Adding solar power to a greenhouse - question on battery chemistry and options

Adding solar power to a greenhouse - question on battery chemistry and options

I have a small greenhouse (12*12) with solar powered vents. Today I run an extension cord out there in the winter but in the summer this will be a mowing hazard.

I'm looking to set up a system that operates a pump (12V, 5.2A) for 2 hours a day, plus some lights (24W, 2-3 hours at most but unlikely to need pump on days I need light). Load per day will be 125Wh to 200Wh. Actual load will likely be less because if it's raining I won't need to run the irrigation pump for the garden, just in the greenhouse. And if it's cloudy for too many days in a row I can choose to not use the interior lights, and use my headlamp instead.

Here's the two areas I need help with:

1- Battery Chemistry. If I use Li ion I can discharge them more fully, so I could get a 400W battery and know I could run the pump for the greenhouse for 3 days before having a problem. But Li ion batteries have a serious warning about keeping them dry to avoid fire. A greenhouse is damp, often approaching 100% humidity. Is that a problem?

Lead-acid batteries are less expensive BUT shouldn't be discharged more than 40% of their capacity, which means I'd be looking at a much larger battery capacity to serve the same requirement of "supply 60-70Wh per day for 3+ days". Am I right that you take daily load times how many days needed and divide by 0.4? So I'd need 525W of capacity to serve 70W of use for 3 days?

2- All-in-one systems vs. roll your own. Looking at GoalZero currently, systems appear well made but price limits how much capacity I can acquire. I've found that I can limit the power I need from batteries if I run some items off the panels directly. Fans are the best example of this - you need them the most when it's sunny and hot. But my question is ... If I don't plan to need/use anything but the 12V power from a GoalZero/Yeti system ... Would I be better off buying components individually (or looking for a kit that is only 12V/24V)? Seems like everything I need is available in a 12V or 24V version. If generating 120VAC off the battery is adding a lot to the cost of a system, I'd just as soon skip it.

**** Answering expected questions ***

• No, I don't have solar today. A few massive oaks make enough shade on the house that it wouldn't work well.

• I don't want to add a dedicated electrical line to the greenhouse because that would make it qualify as a permanent structure and then it has the same setback requirements as a barn or garage (15'-20' from property line). Without power, it has zero setback requirements because it's not permanent. Zoning is weird.

• I'm an electrical engineer, but microelectronics the last 15+ yrs so I'm more comfortable with DIY on the install than getting all creative on the design side. I know enough not to kill myself and have a healthy respect for power (lock out, tag out).

submitted by /u/Engineerchic
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