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- TL;DR Summary
- Looking for a budgetary de-rate to estimate solar capacity needed

I am working on a sanity check for capacity vs load and if anyone here has experience with off-grid solar, any feedback would be much appreciated.

The application is to run a fridge and a modem off-grid on a piece of land in New Mexico. I read that in December (worst case month) the area gets 6.6 hrs of sunshine per day on average.

The stake-in-the ground is to install 600W of solar panels.

End-to-end (panel output to system 12V high-side through a charger) de-rate, intended to include panel over-rating by manufacturer, wiring losses, dirt, charger inefficiencies, etc, I am using 40% loss as my estimate, so I am assuming I will get 360W of usable power for 6.6 hours in December, the worst case month.

This gives me 99W over a 24 hour period, or 2400 Watt-hours per solar cycle.

I will have 600 Ah (@12V) of battery capacity. If I am fully charged on Dec 1, that is 7200 Watt-hours of buffer, less 1800 Watt-hours I need to store each day and use each night (5400 Watt-hours of net buffer) So if I keep the load to 100W average then in the month of December, I have roughly 2.25 solar cycles (2.25 24 hour periods) cumulative of "missing" sun during my expected 6.6 hr charge periods that I can tolerate before the system is drained.

Specific questions from anyone with experience, @anorlunda, perhaps?

Is my thinking on estimating capacity sound? Is my 60% efficiency swag conservative / aggressive / nominal? Of course my need for a fridge in December is not a real thing, but I'm trying to estimate the worst case anyway. All of the other months, on average, should be just fine if December is fine.

The application is to run a fridge and a modem off-grid on a piece of land in New Mexico. I read that in December (worst case month) the area gets 6.6 hrs of sunshine per day on average.

The stake-in-the ground is to install 600W of solar panels.

End-to-end (panel output to system 12V high-side through a charger) de-rate, intended to include panel over-rating by manufacturer, wiring losses, dirt, charger inefficiencies, etc, I am using 40% loss as my estimate, so I am assuming I will get 360W of usable power for 6.6 hours in December, the worst case month.

This gives me 99W over a 24 hour period, or 2400 Watt-hours per solar cycle.

I will have 600 Ah (@12V) of battery capacity. If I am fully charged on Dec 1, that is 7200 Watt-hours of buffer, less 1800 Watt-hours I need to store each day and use each night (5400 Watt-hours of net buffer) So if I keep the load to 100W average then in the month of December, I have roughly 2.25 solar cycles (2.25 24 hour periods) cumulative of "missing" sun during my expected 6.6 hr charge periods that I can tolerate before the system is drained.

Specific questions from anyone with experience, @anorlunda, perhaps?

Is my thinking on estimating capacity sound? Is my 60% efficiency swag conservative / aggressive / nominal? Of course my need for a fridge in December is not a real thing, but I'm trying to estimate the worst case anyway. All of the other months, on average, should be just fine if December is fine.