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Solar sizing second / vacation home

sjg98

Member
Aug 17, 2018
123
96
Brooklyn, NY
My family has a secondary residence (though currently primary during COVID) and would love to put solar on it.
Objective is to be green and to have backup during outages, so grid tied & Powerwalls.

Located in Long Island, NY
Utility has net metering with power banking across months.

Roof is North facing & above the tree line (Google Project Sunroof says we have 850 sq ft of space up there, for reference)

House is oil heat/hot water, and fully electric appliances for the rest.. no cooking gas.
Cooling is via a central air system on 1 level, plus 2 mini splits.

Fall/Winter electric usage has been ~30kWh/day (10kWh/day is EV charging / 20kWh is the house).
Estimating..
Summer usage would be 50kWh/day for 2 peak months based on prior experience.
Unoccupied usage will be closer to 5-10kWh/day based on appliance non-use.

Estimate 100% occupied during summer, 25% occupied remainder of year.
So works out to-
91 summer days at 50kWh/day
205 non-summer unoccupied days at 10kWh/day
70 non-summer occupied days at 30kWh/day
=24kWh/day average year round


I was planning to err on the side of under-sizing to an 8kW + 2 Powerall system.. I would over-generate for 205 days, break even for 70 days and then deduct from my utility power bank for 91 days, and the Powerwall would get me through a day or two of utility outage year round.

Does my math seem off somewhere or are there factors I may be missing?
 

wjgjr

Active Member
May 11, 2020
1,246
985
Silver Spring, MD
On the usage side, you can check utility bills to verify the numbers, but you seem to have a good handle on expected usage. With that, it is just a matter of installing solar that matches that usage over the course of the year - does not really matter how many days are over and how many are under (and you may actually find that 8kW in June will get you 50 kWh, for example, while you won't get 10 kWh due to weather on a number of days, particularly in winter.) Tesla should give you production estimates, but an 8kW system sounds about right, depending on orientation and shading (and Tesla can add/remove panels if you specifically request that.) PWs will add a small amount of usage just to run, but if you are operating in backup-only with rare outages, you won't have to worry much about the 90% efficiency when running on battery.

How close to 100% offset you want to be (assuming the goal is to implement the most cost-effective system) does depend a bit on the specifics of your net metering rules. It sounds like you do not have time of use rates, and you can roll over credits. Is this indefinitely? If so, can you ever get paid for excess credits? If not, how does the presumably-annual reconciliation work? The way CA handles it, for example, is different from (and better for customers) how MD handles it. Some of these considerations, and what the payment amount is for excess credits, may affect the economics of the last couple panels that take you from slightly below 100% offset to at or just above 100%. (Longer-term, you would also have uncertainties around changing usage at the house, changes in rates, and that panels do degrade over time.)
 
Last edited:

trautmane2

Member
Jul 13, 2019
234
161
Leesburg, VA
When sizing, one other thing to consider is whether future electrical usage might be greater because you've replaced the heating and/or water heater with electric options or you'll have EVs (or more of them) parked at the house.
 

sjg98

Member
Aug 17, 2018
123
96
Brooklyn, NY
On the usage side, you can check utility bills to verify the numbers, but you seem to have a good handle on expected usage. With that, it is just a matter of installing solar that matches that usage over the course of the year - does not really matter how many days are over and how many are under (and you may actually find that 8kW in June will get you 50 kWh, for example, while you won't get 10 kWh due to weather on a number of days, particularly in winter.) Tesla should give you production estimates, but an 8kW system sounds about right, depending on orientation and shading (and Tesla can add/remove panels if you specifically request that.) PWs will add a small amount of usage just to run, but if you are operating in backup-only with rare outages, you won't have to worry much about the 90% efficiency when running on battery.

How close to 100% offset you want to be (assuming the goal is to implement the most cost-effective system) does depend a bit on the specifics of your net metering rules. It sounds like you do not have time of use rates, and you can roll over credits. Is this indefinitely? If so, can you ever get paid for excess credits? If not, how does the presumably-annual reconciliation work? The way CA handles it, for example, is different from (and better for customers) how MD handles it. Some of these considerations, and what the payment amount is for excess credits, may affect the economics of the last couple panels that take you from slightly below 100% offset to at or just above 100%. (Longer-term, you would also have uncertainties around changing usage at the house, changes in rates, and that panels do degrade over time.)

The specifics of the rate rules
- Not TOU
- Rollover seems to be indefinite (for 20 years from install), can't find any rules for expiration within that period
- Credits can not be cashed out

Given that rates can't be cashed out, and rollover rules are more likely to become more restrictive with time.. I think this pushes more towards undersizing?
 

wjgjr

Active Member
May 11, 2020
1,246
985
Silver Spring, MD
The specifics of the rate rules
- Not TOU
- Rollover seems to be indefinite (for 20 years from install), can't find any rules for expiration within that period
- Credits can not be cashed out

Given that rates can't be cashed out, and rollover rules are more likely to become more restrictive with time.. I think this pushes more towards undersizing?
It certainly means there is no benefit to oversizing. Essentially, as soon as you hit your annualized usage, you are giving free electricity to the utility. It is sort of like the Price is Right - try to get closest to your actual usage without going over. But, this is only the initial analysis. Over the 25 years (and hopefully more) the panels should be functional, they will degrade (though the present value of banking electricity today to use it years later may be lower.) And, as I mentioned, and @trautmane2 gave the specific example of an EV, usage can change. It would be pretty easy with an EV to use any extra generation. Of course, you could also add panels later if usage increases, though that may be more expensive to do.

It will be interesting to see what Tesla or other installers would propose for your situation. Realizing you said you had a north-facing roof, you may find that you need more panels than you expect to meet your usage since those panels will likely perform very poorly in the winter.
 

sjg98

Member
Aug 17, 2018
123
96
Brooklyn, NY
It certainly means there is no benefit to oversizing. Essentially, as soon as you hit your annualized usage, you are giving free electricity to the utility. It is sort of like the Price is Right - try to get closest to your actual usage without going over. But, this is only the initial analysis. Over the 25 years (and hopefully more) the panels should be functional, they will degrade (though the present value of banking electricity today to use it years later may be lower.) And, as I mentioned, and @trautmane2 gave the specific example of an EV, usage can change. It would be pretty easy with an EV to use any extra generation. Of course, you could also add panels later if usage increases, though that may be more expensive to do.

It will be interesting to see what Tesla or other installers would propose for your situation. Realizing you said you had a north-facing roof, you may find that you need more panels than you expect to meet your usage since those panels will likely perform very poorly in the winter.

Thanks, I also realize many hours later that my brain was taking a nap.
I have a SOUTH facing roof...! So I imagine that puts me even more in the undersize camp as South facing is the most productive.
 

wjgjr

Active Member
May 11, 2020
1,246
985
Silver Spring, MD
Thanks, I also realize many hours later that my brain was taking a nap.
I have a SOUTH facing roof...! So I imagine that puts me even more in the undersize camp as South facing is the most productive.
Yeah - that changes things in terms of the size. But you still want to be looking at the annual production estimate number in kWh that Tesla provides and not just the nameplate (in kW) number. It's just that in order to balance your annual production, you will need fewer panels if pointing south than north. However, your latitude, local weather, and any shading will affect the numbers. To get an idea of the production numbers, use PVWatts Calculator for your location. Knowing Tesla panels are 340W, you can change the settings in those increments to find the number of panels that might make sense. (Though typically, PVWatts provides somewhat higher estimates than Tesla does.)
 

arnolddeleon

Supporting Member
Jul 21, 2012
838
909
SF Bay Area
Are you expecting to be able to grid charge the powerwalls? Make sure that you get this in writing if you need this from your installer/Tesla.

I believe Tesla by default prevents grid charging (except during storm watch) even if you don't plan to take the investment tax credit (ITC).
 

sjg98

Member
Aug 17, 2018
123
96
Brooklyn, NY
Yeah - that changes things in terms of the size. But you still want to be looking at the annual production estimate number in kWh that Tesla provides and not just the nameplate (in kW) number. It's just that in order to balance your annual production, you will need fewer panels if pointing south than north. However, your latitude, local weather, and any shading will affect the numbers. To get an idea of the production numbers, use PVWatts Calculator for your location. Knowing Tesla panels are 340W, you can change the settings in those increments to find the number of panels that might make sense. (Though typically, PVWatts provides somewhat higher estimates than Tesla does.)

Thanks, very cool site.
Given my specs it is showing the 8.13kW Tesla system as being able to produce 30kWh/day on average, with winter trough of 15kWh/day and summer peak of 40kWh/day.
This is actually pretty close to my estimated need of 24kWh/day average, 5-10kWh/day trough and 50kWh/day peak.
 

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