Another way to phrase it is that your panels need to be enough to supply your entire day’s needs, plus a little margin for safety and losses.
Let’s say you use 20kWh per day. Then you might choose to put a margin on 10kWh it, and size your array so that it can produce 30kWh (seasonality depends on your location, and in reality it is therefore more complex when it comes to the length of the day, night, clouds, etc).
Now your panels can provide for all your daily needs. But, you don’t use all 20kWh while the sun is out. You use 5kWh when the sun is down (you calculated this).
Great, then you need a battery bank that can responsibly supply the 5kWh. Taking account of 10% DC to AC losses, and a minimum SoC of 30% gets us to an 8kWh bank.
As you can see this calc’s bank size is independent of the panels, because you’ve already calculated you panels to be enough to supply your entire day’s energy, so you know that it’ll be enough to recharge your batteries.
But as I’ve mentioned in a previous post, this is also if you actually want to take your night time loads fully off-grid. If you don’t want to, then the battery bank becomes a question of backup needs (subject to certain minimums).
Oh and obviously the size of your inverter should come into these equations etc.
So a rule of thumb is really just a rough estimate that you can do on a napkin. But it is subject to so many underlying assumptions, or simplifications, that it can only responsibly be used as a starting point for your calculations.
Even if you only have a grid tie system this question remains.
The problem with PV panels is matching them if they are bought progressively. Better to buy all you need at the same time.
A lesson I learned early on. These days I buy panels by the pallet. It is cheaper, and in years gone by, it was worth waiting for Black Friday. — Not so much now.
For my next big project, I am toying with importing panels direct, because I am probably going to use about 50kWp. But we’ll see closer to the time.
Panel technology is always improving so that is going to be one of my last purchases for the project.
currently have 16 x 550w panels with a 21kWh battery and the solar is battling to get the battery from 50% to 100%
I have a similar setup 16 x 400w panels and ~20kWh battery - I used 80% overnight so ~16kWh
On most days, I’m able to charge from 20% to 100% by about 13:00pm
From 11:00-13:00, the panels average generate ~5.5kW per hour, 11kWh in two hours (peaks at >6.4kW)
From 09:00-11:00, the panels average generate ~4.5kW per hour, 9kWh in two hours.
I have a typical load of ~1kW during the day.
What is the “C” rating of your batteries, can the system handle 16 x 550W = 8800W @ 48V = 180A?
What other loads are on during the day?
I have a timer for my two geysers to come on from 11am and one geyser is solar
Are there any obstructions on the panels?
Are the panels north facing?
What is the lean angle?
