Jerry's cautionary tale of crispy wires in the j-box should be well considered. NEC says storage water heaters shall use wire sized to 125% of 'nameplate' current. Assume your 2000w element is 'nameplate', then: 2,000w / 120v = 16.66 amps current, x 125% = 20.83amps. 12 ga is rated 20 amps, 10 ga = 30a.
Maybe run the diesel when its time for a shower!