I think perhaps rather than focus on numbers per se, it might help if we work towards an agreed process for calculating the per kWh cost which anyone can use and plug in the values for the variables applicable in their individual case.
Agreed, everyone will be different so it's the math that's important. In the prior examples (and this one ; -) the numbers are meant as
examples. Here's the math and some examples presented again so folks can suggest fine-tuning them:
Math
Ultimately, the math is: value - cost. Where the value is the $ paid to the utility and the cost is that of a total replacement solution.
In order to replace the grid you need power generation and energy storage if it's not dependable.
The value is fairly straightforward, it's the savings between paying the grid and paying for the energy generation adjusted for inflation.
The costs are a bit more complicated. It's the cost of the energy storage plus backup generator adjusted to the same timeframe as the solar (e.g., if the ESS calendar ages to 10 years you double its cost to get to 20 years).
Value Example
In post
#36 we calculated that solar with a DIY installed cost of $1/W had a lifetime per kWh cost of $0.034.
Let's say the $total bill amount / total monthly kWh for the lowest monthly value on your bill comes to $0.15/kWh. Then
($0.15 - 0.034) * 365 * 20 = $846.80
This is the
minimum amount of money saved per kWh of power over the lifetime of the equipment (20 years)
if you could switch away from the grid. That is it's the money available to replace the grid after solar is installed.
If you think 2% inflation is reasonable, then adjust that to $1092, or $1214 at 3%. Everyone has to plug their own numbers that they're comfortable with to get the real value.
Cost Example
As an example of the costs, let's use
$0.156/wh as the price of the Energy storage. For LFP to last 20 years cycle wise you need 3d autonomy (see post
#1). But, there's doubt they'll calendar-age that long, so to be conservative we can use a decade. From the cycle life the minimum capacity for a decade is 112%, but let's use 200% (2-day autonomy) to again be conservative. For the generator let's say it'll last 20 years and costs $1000.
So, thats $156 kWh battery costs * 2 day autonomy * 20 years needed / 10 year life + $1000 for generator / 20 years = $674/kWh.
In this example, $846 savings - $674 costs = $172 > 0, so going completely off-grid would more than break even if your power usage/consumption were consistent throughout the year. Would it save you money? It depends. For example, if you invested that $647 at 1.5% then no.
The goal for me was to get a ballpark number to know if I should do a more detailed analysis. It looks like I should ; -)
So hopefully you all keep coming up with ideas to refine the math. Possibly it can be turned into a spreadsheet or something. The OP used a specific number for the maximum kWh used which seems to be more useful than assuming every month is the same. The devil is in the details (e.g., batteries have to cover maximum seasonal usage, round trip losses need to be accounted for, will a generator really last 20 years). In my situation power usage is very asymetric (lots in summer and nothing in winter) and sizing for the worse case is going to favor staying on-grid.
Let's say you do your energy audit and recognise that to manage through periods of poor insolation, and/or times of year where daily consumption is seasonally much higher the battery needs to be (say) 3x your average daily consumption.
I think you need a generator if you're going off-grid, in which case days of autonomy isn't as important as DoD to hypermile the battery. So, calendar aging is the real critical factor.
That's great. Except for large parts of the year you won't use the full capacity of the battery every day.
No help for it. But, it does play into DoD and therefore battery longevity.
