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> I've recently got a roof solar system. Not surprisingly it's new so I'm checking production rates, battery charge, and grid consumption regularly You will al

> I've recently got a roof solar system. Not surprisingly it's new so I'm checking production rates, battery charge, and grid consumption regularly You will also learn about the realities of solar vs. the fantasy of solar (as I did once I installed my system). Like this reality (my system), cause by clouds and rain: https://i.imgur.com/bo0s7b0.png https://i.imgur.com/bo0s7b0.png Most people think solar is a nice flat line of constant power once the sun is up. Not even close. Here's another example of a day that was close to ideal vs. two day with clouds rolling by: https://i.imgur.com/A0u4I0R.png https://i.imgur.com/A0u4I0R.png

bruce511 3y ago
Yes, I've had a couple days with partial cloud. I've tweaked the battery settings to handle that. (The battery augments during the cloudy time, and charges back up in sun). Before I'd assumed the battery would only be used at night. For the next 6 months I'll gather data to answer the question - should I have more battery (expensive cost, short-term storage, 95% effecient), or rather pump back to the grid (cheap equipment, long-term storage, 40% efficient buy/sell margins).
- robomartin 3y ago
  > should I have more battery The answer to this question will surprise you as well. The answer is: Lots more. Well, this, assuming you want to store enough energy to be reasonably self-sufficient during weather events. The shape of a perfect solar energy day approximates an inverted parabola. The integral of an inverted parabola is the energy you produce. This turns out to be 2/3 of the enclosing rectangle. The height of that enclosing rectangle would be the constant peak power output (think of a nuclear power plant, which is constant power). Not too long ago, we had a period of about four months of fairly consistent mid to heavy rain. That meant heavy cloud cover even when it wasn't raining. If you want to plan for such events and perhaps, assume you might have to deal with the grid going down due to emergencies, you'll need a massive amount of batteries to store the energy. If you depend on an electric car for transportation it's even worse. I installed a 13 kW system. That was nearly five years ago. In other words, I have years of looking at this data and navigating various hypothetical and real scenarios. My conclusion is that I would need to triple my system and have somewhere beyond 300 kWh of storage if I wanted to be self-sufficient enough for a two week event without grid power, particularly with electric vehicles. I'm in CA, so a two week outage, while definitely black swan, is just one earthquake away. During the Northridge quake (I lived less than a mile from the epicenter then) we were without power for three days. Anyhow, I like having solar. I just wish the people pushing it as the solution to the problems of the universe would understand the difference between solar reality and fantasy. Most of my neighbors were sold these small arrays (3 to 5 kW), have expensive leases and are in absolute pain right now as rates have more than doubled and the solar array turns out to be far from an idealized power source.
  - chris222 3y ago
    Yes it is definitely the case that overprovisioning can help here but it would probably be the case that you would conserve in a long power outage.
ziga 3y ago
PVWatts is a good resource to estimate solar production based on location, system size, roof orientation, etc: https://pvwatts.nrel.gov/pvwatts.php https://pvwatts.nrel.gov/pvwatts.php
- robomartin 3y ago
  > PVWatts is a good resource Yes and no. It gives you an idea of average production. What it does not tell you at all is the day-to-day reality, which can be dramatic (check out the screen shots I posted in my comment). I used PVWatts and other tools extensively while designing my system many years ago. They are helpful, yet, again, none of them expose you to the reality of living with solar. In statistics, it is well known that applying averages to the entire population is a bad idea. Any single day, you can have one cloud kill half your system for hours. You'd be surprise to learn how often clouds --not even rain-- cause you to have to buy energy at the most expensive rate of the day. You quickly learn to hate clouds.

bruce511 3y ago

Yes, I've had a couple days with partial cloud. I've tweaked the battery settings to handle that. (The battery augments during the cloudy time, and charges back up in sun). Before I'd assumed the battery would only be used at night. For the next 6 months I'll gather data to answer the question - should I have more battery (expensive cost, short-term storage, 95% effecient), or rather pump back to the grid (cheap equipment, long-term storage, 40% efficient buy/sell margins).

robomartin 3y ago
> should I have more battery The answer to this question will surprise you as well. The answer is: Lots more. Well, this, assuming you want to store enough energy to be reasonably self-sufficient during weather events. The shape of a perfect solar energy day approximates an inverted parabola. The integral of an inverted parabola is the energy you produce. This turns out to be 2/3 of the enclosing rectangle. The height of that enclosing rectangle would be the constant peak power output (think of a nuclear power plant, which is constant power). Not too long ago, we had a period of about four months of fairly consistent mid to heavy rain. That meant heavy cloud cover even when it wasn't raining. If you want to plan for such events and perhaps, assume you might have to deal with the grid going down due to emergencies, you'll need a massive amount of batteries to store the energy. If you depend on an electric car for transportation it's even worse. I installed a 13 kW system. That was nearly five years ago. In other words, I have years of looking at this data and navigating various hypothetical and real scenarios. My conclusion is that I would need to triple my system and have somewhere beyond 300 kWh of storage if I wanted to be self-sufficient enough for a two week event without grid power, particularly with electric vehicles. I'm in CA, so a two week outage, while definitely black swan, is just one earthquake away. During the Northridge quake (I lived less than a mile from the epicenter then) we were without power for three days. Anyhow, I like having solar. I just wish the people pushing it as the solution to the problems of the universe would understand the difference between solar reality and fantasy. Most of my neighbors were sold these small arrays (3 to 5 kW), have expensive leases and are in absolute pain right now as rates have more than doubled and the solar array turns out to be far from an idealized power source.
- chris222 3y ago
  Yes it is definitely the case that overprovisioning can help here but it would probably be the case that you would conserve in a long power outage.

ziga 3y ago

PVWatts is a good resource to estimate solar production based on location, system size, roof orientation, etc: https://pvwatts.nrel.gov/pvwatts.php https://pvwatts.nrel.gov/pvwatts.php

robomartin 3y ago
> PVWatts is a good resource Yes and no. It gives you an idea of average production. What it does not tell you at all is the day-to-day reality, which can be dramatic (check out the screen shots I posted in my comment). I used PVWatts and other tools extensively while designing my system many years ago. They are helpful, yet, again, none of them expose you to the reality of living with solar. In statistics, it is well known that applying averages to the entire population is a bad idea. Any single day, you can have one cloud kill half your system for hours. You'd be surprise to learn how often clouds --not even rain-- cause you to have to buy energy at the most expensive rate of the day. You quickly learn to hate clouds.