Energy.gov Water Heating
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US DOE free resources on efficiency, sizing, and types; rec'd in r/HomeImprovement.
More resources on Water Heaters
familyhandyman.com
Family Handyman is a DIY home-improvement site offering practical, step-by-step guides, tutorials, and project ideas for homeowners. It covers water heaters with articles on buying, installing, maintaining, troubleshooting, and upgrading different types (tank, tankless) plus energy-efficient and safety tips.
lowes.com
Lowe's is a home improvement retailer offering an extensive online catalog of water heaters (gas, electric, tankless, solar). It provides buying guides, product specs, installation resources, and how-to articles/videos to help you choose, compare, and install a water heater.
homedepot.com
Home Depot is a home-improvement retailer with an extensive selection of water heaters (gas, electric, and tankless) and related plumbing supplies. The site provides product listings, specifications, reviews, buying guides, installation instructions, and options for professional installation or in-store pickup.