[njb311]

The article doesn’t say it explicitly, but I’m assuming it heats water on demand, rather than heating a tank which immersion heaters do. You can buy a resistive element electric shower that gives instant hot water and requires up to about 10kW (and costs £100-200 not £3500) but a gas central heating boiler might pump out 22-30kW equivalent of heat. You would need modifications to your electricity supply to achieve that with electric heaters. If the microwave technology was able to give better performance that would be in its favour, but the boiler cost is not competitive (like many new products). Air source heat pumps are more efficient but again the purchase/install costs make the return on investment long, it was going to be more than 40 years in my case (unless of course carbon pricing changed the calculation), which may be longer than the lifetime of the unit. You also have to be mindful of the refrigerant used in ASHPs because if it leaks the warming potential is several tonnes of CO2e, thankfully a lot of change is happening there too.

What will make the difference to all this is the combination of increased efficiency plus production at scale that makes retrofitting attractive to homeowners. Part of that scale could come from mandatory fitting of such equipment to all new builds.

[zeroping]

The only reason to heat a water tank rather than on demand is if your electrical connection can't supply enough power for peak loads. This can be true for domestic hot water, like showers and sinks. Tankless hot water heaters exist (and are cheap), but take 3-4x as much as much instantaneous power, and can be expensive to install for that reason.

If this microwave tech has a COP of less than 1 (meaning it doesn't generate more thermal heating than you put in), then it doesn't solve this problem and better than resistive electric heating. You have the same electrical installation problems.

There may be some other advantage here (long-term maintenance?), but not power efficiency. Thermodynamics says so.

The only way to do better is to make something else colder while you heat your house, using some kind of heat pump.

[SigmundA]

Resistive heating is 100% efficient, they are saying 96% for microwave, meaning you will need 4% more power to achieve the same heating either in a tank or no tank.

[njb311]

Yes, I wasn’t suggesting that this solution was more efficient, but that solutions that are more efficient than what they replace are what we should be focusing on. It’s fine to say that resistive is 100% efficient, but my point was that a 10kW instant electric heater is not enough to meet the demands of many households (and falls well short of a gas boiler output) and you can’t just say install a 20kW electric heater because typically household supply is not built for that. And so if you heat a tank of water (a typical immersion heater is 3kW) then all of a sudden you have heat loss and you are nowhere near 100% efficient.

[dTal]

I am curious what form of energy you think the other 4% turns into?

Both systems are 100% efficient at turning electricity into heat. Neither system is 100% efficient at turning electricity into useful hot water. There are always losses. They're just being honest with their accounting.