I used to use a 30 amp 240VAC mechanical relay to switch the hot water tank but the relays would only last a year or two.

I have now switched to an inexpensive, ($7.32 Cdn with free shipping over $10 so I bought two), DC-AC Solid State Relay instead which should last forever.

I bought one capable of switching 60 amps even though I measured only 15 amps used by the water heater.

Bigger because, why not? 

Difference in cost is insignificant. Same size.

The 3.3V NodeMCU is only 12 mA per output so it can't directly turn on the SSR from an output.

The previous 30 amp mechanical relay needed 5 VDC to turn on so I used a small 4 channel relay for switching the 5V which is what I now use for the SSR. The small 10A 4 channel relay is only switching mA so no issue with durability.

I mounted the SSR to a flat aluminum plate with heat sink grease because I read SSRs can get hot while on.

But it doesn't even get warm.

The datasheet says it has a 1.6 volt drop x 15 amps which should mean 24 watts of heat needs to be dissipated.

I could have bought a heat sink for it which costs $5.25 Cdn.

But I only measured a 0.9 volt drop x 15 amps which means only 13.5 watts of heat, but it doesn't even get warm.

In Ontario there is a 2.8 cent per kWh optional overnight rate.

We switched to the new rate and save another $150/year on top of the fact we have always had timers on the hot water tank and hot tub. We save even though the new low rate comes with a 28.4 cents per kWh 4-9 pm weekdays cost.

2.8 cents per kWh is cheaper energy than natural gas. More expensive to buy and now more expensive to operate natural gas water heaters are obsolete with the new rate.

Also it's only $100/year to "fuel" an EV at the ultra low cost overnight rate.