybe.

Still not willing to bet the farm on it, however.

Tom

[kitestrings]

T,

I think you've got it right.  The "Net" being positive tells me that is the deliver kWh from the utility to you, so it total you used 444 + 658 = 1102 or 40/60%.

Jump ahead to a sunny month, say July (for us), and let's say your SMA is 900 kWh and your Net is -200 (kWh rolled back to utility).  Your use is still the some 900 + (-200) = 700 kWh.

~ks

[Halfcrazy]

Wouldn't NET be the total inbound or outbound for the month AFTER the SMA has done its thing. So if I am thinking right (Probably not havent had coffee yet)

NET = 658KWH
SMA = 444KWH

Total KWH usage for the month would be 1102KWH

444/1102 = 0.403 or 40.3% so the SMA supplied 40.3% for that month

Again this would be my interpretation of NET so I may be all wet

[Westbranch]

Yes, that was my thinking but the utility needs to be contacted to see if the meter IS recognizing the SMA portion... mind you if the bill read $X for 644net or  1102Kwh... depends how you bill reads,  but nice to wonder if they haev you over the table.

[TomW]

First, the numbers are Since we commissioned the SMA with a new meter (showing all zeros) and not for any actual  month. Our use has gone up a LOT since we moved to town but we certainly don't use 1100 KWH a month. Even with a gas furnace blower and electric water heating which we did not have on the farm.

The meter shows received (by us)

Delivered (to them)

and Net. (what we pay for)

The values record properly at least when I checked them against one another.

Not all power goes through the bi-directional meter and that is where I get confused on calculating the % of use.

The odd thing to me is Delivered never equals what the SMA says because we are usually using some power all the time and, of course, the local loads get fed without going through the meter as the SMA feeds into the bottom breakers in the AC main panel.

This seems to be an accurate way to measure the net power but leaves me with a headache trying to sort the % of use number. I may need to use other values from the meter?

I have only been watching the Net value and probably not the right value to watch.

Thanks for the input, guys!

Tom

[russ_drinkwater]

Some grid tie inverters log total life production.
However things get a bit confused when using power accounts and the figures from the smart meters.
Over here they seem to bill me for the power I produce and use and then pay me for the excess???

[Westbranch]

Tom what about an analog meter in the line to the dist box?