I own a largish building of about 360 sq.m., of divided use, some of which is my home. It was built about 110 years ago. Heating it is an issue. Heating and hot water cost me about 40,000 kWh per annum, about 110 kWh per day. I use a hyperefficient gas heater and pay the city for the gas. The heater is nominal-20kW, actual 22kW, and was installed in anticipation that the building would be insulated. As it turns out, only the roof was insulated (which in my case yielded almost exactly the rule of thumb that it saves 25% of the building’s heating energy). To heat the entire building to 20°C would take somewhat more, although less than 30kW. But burning at a rate of 30kW is a third more than what I can at present, and I am not sure I need, or want to pay for, a heating paradigm that uses 150 kWh per day. I paid 6.3¢ per kWh in the last year, so that would be about €75 more a month, €900 more per year.

One way of reducing gas burning is to replace gas with a geothermic installation. I haven’t had a geologic inspection, but even if my site is suitable, not only are installation costs high but maintenance costs also. My architect thinks it will be another decade before the technology is mature. Besides, geothermic heating works at lower running temperatures than what I need, so I would have to insulate my building. A good idea in theory, but it would probably cost a six-figure sum (there is not only padding the outside walls, but replacing 21 windows out of 28 total).

Just looking purely at finances, rather than the moral goodness of burning less gas, insulation is a questionable action. Over the next 25 years, €900 more in gas bills per year costs a quarter, or less than a quarter, of what it would cost me to insulate thoroughly right now. Of course, gas prices can change significantly (and have in the past) in ways which are not now foreseeable. Insulation adds to the value of a building, but it is not at all clear that it would add €70,000-75,000, which is what would have to happen to amortise the cost over 25 years using constant-2016 gas prices.

Last winter, I bought and used a Dyson AM09 hot-air heater for spot heating. Much of my building is not in constant, but in sporadic use (that is, for a couple of hours per day) and it seems to make sense to keep such space at a base temperature of 14-16° and use the Dyson to warm up individual rooms when they are in use. Practically, this works.

I told my heating engineer’s lady what I was doing, and got a five-minute lecture on proper heating behaviour. I talked to her husband a week after, and got another lecture. His senior tech person then called up about my heating needs and I got a third lecture. Heating from household electric supplies is regarded as inappropriate in Germany!

I am not so sure. I have been thinking about PV roof panels feeding a decent battery installation in the basement which could support my spot-heating paradigm. The AM09 is apparently a 2kW machine https://www.theguardian.com/technology/2015/apr/15/dyson-hot-cool-am09-review-worlds-best-heater-fan (it’s odd that I couldn’t find this datum on the Dyson www site). The Tesla Powerwall battery has a capacity of 6.4 kWh, but is not yet available in Germany https://www.tesla.com/powerwall . So 1 Powerwall = 3 hours AM09 spot-heating.

There is a safety issue with high-capacity batteries in buildings. I don’t know whether sustainability in general is a safety topic, although I have some sympathy with those who argue it is THE safety topic. Safety+sustainability does come together in debates about power, notably the viability of nuclear power. With buildings using a radiation-into-electricity-into-heat paradigm, battery safety would be an issue. Tesla’s design, using many single, well-separated lithium-ion cells in a design which is tolerant of thermal runaway in individual cells, is attractive.

Then there is the practical matter of having enough roof space for PV panels. I have a one roof with north and south faces, and another, lower, with east-west. Using south and west faces would be possible.

One Powerwall gives me about 6% of my average current daily use. There is no way I will be buying or mounting 17 of them to substitute fully for the gas heating, even if I had the space, and there is no way my roof has enough space to drive that quantity from PVs. It follows that such a system cannot substitute for my gas heating. I haven’t done the geometry, but I think I might be able to drive 2 or 3 using my available roof space. If they are charged and discharged once per day, that amounts to just under 12% of my current daily energy use on heating.

Two Powerwalls would give me 6 unit-hours of spot heating per charge, which might well suffice along with my current 20 kW gas heating, providing I can reasonably expect to recharge both Powerwalls each day. I used 3,000 kWh less in the last annual period, with the AM09, than in the period before, but I have no idea how much of that is the annual variation in the weather, and how much is down to using spot heating.

I haven’t done the costing for a PV+battery installation yet. It would also need protection to stop any local juice feeding into the grid. But if it costs less than €28,000 then it passes the 25-year amortisation test against the 30kW gas heater (which itself would cost in the region of €5,000, likely more, to install).

There is a small company selling roof-mounted home windmills to generate electricity, but I am doubtful of the maturity of that technology. Robustness is an issue, especially given some of the wind storms we have had around here. Then there is mechanical effectiveness, how it orients itself to the wind. It’s not clear how they deal with orographic eddies, of which there are plenty in my little cluster of buildings. Then there is noise – neighbours are quite close. And, finally, how much electricity would practically be generated in an installation on my building.

I note that a while ago, Mitsubishi floated the idea of turning its i vehicle into an emergency power supply. It has 16 kWh of storage https://www.theguardian.com/technology/2015/apr/15/dyson-hot-cool-am09-review-worlds-best-heater-fan In a power outage, that could run one spot-heater for 8 hours, or a spot heater plus cooker, kettle and some lights for, say, 6-7 hours. And you would have to buy kit to attach it to your home network as well as the protection to stop it feeding out of the building into the grid. It seems to me more of a novelty, but maybe if you live in a country occasionally but regularly ravaged by typhoons it may be more attractive.

Finally, there’s not much to be done about the radiation -> electricity -> heat conversion factor. That is why my heating engineer deprecates electrical heating, and it is a powerful point.