You have an electric water heater and the two blue panels are your heating elements.
They each have separate thermostats and will turn on and off at different times.
In most residential water heaters only one element is on at any given time. The lower element does 90% or more of the heating. During periods of heavy use, the upper element switches on (the lower switches off) and heats the upper part of the tank to 120°F. It then shuts off, turns on the lower, and it heats the water to the setting of the lower thermostat.

TWO PACK OF HOLMES PORTABLE ELECTRIC HEATERS - BOX NEVER OPENED $22.99

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Broan Nutone Two Bulb Heater 163 $48.75

LOT OF TWO WARMWAVE AND OSICILLATING REFLECTIVE HEATERS $4.25

Infrared Two Person Sauna Energize Duet Series with 6 Carbon Heaters $899.95

Infrared 2 Person Sauna Energize Duet Series with 6 Carbon Heaters Two Infared $899.95

Nutone Bulb Heaters #9422P Two Bulb Heat A Lamp $43.50

TWO SYLVANIA ELECTRIC STOVE HEATER MODEL SO111R-MBK $6.50

TWO ALLEN-BRADLEY TYPE. N 11 HEATERS $4.00

Holmes HQH307 Prismatic Quartz Tower Heater w/Two Heat Settings $105.99

Propane Auto Changeover Two-Stage Regulator (Mr Heater # F273766) $24.99

HEATER Propane - Tank Mounted - Two Head Piezo Ignitor $213.44

SET brand.Two seats,round switch seat heater,heated seat,car cushion. $79.99

NEW BAINS MARIE TWO ROUND POT COOKER WARMER HEATER BUFFET SERVICE SAUCE HOLDER $197.72

Prismatic Quartz Tower Heater w/Two Heat Settings, 14w $99.99

TWO NOS WEIL-MCLEAN THERMA TRIM HOT WATER BASEBOURD HEATERS 5FT & 6FT $55.00

Two Brand New Army USMC Canteens with Cup and Covers and Heater Device, LBE LBC $15.50

SET brand.Two seats,round switch seat heater,heated seat,car cushion. $79.99

SET brand.Two seats,round switch seat heater,heated seat,car cushion.heated pad $79.99

SET brand.Two seats,round switch seat heater,heated seat,car cushion.heated pad $79.99

Universal Five Level Two In one Switch Carbon Fiber Seat Heater Cushion Warmer $73.99

Ambia Ach-120 Desgin Winter Portable Two Zone Ceramic Space Heater $63.82

MERCEDES W201 190E 190D TWO SWITCH AC / HEATER CENTER VENT 1984-1993 $89.90

TWO packages BackDraft Indicator for Natural Draft Gas Furnaces & Water Heaters $15.00

Lot of two New Element Gasket Kits for Electric Water Heater $4.99

Chevy Silverado/GMC Heater Control tahoe 96-97-98-98 only two months old.. $129.99

SET brand.Two seats,round switch seat heater,heated seat,car cushion. $79.99

Two ALLEN BRADLEY Type N HEATER ELEMENTS N23 $8.99

Two ALLEN BRADLEY Type N HEATER ELEMENTS N21 $8.99

Two ALLEN BRADLEY Type N HEATER ELEMENTS N17 $8.99

Two ALLEN BRADLEY N44 HEATER ELEMENT OVERLOAD RELAY $8.99

Two JEBO 300W Aquarium Submersible Heaters 300 Watts $33.99

Two 200W JEBO Aquarium Submersible Heaters 200 Watts $32.99

SET brand.Two seats,round switch seat heater,heated seat,car cushion. $79.99

SET brand.Two seats,round switch seat heater,heated seat,car cushion. $79.99

SET brand.Two seats,round switch seat heater,heated seat,car cushion. $79.99

Electric Instantaneous DAFI Water Heater 9 kW 400V two phases - under sink $110.72

TWO- K&H Manufactuting Thermo Pond 3.0 Heater / De-Icer 100 watt $64.68

TWO SYLVANIA ELECTRIC STOVE HEATER MODEL SO111R-MBK $0.99

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Stewart Warner Aviation heater Lauson RSC carb two gaskets Tillotson $1.00

Water Heater 6A AC 250V Two Pin Male Connector Boat Rocker Switch 2 Pcs $3.76

SET brand.Two seats,round switch seat heater,heated seat,car cushion. $79.99

TWO (2) X Aquarium Overflow Boxes with CA 6000 pump & Titanitum Heater PACKAGE $129.99

SET brand.Two seats,round switch seat heater,heated seat,car cushion. $79.99

SET brand.Two seats,round switch seat heater,heated seat,car cushion.heated pad $79.99

Universal Fit Seat Heater 2 Heating Pads Five Level Two in One Switch NEW! $75.99

LOT OF TWO WARMWAVE OSCILLATING CERAMIC HEATERS HOQ15-M $0.99

TWO SYLVANIA ELECTRIC STOVE HEATER MODEL SO111R-MBK $0.99

NEW PROFESSIONAL TWO HEAD WAX HEATER FACIAL SPA BEAUTY $66.00

Johnny Toymaker Heater and Two Molds - 1 Car and 1 Monster - Vintage 1970's $18.50

TWO SYLVANIA ELECTRIC STOVE HEATER MODEL SO111R-MBK $0.99

VINTAGE LAKEWOOD TWO HEAT UTILITY HEATER MODEL 792/792-A $79.99

New Nautilus N164 Two Bulb Heater Fan  $39.99

Lloytron 2000w Fan Heater with 2 Heat Settings & Cool Blow F2003WH £8.41 Key features 2 heat settings Cool blow function Variable thermostat Anti freeze protection Auto safety cut-off Integral carry handle Prone or upright positions Approx. 1.5m BS approved mains chord...

De'Longhi HS20/F Convector Heater with Fan Boost, 2kW £34.99 ...

De'Longhi HCM2030 Convector Heater with Thermostat, 2kW £28.99 ...

SWAN SW1800HEAT AND LIGHT 2 BAR RADIANT FIRE 1.8kW £29.95 2 bar traditional electric radiant fire 2 heat se...

DIMPLEX IRX200NINFRA RED HEATER 2kW £65.50 Pull cord operation white silver grill IP rating...