Electric water heater on solar: the cheapest 'battery' for surplus

An electric water heater is the most cost-effective 'thermal battery' there is. While a lithium battery costs $450/kWh, a 50-gallon tank stores 12 kWh for $350. The catch: it stores as heat, not reusable electricity.

How much energy to heat water

25-gallon tank from 32 °F to 140 °F = 7 kWh. 50-gallon = 14 kWh. If you only reheat what you use (10 gal hot daily): 3 kWh/day. Easy to cover with 1-2 solar panels.

Types: resistance vs heat pump

Resistance: 1500-2500 W, COP 1 (1 kWh electric = 1 kWh heat). Heat pump (HPWH): 500-800 W, COP 3-4 (1 kWh electric = 3-4 kWh heat). HPWH costs 4× more but uses 75% less.

Strategy with panels

Set the heater to run at 12 PM under full sun. 2-3 hours at 1500 W = 4 kWh stored as heat. By night you have a hot shower without using grid or lithium battery.

Solar surplus diverter

A device ($250-450) that detects solar surplus and modulates exactly enough power to the heater. Brands: MyEnergi Eddi, SolarEdge Smart Energy, Aurora Solis. Uses every last watt that would have been exported.

Cost vs stored energy

5 kWh usable LiFePO4: $2700. 50-gal tank with 12 kWh storable: $400. Heat pump water heater: $1700. Combining tank + small battery is more efficient than only a big battery.

Maintenance

Electric tank: replace anode rod every 4-5 years ($20). Drain calcium every 5-7 years. Lifespan 12-15 years. HPWH: filter every 6 months, lifespan 15-20 years.

Combine with heat pump for HVAC and what to do with surplus.