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When considering your gross savings from PV generation, you'll find that because of T.O.U. pricing/times, the value of the electricity your PV system generates will be less than the peak pricing value. For example, using PVWatts numbers for my system, at current SDG & E rates - not too unlike PG & E rates - my system generates ~ 1,720 kWh/yr per installed STC kW. If I feed all of it to the grid, and using PVWatts hourly option and published SDG & E T.O.U. rate sheets and times for T.O.U. rates and times it will have a value of about $414.
$414/1,720 kW = ~ $0.240/kWh.
I think I'm pretty informed about CA mandated NEM policies and SDG & E rates, and somewhat but admittedly less informed about PG & E rates. I cite my information as an example only. But my suspicion is that your rates, times and billing situation may not be so different than mine.
As for the cost of storage, everyone throws numbers around, mostly without any explanation of how the numbers are arrived at. Maybe the "cost" is a buck per stored kWh. Maybe it is $0.139/stored kWh. Or delivered kWh per year. Or something else. Unless the basics of the analysis are defined, the costs quoted are mostly useless. Problem is most folks don't know that the concept of something called the time value of money even exits, much less know what to do with it, so they usually grab the last thing they heard someone say and see if it sticks when the throw it against the wall.
You provide some explanation of your costing logic, but it's pretty simplistic compared to what's considered necessary if one claims to know something or attempt some verification with respect to solar process economics.
As long as you're claiming a cost for storage, I'd respectfully suggest you consider digging into what the possible (+/- some model's tolerance might be) generation might be for your system by hour and then put a NPV on that generation based on some form of recognized life cycle costing method(s), and then compare that to the NPV of the cost of what you are considering doing, including maintenance and battery replacement.
One other thing to perhaps consider in your analysis: Electricity rates do actually go down as well as up. Example (only): on 12/01/2017, the peak T.O.U. rate for SDG & E , T.O.U. for residential PV systems was $0.54297/kWh. On 07/02/18 it was $0.53773/kWh for the same billing schedule. On 09/17/18, it dropped to $0.44566/kWh. That doesn't make any analysis any easier and adds uncertainty (or perhaps more accurately said, highlights the problem of assumptions always present in any cost analysis).
Lastly, and only FWIW, suit yourself, even with accurate input, PVWatts is already rather conservative in terms of annual output. Many folks have found the 14 % system loss parameter to be a source of what's usually a low annual output number from the model. Many have found changing that parameter from 14% to 10% gives a better match of the model's output to what their system actually seems to produce. Sill +/- maybe 10 % on individual years, but the running day/day 30 day model averages get closer to the actual running 30 day average outputs. There is such a thing as compounding conservative assumptions , and that may not always be the best way to go. Just sayin'.
Take what you want of the above. Scrap the rest.
$414/1,720 kW = ~ $0.240/kWh.
I think I'm pretty informed about CA mandated NEM policies and SDG & E rates, and somewhat but admittedly less informed about PG & E rates. I cite my information as an example only. But my suspicion is that your rates, times and billing situation may not be so different than mine.
As for the cost of storage, everyone throws numbers around, mostly without any explanation of how the numbers are arrived at. Maybe the "cost" is a buck per stored kWh. Maybe it is $0.139/stored kWh. Or delivered kWh per year. Or something else. Unless the basics of the analysis are defined, the costs quoted are mostly useless. Problem is most folks don't know that the concept of something called the time value of money even exits, much less know what to do with it, so they usually grab the last thing they heard someone say and see if it sticks when the throw it against the wall.
You provide some explanation of your costing logic, but it's pretty simplistic compared to what's considered necessary if one claims to know something or attempt some verification with respect to solar process economics.
As long as you're claiming a cost for storage, I'd respectfully suggest you consider digging into what the possible (+/- some model's tolerance might be) generation might be for your system by hour and then put a NPV on that generation based on some form of recognized life cycle costing method(s), and then compare that to the NPV of the cost of what you are considering doing, including maintenance and battery replacement.
One other thing to perhaps consider in your analysis: Electricity rates do actually go down as well as up. Example (only): on 12/01/2017, the peak T.O.U. rate for SDG & E , T.O.U. for residential PV systems was $0.54297/kWh. On 07/02/18 it was $0.53773/kWh for the same billing schedule. On 09/17/18, it dropped to $0.44566/kWh. That doesn't make any analysis any easier and adds uncertainty (or perhaps more accurately said, highlights the problem of assumptions always present in any cost analysis).
Lastly, and only FWIW, suit yourself, even with accurate input, PVWatts is already rather conservative in terms of annual output. Many folks have found the 14 % system loss parameter to be a source of what's usually a low annual output number from the model. Many have found changing that parameter from 14% to 10% gives a better match of the model's output to what their system actually seems to produce. Sill +/- maybe 10 % on individual years, but the running day/day 30 day model averages get closer to the actual running 30 day average outputs. There is such a thing as compounding conservative assumptions , and that may not always be the best way to go. Just sayin'.
Take what you want of the above. Scrap the rest.
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