This site has some handy heating calculators. https://bloglocation.com/art/water-heating-calculator-for-time-energy-power
I stumbled upon it yesterday after spending half my day plumbing in a new hot water tank. (did a major redo of the pipes from the pump , filter and new tank placement location)This tank is a Rheem 30gal. tank that came with 2-4500w elements wired for 240v. I removed the factory elements and replaced them with 120v, 2000w elements instead so that I could use it with my solar power system via an individual transfer switch. One thing I was curious about was just how much time it would take to heat the water up to 135F from the base temperature of around 65F.
Using the calculator for time to heat up the numbers were 30gal., 65F, 2000w and the results were 2h-35m. My measured time from energizing the tank to temperature shutoff came in at 2h-40m being powered from the grid. The approximate power used was 5.3Kwh. The calculator on the site for the same numbers comes in at 5.14Kwh Considering my present solar arrays values of 2000w it would be unlikely to that I would be using it as a every day situation. I expect I mostly will continue to use grid but I now can supplement water heating on those days when my batteries get fully charged by noon.
I stumbled upon it yesterday after spending half my day plumbing in a new hot water tank. (did a major redo of the pipes from the pump , filter and new tank placement location)This tank is a Rheem 30gal. tank that came with 2-4500w elements wired for 240v. I removed the factory elements and replaced them with 120v, 2000w elements instead so that I could use it with my solar power system via an individual transfer switch. One thing I was curious about was just how much time it would take to heat the water up to 135F from the base temperature of around 65F.
Using the calculator for time to heat up the numbers were 30gal., 65F, 2000w and the results were 2h-35m. My measured time from energizing the tank to temperature shutoff came in at 2h-40m being powered from the grid. The approximate power used was 5.3Kwh. The calculator on the site for the same numbers comes in at 5.14Kwh Considering my present solar arrays values of 2000w it would be unlikely to that I would be using it as a every day situation. I expect I mostly will continue to use grid but I now can supplement water heating on those days when my batteries get fully charged by noon.
it even handles our proper-sized gallons compared to your small ones 
