Insulation water heater tank can be is simple and inexpensive, and it will benefit you in about a year. Find some pre-cut jackets or blankets available costing around $10–$20. Choose one with an insulating value of at least R-8.
Insulating an Electric Water Heater Tank

American Heat ADK 16 KW Whole House Tankless Water Heater US $269.00

Noritz SC-201-6M Tankless Water Heater System Control US $264.95

Tankless Water Heater Olympia 18 KW Whole House New US $205.00

American Heat AH-18 Whole House Tankless Water Heater US $391.00

TITAN N-160 SCR-3 Electronic Tankless Water Heater BRAND NEW US $309.99

Rheem RTGL74XP Outdoor 7.4 GPM Tankless Water Heater for Liquid Propane US $699.88

Bosch 1600H LP AquaStar 4.3 GPM Indoor Tankless Liquid Propane Water Heater NIB US $580.00

NIB GE INDOOR TANKLESS GAS WATER HEATER 7.5 MODEL GN75DNSRSA US $649.88

NORITZ 8 GPM INDOOR NATURAL GAS TANKLESS WATER HEATER US $1,999.99

NORITZ 8.3 gpm INDOOR HOME TANKLESS WATER HEATER NATURAL GAS NG NR83-DVC-NG US $1,319.99

NEW NATURAL GAS TANKLESS WATER HEATER 4.23 GPM / 16 L US $297.95

American Heat 16 KW Whole House Tankless Water Heater US $269.00

Noritz N-0751M-DVC (LP) - 9.8 GPM - 199,900 BTU/H Tankless Water Heater Propane US $910.00

Rinnai Tankless Water Heater R75LSIN Interior Natl Gas US $1,125.00

8 Valve kits for Tankless Water Heater LEAD FREE (T) US $590.00

8 Valve kits for Tankless Water Heater (S) US $460.00

8 Valve kits for Tankless Water Heater LEAD FREE (S) US $590.00

8 Valve kits for Tankless Water Heater (T) US $460.00

Rinnai Tankless Water Heater R94LSIN Interior Natl Gas US $1,299.00

Rinnai Tankless Water Heater R75LSIP Interior LP Gas US $1,125.00

Marey® 4.2GPM/16L Natural Gas Whole House ON DEMAND Tankless Hot Water Heater US $296.95

Rinnai Tankless Water Heater R75LSe Exterior Natl Gas US $1,090.00

Rinnai Tankless Water Heater R94LSe Exterior Natl Gas US $1,290.00

Eccotemp 40HI 6.3 GPM LP Intdoor Tankless Water Heater US $429.00

Eccotemp 40H 6.3 GPM LPG Outdoor Tankless Water Heater US $429.00

Eccotemp FVI12-LP 3.4 GPM Gas Tankless Water Heater US $269.00

Eccotemp 40H 6.3 GPM NG Outdoor Tankless Water Heater US $429.00

Rinnai Tankless Water Heater R75LSEP Exterior LP Gas US $1,090.00

Rinnai Tankless Water Heater R94LSEP Exterior LP Gas US $1,290.00

Rinnai Tankless Water Heater R94LSIP Interior LP Gas US $1,299.00

Noritz N-132M-ASME (LP) - 13.2 GPM - 380,000 BTU/H Tankless Water Heater Propane US $2,905.00

NIB Rheem Tankless Water Heater US $580.00

Bosch Tankless Water Heater ES VVT Vertical Vent Kit for AquaStar 2700ES 2400ES US $270.00

Marey Whole House On Demand 4.2 GPM Propane LP Gas 16L Tankless Hot Water Heater US $296.95

New Rheem/Paloma Tankless Water Heater 7.4 GPM Propane Gas Power Vent PTG-74PVP US $799.99

NEW Rheem/Paloma 5.3 GPM Tankless Water Heater Power Vent Propane RTG/PTG-53PVP US $529.95

HHP HH19 Single Shower Tankless Water Heater US $434.12

HHP HH28 Tankless Water Heater Whole House US $704.95

HHP HH12 Single Shower Tankless Water Heater US $412.47

Stiebel Eltron Tankless Water Heater Tempra 12B US $435.00

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You can probably install an insulating pre-cut jacket or blanket on your electric water heater tank yourself. Leave the thermostat access panel(s) uncovered. Don't set the thermostat above 130ºF on electric water heater with an insulating jacket or blanket—the wiring may overheat. You also might consider placing a piece of rigid insulation—a bottom board— under the tank of your electric water heater.

This will help prevent heat loss into the floor, saving another 4%–9% of water heating energy. It's best done when installing a new water heater. Water heater gas and oil-fired water heater tanks is more difficult than those for electric water heater tanks. It's best to have a qualified plumbing and heating contractor add the insulation.