11-13yr payback on something that is going to degrade ~3-4%/yr for 10 years (and limited terminal value) doesn't seem like the greatest investment unless you think the arb is going to widen a ton.

Alternative systems are closer to 5 years payback if you have access to off peak energy. 3-4%/year degradation is very high, battery packs available in the UK from Fogstar are rated at 8,000 cycles with a 95% depth of discharge.

FWIW my high voltage battery pack has almost 365 cycles and is still at 100% state of health.

And any arbitrage is also trivially exploited by the electricity company, who has a much greater financial incentive and scale, so there's really no reason to think that it's going to widen in the future either.

Add in that my freezer probably contains over $500 of food (and the fridge has less, but not none.)

Usually you can buy ice, but when I had a ~36 hour outage a couple years ago, I couldn't. Most stuff was fine for that long, but 2-3 days would've meant tossing pretty much the entire contents.

(Plus, I don't have cell service where I live, so being able to power my router is pretty useful.)

This is probably a bad move if you live in a city. It makes more sense in rural areas.

Let's assume the calculation is correct and you actually earn back in 11-13 years. You are better off investing the money in something like the S&P 500 where you'd double your investment in about a decade without the hassle.

Things will start to get interesting where €1600 don't buy you 5kWh but double or triple the amount.

I can imagine most people are better off either investing like I said or in insulation.