Photovoltaic panels are now down to €0.19 per peak watt:

That's down from €0.65 or so per peak watt five years ago:

This is so cheap that now it might actually be cheaper to heat your water with photovoltaic panels than with passive solar collectors under some circumstances:

@kragen And if you're heating the water with a heat pump water heater that's either drawing its heat from a space that needs cooling, or at least from outside if the space doesn't need cooling, things start looking *REALLY* good for solar PV water heating...

(Of course, I'd still like to see systems that integrate cooling of the solar panels in summer conditions, which improves their efficiency while also reducing heating demand for hot water...)

@bhtooefr That's a really good point! Even if it's drawing the heat from your cold water, a heat pump can have a CoP of 6 or so. But the heat pump itself is a potentially significant cost.

@kragen Heat pump water heaters are now more of a thing in the US, especially after some 2015 efficiency mandates, so you might already have one. However, AFAIK they all pull heat from the air in the room they're in.

@kragen (or rather, so a US person might already have one)

@bhtooefr Cool! Yeah, I didn't know they were a thing on the market already. Any idea how much they cost per output thermal watt?

@kragen So the cheapest one I can see from a big box hardware store is this:

The compressors on all three are rated at 4200 BTU/h, which would translate to 1231 W. (That said, being hybrid, it also has conventional elements and can run those if needed to get hot water faster.)

@bhtooefr Possibly because I'm in Argentina, for me, that page says, "Access Denied
You don't have permission to access " on this server.

Reference #18.344a2cb5.1546286921.372a8c8f ".

How much are they asking for it?

@bhtooefr Oh interesting, almost exactly US$1 per watt. Well, that's five times the price of the photovoltaic panel to power it, and ten times the price of laying a black plastic pipe in the sun. Probably you'd be better off spending US$600 on 3000 watts of photovoltaic panels and then using them to power a 2000-watt resistor to heat your water tank. Better heat exchangers might close the gap.

@kragen Worth comparing to conventional water heaters, though.

From the same manufacturer, for resistive heaters, same capacity:

* Same series and feature set (LCD control panel and mobile app and all), 12 year warranty (2 years longer), dual 5500 watt resistive elements, $604
* One series down (LED indicator), 9 year warranty, dual 5500 watt for $538, or dual 4500 watt for $520.70
* Entry level (just a basic water heater), 6 year warranty, dual 4500 watt for $409

@kragen So the price premium, depending on what you value, is between $695 and $890, if you need to replace your water heater anyway.

Still better to buy more panels and use resistive, but if you're out of room for panels...

@bhtooefr Oh yeah, that's a great comparison! That's 4.5¢–5.4¢ per (peak) watt, about 20 times cheaper, though still significant compared to the 19¢/watt cost of the energy itself (during a presumably much longer period of time, which matters with sufficiently low discount rates). Maybe a heated-pebble-bed design that doesn't need a giant stainless steel tank to hold the water could improve the cost?

@bhtooefr Because is a "Dernord" 3500-watt "immersion water heater", which is to say, just the resistor in a waterproof coating to dunk in your thermos and burn your fingers on, and it costs US$25 (0.7¢/watt).

(I'm assuming it makes more sense to store the energy until nighttime in an insulated hot-water tank rather than in batteries, or a tankless heater would be clearly better.)

@bhtooefr This is maybe a mild preview of the massive change in energy efficiency tradeoffs due to plunging PV costs that I predicted in

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