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Comment by ben_w | Hacker News Reader

Only a problem when you're limiting yourself to resistive heating. Heat pumps can heat water, not just air, and are several times more efficient than resistive.

I've got a heat pump, and I'm in Germany.

Also, if you're in Germany, you can get a balcony PV system from half the supermarkets a few hundred euros, and those are designed to be installed DIY without needing an electrician. Limited power, sure, but way cheaper than €0.39/kWh delivered:

• https://www.lidl.de/p/vale-balkonkraftwerk-ecoflow-820-w-800...

• https://www.kaufland.de/product/502015379/?search_value=balk...

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Your first link seems to be 349€ for 800Wp including microinverter. German utility-scale PV has a capacity factor averaging about 12% IIRC, but presumably a balcony system will be lower because it isn't optimally angled for the sun, say 8%. Then that's about 64W, which is 560 kWh per year. 349€ over 10 years would be 35€ per year, about 0.06€/kWh.

That's still about six times the cost of wholesale low-cost solar panels: https://www.solarserver.de/photovoltaik-preis-pv-modul-preis...

64 watts is about 40–50 liters per day of hot water heated resistively, presumably closer to 150 liters per day with a heat pump. But it seems like the heat pump is only saving you the 700€ for two more such balcony systems, assuming you have the space. Moreover, you don't need a microinverter for a resistive heater.

Oh indeed, balcony systems are small and limited — the constraints that led to them are: (1) DIY-safe, (2) useful for rented apartments (limited window or balcony fencing space, may not be allowed to fix something to an exterior wall).

I'm not sure if you're allowed to just resistively dump an off-grid PV system into a resistive heating system, but I guess if you did, you could indeed save on the cost of the inverter.

I believe that even in Germany you can connect a low-voltage solar panel to a battery or heating element without any licensing approvals beyond CE. Low-voltage wiring poses much less risk of electrical shock and is consequently exempt from most safety regulations.

efficiency of no MPPT is around 10 percent lower then with MPPT system so no issue. 2000 watt is maybe max what you can have on balcony - physical size, and 2000 watt inverters are like 20 $. well connecting anything to grid is/should issue, offgrid noone cares.

You might be able to hang an additional balcony system out a non-balcony window or something, or just keep it inside the window. Less insolation, but maybe not so much less that it's unprofitable.

Interesting note: this is about six times cheaper per watt than residential solar systems in the US.

We actually do have a Balkonkraftwerk im Garten :-)

Now that the sun is out for longer periods each day we are "wasting" energy to the grid a lot. I don't really see how to capture that energy though.

1. Buying a battery quickly shifts the break even points to decades. Without a battery I estimate 3-4 years. 2. I would love to heat water, but renting a place limits my options a lot. I was looking at electrical boilers to supplement the gas heater. But we are limited on space for small heaters below the sink and big heaters in the main water path. (Also we can't change the plumbing for legal reasons.) 3. The next best thing is some imaginary insulated water heating kettle that I can control to only use exactly the excess energy. No idea if such a thing exists.

You should calculate the battery parts again. I'm currently installing mine in Germany and the cost parity is getting close.

You probably thought of this already but we mostly do load shifting. If you have moderate PV output (like with balcony solar) that can probably use up most of your production.

Consider running the dishwasher (if you have one) or washing machine / dryer (if you don't dry that in the sun directly) during the day.

Granted, we work from home _a lot_ and also have an EV so it's a lot easier to do load shifting for us, but just shifting the dishwasher and washing machine to 'sunlight hours' already made a pretty decent difference.

Yeah, we do that. Given that most home appliances use more than 800W (even in Eco modes) we often use more than our production.

E.g. our washing machine uses 1000W over a prolonged period of time which would be perfect to run on a sunny day. But it does so by switching the 2000W heating element so it averages to 1000W ...

So we repeatedly export 800W (without any form of reimbursement) and import the missing 1200W back.

And that is the case for all of our appliances. (I have a sensor to monitor that)

Don't know if more modern machines are better in this regard, our machines are about 5 years old now.

edit: I don't want to sound bitter about it. The Balkonkraftwerk works perfectly fine to power our base energy load.

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