Panels vs. Battery Bank Size

[Dennis Gondola]

:sad2: I really need to figure out a good free tool to convert how many solar panels I will be needing to manage my off grid batteries

[Rob Beckers]

Hi Dennis,

It helps to see panels and batteries as separate items: The panels need to bring in as much energy as you use per (average) day. That is regardless of the battery bank size. If your panels fall short you'll just deplete the batteries more and more, and they'll never get a full recharge. So the answer to this part of the question is to figure out what your daily loads are (in kilo-Watt-hours).

The battery bank size is what tills you over when the sun falls short. On average the panels should (at least) bring in as much as you use, but there will of course be days with little sun and that's when the batteries keep you going. Normal off-grid sizing is for 3 days of autonomy (no energy coming in from the panels) and at the end of that the batteries are down to 50% State-Of-Charge. In practice this will give you more than 3 days, which is a good thing in our winters here, the rule is for sizing.

There is a relationship between battery bank size and solar PV size: Lead-acid batteries do very poorly if they have to sit at partial charge for long periods of time. They absolutely need a full 100% recharge at least once in 2 to 4 weeks. If you have a very large battery bank and relatively little solar PV that would never happen; a sunny day will bump up the batteries a little, but it just takes too long to reach 100% SOC.

I've written an article that helps explain this a bit more: https://www.solacity.com/lead-acid-battery-care/

I'll leave it at this for now. If you have a number for your daily energy use, and a location, I can tell you what it'll take in solar panels to provide that amount, and what battery bank size would fit.

-RoB-