A tax reminder: The tax credit for installation of energy-saving devices is scheduled to expire at the end of 1985. So if you want to take advantage of either of the two energy credits, better get started.

The first credit is for installation of certain components in your principal residence. The residence must have been built before April 20, 1977; but you don't have to own the property -- rental property qualifies too, as long as it's your principal residence and you pay for the installation.

Among the items that qualify for the credit are insulation, caulking and weather-stripping; storm or thermal doors and windows; energy-saving furnace ignition systems or replacement burners; and an automatic setback thermostat.

You may claim a tax credit equal to 15 percent of the first $2,000 of such expenditures, for a maximum credit of $300. This is not an annual ceiling, but rather the total cumulative maximum allowed since the program started several years ago.

The second energy tax credit is allowed for what are called "renewable energy sources," such as solar, wind or geothermal energy used for heating or cooling or to provide hot water or electricity.

The amount of this credit is 40 percent of expenditures up to $10,000, for a maximum cumulative credit of $4,000. Again, the equipment must be installed in your principal residence, but the 1977 construction requirement does not apply.

In either case, the installation must be completed and paid for not later than Dec. 31, 1985 -- so you don't have much time to fool around if you want to get in under the wire.

Q As a youth (that was ages ago!) I had a stamp collection. Over the years since, I would periodically get interested again, but never thought of it as anything but a hobby. As a result of the inflation of the '70s, the collection now has substantial value. If I sell it now, what's the best approach for me to take to report the gain for tax purposes? I don't have anything at all in the way of records of cost, over a period of some 50 years. Can I expect the IRS to be reasonable in accepting an estimate, rather than applying tax on the whole selling price because I have no records to prove otherwise?

A Your question struck a responsive chord because I'm in the same situation. I, too, have returned intermittently to a stamp collection that I began more than 50 years ago. The only difference is that I'm still interested and not contemplating a sale. But I suspect other readers may have a similar question regarding old stamp and coin collections.

According to the tax laws, you must be able to support your cost basis with some documentation in the event of an audit. The IRS can give no assurances that it will do anything other than what is required by the law.

As a practical matter, however, your return may never be audited. If it is, I think most auditors would accept an estimate that appears to have been arrived at reasonably. You might, for example, obtain from a stamp dealer a written statement of the average appreciation in value of collectible stamps in the past 25 years (the mid-point of your 50-year span).

Then subtract that appreciation from the selling price of your collection to arrive at a cost basis to use for calculating gain. Or perhaps after examining your collection, a dealer might be willing to give you his estimate of what the collection was worth 25 years ago.

The bottom line is that the IRS is not required to accept anything other than documented proof of cost. But IRS auditors will recognize that, unless you got the collection as a gift or bequest, you undoubtedly paid something for the stamps. In most cases, in an understandable absence of hard facts, they are likely to accept an estimate that was derived in some reasonable manner.

Abramson is a family financial counselor and tax adviser. Questions of general interest on tax matters, insurance, investments, estate planning and other aspects of family finances will be answered in this column. Advice cannot be given on an individual basis. Address all questions to E. M. Abramson, The Washington Post, Business News, 1150 15th St. NW, Washington, D.C., 20071.