Solar tips: Why 1 watt to 1 amp

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1 watt of solar to 1 amp of battery storage

I developed this basic rule of thumb about 5 years ago. When I did my initial solar research I noticed that off grid, either full time or RV boondocking, the folks who were happy with their systems typically had 1 watt of solar panel per 1 amp of battery capacity (measured at 12v/lead acid battery packs).

Lead acid. Yuck. But I've still got mine. 4 years and still chugging along.
Lead acid. Yuck. But I’ve still got mine. 4 years and still chugging along.

Those with less panel grumbled about “genny time” and worse. The consequence of lead acid battery packs: over discharge & under charge = dead batteries.

So let’s look at a small system

A small camp system I looked at recently has 235 watts of solar input and 370 amps of 12v battery.

This is fine in the summer. Six hours of sunshine x 235w less line/battery losses. He’s all good. But in the shoulder seasons and winter. He’s gonna want more panel. Couple of rainy days… etc.

Math

Let’s look at some math: with 370 amps x 12.7v = 5kw power. His usable power for max. lifespan on the lead acid batteries is then @ 20% discharge. 1kw.

This camp was using .75kw/day. That means they need to fill the batteries daily.

I don’t have a problem with that. Basically that’s what I need to do too. And I’ve been doing that for the last two years. 365/24/7. So it works. (previous 2 years with this system my fridge was propane so I wasn’t using as much power. I had 2-3 days if nec. And spent $450/yr on propane…)

But back to the few rainy days. Now you really want extra power. A rainy/cloudy day. His panels are making 1/2 power, 1/4 power, 1/20th the power. And he needs to make up 1/2 kw min.

I think that’s the thing about off grid power. You, to some extent, have to build for the 30-70% days. The top 30% is gravy. The bottom 30% is genny time. pick a ratio…

Other issues

There is also the issue of getting enough power to quickly charge the batteries (avoiding sulphation) and equalizing. Which is best done with solar, imho.

Thoughts on Lithium Iron Phosphate (LFP) vs Lead acid

In RV and marine use batteries last 1.5 to 2 years. Oh sure, there are those who do better. But you’ll see on the web the number of folks who replace their lead acid battery packs quite frequently.

The issues are over discharge. Lead acid doesn’t like it. No battery does. But lead acid batteries die faster the deeper you discharge.

And folks with camps/cabins and RVs often leave their homes unattended for long periods. This leads to death by sulphation for lead acid.

This all points to why I like LFP batteries so much. Didn’t get a full charge on your batteries? LFP doesn’t care. Lead acid … yikes! Left your batteries partially charged. Recommended for LFP. Lead acid. Dead!

2 thoughts on “Solar tips: Why 1 watt to 1 amp”

  1. what about the newer lithium types of battery. How do they compare? Certainily I wouldn’t need 1w input for 1amp storage?

    1. great point. I haven’t really thought about it. But as lithium ferrous phosphate batteries have three to four times deeper discharge available. For batteries like LifeP04 you can probably say 1 watt to 1/3 amp storage (at 12v). This will be interesting to observe over the coming decade.

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