That isn't the efficiency, which is a number somewhere under 23%, dividing the number of joules of light into the panel by the number of joules of electrical power out of it. Maybe you mean that at peak he gets 91% of the advertised 6kW, i.e., 5500W?
Anyway, you said they paid €7000 and get back €500 per year, which is enough to calculate the ROI at 7.1% per year.
Powerwalls are indeed extremely overpriced. But battery prices are down to below US$70/kWh (US$19/MJ, €17/MJ); if we assume the maximum production in a day is about 25%, that's 36 kWh (130MJ), so it would be about US$2500 for that amount of battery. But probably even €500 of battery (7.2kWh) would make a big dent. That would be 1800 watts for four hours in the evening.
Depending on the type of evening consumption, it might be possible to ameliorate it dramatically with various kinds of thermal storage, or even plugging the washing machine into a timer. Those are much cheaper than batteries.
yes 7% ROI if no depreciation, but neither panels nor inverter are expected to last much beyond ten years. I was actually chatting with him just now since I got into this discussion, and he said he also needs a maintenance/emergency callout and cleaning contract, thats about €250 a year. In spite of this, he was still happy with doing this, good for the planet, even if not for this wallet. He also put up a large screen with the relevant numbers to train others in the house to use appliances at optimal times.
Panels are normally warranted not to drop below 80% of rated output after 25 or 30 years, although some are defective and crack within a year or so. But they don't start dropping off more rapidly at that point; rather, their output declines more slowly after that point. Those panels will be fine 50 years from now if he doesn't throw them away.
Inverters are a different problem, and they do in fact die after a few years. But you don't need the inverter to use the energy.
Anyway, you said they paid €7000 and get back €500 per year, which is enough to calculate the ROI at 7.1% per year.
Powerwalls are indeed extremely overpriced. But battery prices are down to below US$70/kWh (US$19/MJ, €17/MJ); if we assume the maximum production in a day is about 25%, that's 36 kWh (130MJ), so it would be about US$2500 for that amount of battery. But probably even €500 of battery (7.2kWh) would make a big dent. That would be 1800 watts for four hours in the evening.
Depending on the type of evening consumption, it might be possible to ameliorate it dramatically with various kinds of thermal storage, or even plugging the washing machine into a timer. Those are much cheaper than batteries.