Help calculating self-powered costs

[zƬesla]

I'm in Eversource/NH country where net metering is not quite 1:1 and I'm trying to figure out whether self-powered makes monetary sense. The only difference in net metering is the distribution charge, where I pay $0.04508/kWh from the grid and get a credit of $0.01127/kWh to the grid. So, if I'm calculating correctly, if I send then use 1mW to/from the grid, I will be at a loss of $33.81.

Now, if the PWs are in self-powered mode, which has a 10% loss, then for 1mW use, I'd need an extra 100kW from the grid. At a cost (including all kWh charges) of $0.16312/kWh, that would mean a loss of $16.312.

Do these calculations look correct? If so, then it seems that running self-powered as much as possible makes sense just looking at $.

Here's the full rate breakdown:
https://www.eversource.com/content/docs/default-source/rates-tariffs/nh-summary-rates.pdf

 

[wjgjr]

I did not validate your numbers from your utility, which I assume are correct. For each kWh you export to the grid and re-import, the cost is $0.03381. For every kWh you store instead, the cost is $0.16312 / 0.9 - $0.16312 = $0.01812. The reason for this slightly higher number is to have 1 kWh of usable energy, you need to generate 1.11111 since 1.11111 * 0.9 = 1. (1.10000 * 0.9 is only 0.99.) Despite that small adjustment, it appears you are correct that from an economics standpoint self powered makes more sense. The only caveat would be that if more frequent charging/discharging of the PWs does shorten its lifespan and/or capacity more quickly, there may be long-term costs associated with a replacement.

As a side note, for somebody in a similar situation where the 90% efficiency issue takes you from being above to below 100% of your annual usage, it is possible there are additional benefits, depending on your utility's net metering rules for how they handle excess generation.

 

[zƬesla]

Thanks for the confirmation @wjgjr. The lifespan degradation seems to be a big ? at this point.

 

[wjgjr]

Thanks for the confirmation @wjgjr. The lifespan degradation seems to be a big ? at this point.

Yeah - that's why I just tossed it in as a caveat and made no effort to estimate its value. It may be nothing, it may be a small difference, and it could even be better than backup only if it turns out after all that keeping PWs fully charged has a negative impact. And it could even be such a big issue that you end up with a replacement from Tesla because you drop below 70% capacity before 10 years is up.

 

[jjrandorin]

Thanks for the confirmation @wjgjr. The lifespan degradation seems to be a big ? at this point.

I am using my battery in self powered mode, mostly because I want to use as little energy from my utility as possible.

On the " lifespan" issue, I am rolling the dice, for sure. In my mind however, the best case scenario would be for my usage to cause more than 70% degradation somewhere before year 10, so they would need to repair / replace the battery unit.

No idea on how much degradation will occur over that span, but i am running in self powered mode and have been since January of this year, and likely to continue into the future because it matches my desire to be as self sufficient as possible from the utility.

 

[charlesj]

My only question right now is, do you or will you use more or less than what you solar generate on a year?
If you generate more that may offset the losses of battery?

 

[wjgjr]

My only question right now is, do you or will you use more or less than what you solar generate on a year?
If you generate more that may offset the losses of battery?

If you generate more than you use, that would potentially strengthen the case for self-powered mode if the utility doesn't pay the near-full net metering described above. So it does not change the answer, only makes the benefit bigger.

 

[zƬesla]

PTO was this past June 2nd. Had some great days generating >100kWh, however this past month I've had several days at < 10kWh. So, will have to wait and see where I'm at next spring with a full year of production.

 

[charlesj]

PTO was this past June 2nd. Had some great days generating >100kWh, however this past month I've had several days at < 10kWh. So, will have to wait and see where I'm at next spring with a full year of production.

What? Only a 10% generation what you had? Weather? Knowing you can follow the sun, weather must be bad already way up there.

 

[zƬesla]

Several all rain or heavy overcast days. Today was pretty much sunny all day, first in a while, and we produced around 50. Been getting into the 20s F at night so my mini-split heat pumps have been eating up everything I’m producing and then some. Sadly this is my first month since PTO that we used more (1.41) than produced (1.06).

 

[zƬesla]

On the 10% PW roundtrip loss, taking the battery_energy_exported and dividing it by the battery_energy_imported, I get ~85%. Shouldn't this be 90% if it's only a 10% loss? Maybe calculating lifetime isn't accurate, or am I mixing #s up?

 

[wjgjr]

On the 10% PW roundtrip loss, taking the battery_energy_exported and dividing it by the battery_energy_imported, I get ~85%. Shouldn't this be 90% if it's only a 10% loss? Maybe calculating lifetime isn't accurate, or am I mixing #s up?

This difference also includes the energy cost of maintaining the PWs (which it has generally been reported as 1%-2% of capacity, daily, and potentially more if the PWs are in a location requiring heating/cooling.) Since this loss would likely be the same regardless of the operating mode for the PWs, it should not be factored in to evaluating cost savings vs. backup only. (But, if trying to calculate the overall cost of the PW, it should be considered.)

Additionally, the difference needs to be adjusted for your PWs likely arriving with a low state of charge, so if they are currently full, that may increase the difference by roughly 10kWh per PW (assuming they arrived around 1/4 full.) Over time, this difference becomes negligible, but for a newer system, it might make a difference in the calculation.

 

[zƬesla]

Additionally, the difference needs to be adjusted for your PWs likely arriving with a low state of charge, so if they are currently full, that may increase the difference by roughly 10kWh per PW (assuming they arrived around 1/4 full.) Over time, this difference becomes negligible, but for a newer system, it might make a difference in the calculation.

I had looked at that, and with a May install, adding the current PW charge only bumped it from 85% to 86%. PWs are in a basement that's usually around 55-65F, so I don't think there's much required heating/cooling.

 

[wjgjr]

I had looked at that, and with a May install, adding the current PW charge only bumped it from 85% to 86%. PWs are in a basement that's usually around 55-65F, so I don't think there's much required heating/cooling.

Right, but even without heating/cooling, they still lose that 1%-2% daily. That is what I have seen in my data (around 2%) for my 2 PWs sitting in a basement in backup only mode. They fluctuate in the 97%-99% range, and when they are re-charging, the energy imported number rises. Because I am in backup only mode, the energy exported does not change.