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As long as you've got electronics and appliances in your home, you're inevitably going to be faced with a choice: Something breaks. Now what?

From a price perspective, if the cost to repair a household appliance is more than half the price of a new product, advances in energy efficiency will generally make buying a newer model the cheaper choice. Based on these numbers and considering today's more environmentally friendly technologies, here's a guide to when you should repair or replace major appliances.

Air conditioners

* Replace window units older than 10 years and central-air systems older than 10, but consider alternative cooling methods.

Upgrading your window units to a more efficient model can cut energy bills by an average of $14 a year, estimates the Energy Star program. The most efficient room air conditioners have higher-efficiency compressors, fan motors and heat-transfer surfaces than previous models. Central ACs are rated according to their seasonal energy efficiency ratio (SEER)—for which most 1992 to 2005 models score about a 10; older ACs have ratings of only 6 or 7. New minimum standards set in 2006 require current central-air units to have a SEER of at least 13. Because of the coolants used, old room-AC units need to be disposed of in hazardous waste facilities; old central units are usually disposed of by the contractor hired to install the new unit, but always ask ahead of time to ensure proper disposal.

Before you buy, however, consider alternatives such as ceiling fans, evaporative coolers (if you live in a dry climate), whole-house fans and landscaping or decorating changes, all of which can keep your home comfortable for a fraction of the cost.

Water heaters

* Replace all electric heaters, and any gas heaters older than 10 years.

If you have an electric heating system, you can achieve a 50 percent energy savings used by switching to a high-efficiency gas model. Gas heating systems can last for about 25 years but will operate for years at very low efficiency before they finally fail; if yours is more than 10 years old, it probably operates at less than 50 percent efficiency and deserves to be replaced. Consider a "demand," or tankless, system, in which water is circulated through a large coil and heated only when needed. Although EnergyStar doesn't certify these models, the government estimates that they can save between 45 and 60 percent of water heating energy and up to $1,800 a year when compared to standard, minimum-efficiency heaters.