So, I'm a newbie, trying to learn fast because I'm about to buy an off-grid cabin in Maine. I've worked through this and used the number I think fit my situation, but I have a question...The process is:
- determine the number of what-hours the equipment consumes in a day (say 800 wh/d).
- Divide that number by .8 for losses (800 / .8 = 1000 wh/d)
- Divide that value by .5 to get the base battery size if lead acid or .9 if LiFePO4 batteries (100 / .5 = 2000 wh/d). Divide by system voltage to get battery amp-hours (e.g., a 12V system would be 2000 / 12 = 166 Ah battery).
- Multiply the watts in step 3 by the number of days of backup (e.g., 3 days is 3x2000=6000 wh, or 500 Ah).
- Get the value from an insolation map for your location that represents hours-of-sun-at-100%-output (say 4).
- Divide step 3 by the value from step 5 and again by .8 for losses (2000 / 4 / .8 = 625 watts) to get the minimum number of watts the panels must supply to meet your needs (minimum as clouds/shade will reduce output). If you expect 50% clouds, double the panel wattage. Keep in mind that in winter the cold weather will negatively impact the battery and solar output, see the battery FAQ for more information. If you get a larger inverter than you'll think you need (or use microinverters) you'll be able to add more panels later if needed.
I understand that the idea is to get the minimum number of watts the panels must supply to meet someone's needs, but that would be charging batteries up during the day, and draining the batteries at night (and day depending on usage). So it's essentially a yo-yo effect, which uses batteries and therefore cuts down on the life of the battery. I know, I know, that's what they are there for. But my question is this...
Would it make more sense to buy more panels so that it charges the batteries faster, and all my daily usage is taken care of from the over abundance of power coming in from the array, while the batteries sit there topped off? Also, when it is a little cloudy and you're only getting 25%-50% from the panels, you have so many that your batteries are still topped off and extra power for daily usage is there? I get that at night, depending on usage, you will be using the batteries...just trying to think of a way to use them less on a daily basis, and have them for longer periods of down-time when it's really needed. Trying to think of extending battery life, and making sure that I have a good sized array so that even on cloudy days, I'm pulling in enough power.
Even if there were 3 days of super cloudy days, if I had a big array of panels pulling in that low percentage rate, added up it would still be a fairly good amount that might even cover all my daily needs, therefore not really needing such a large battery bank...which is the most expensive part of all of this. Does that make sense?