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How to calculate savings

Southpasfan

Member
Jun 2, 2019
426
540
Pasadena
Since savings and how to calculate them comes up quite a bit, I thought I would weigh in now that I see how my 16.32 system with 3 PWs works in reality.

1. Figure your annual energy usage and cost. In my case, pretty easy. Its 27,000 kwh. 3,000 sf house, southern cal, with two electric cars plus spa.

2. Figure your cost. Also easy, its an average of 25 cents per kwh, or $6750 or $562 avg per month.

3. Figure out what type of system your house can handle. In my case, a large enough system to produce the 27k per year.

4. Point 3 includes going to PV watts or using Tesla's estimate to not only figure out how large a system you can get, but what the systems production will actually be. This obviously varies by location and house.

5. Figure out cost. In my case, financed over twenty years, panels alone would have been about $150 per month, panels and powerwalls $300. That's after fed credits but with no other state credits. Again, your state may be different.

So one could stop there, Tesla's pricing is around $13 cents per kwh, for me. So around 7 cents just for panels.

With my SoCal average only due to go up, it was clear that the economics are favorable.

How favorable? Or, "should I get the powerwalls?"

Let's assume its not a case of actually expecting to need them during an average year, just looking at the economics:

6. Powerwalls are the only way to actually use all the energy your system produces. Unless you could monitor your draw throughout the day, without PWs a large system would overproduce at midday and go off entirely during what many utilities charge peak prices for. That's why in steps 1-5 above I used the price for both panels and powerwalls. There are two additional calculations. First, with true net energy metering your utility might give you 1 for 1 credit for all energy you produce. In that case, the additional cost of powerwalls is only for backup. So figure out what your utility does. Second, if the utility does not have true net metering, check the spread between what the utility would pay at noon vs. what they would charge during evening peak periods. In some cases that spread alone would be 20 cents per kwh. If you have such a spread, the PWs might be a necessary component to getting what I would call "full value" from your system, whatever its size.

7. Finally, even with PWs one would keep track of usage throughout the day, and shift, to the extent possible, large draws (like A/C) to the time of day when the sun is out. In my case, with two EV's that's not hard to do as the Model 3 batteries have plenty of capacity.

There you go. :)
 

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