AFTER 15 experiments, 51 hours of sunlight, nearly 29 quarts of water and 110 tea bags, I've come up with the best iced tea under the sun. The refreshing results obviously didn't come easy. The sun didn't always cooperate and Mother Nature sometimes drizzled on my parade of tea concoctions. Though when they weren't dumped down the kitchen sink, I often drank my mistakes.

Each experiment began the same way: wide-mouth solar tea jar filled with water, from 30 ounces to just under two quarts; tea bags immersed in water, tags removed; jar placed in back yard under sun, from four to six hours.

Each experiment ended in the same manner: tea bags squeezed against inside of jar, then discarded; cold tap water added, almost always; ice cubes popped into tall glass, followed by tea, sugar or sweetener and lemon juice, except on several occasions, stirred in.

I used regular and family-size tea bags and three types of tea, but relied most often on the plain black tea available at every store: orange pekoe. I tried flavored tea, the mint variety. I even sampled expensive herbal teas.

By experiment No. 7, I arrived at the correct amount of water to use: one quart to brew the tea, one quart to stir in later.

By experiment No. 8, I had the solar time down pat: 4 1/2 hours. I learned See TEA, E3, Col. 3 The Best --Tea Under -The Sun --TEA, From E1 that constant bright sunlight isn't required. While an overcast sky that produces cool breezes inhibits brewing, passing clouds don't--as long as the temperature is warm.

By experiment No. 12, I deduced that nine regular-size tea bags produce the best results.

By experiment No. 15, I was sure I had the correct blend of tea for the best flavor: orange pekoe and mint.

I also made several other discoveries:

* The best time to brew solar tea is between noon and 5 p.m.

* Family-size tea bags offer less leeway to experiment for taste.

* If using artificial sweetener instead of sugar, use a smaller amount, as its sweetness is more concentrated.

* There's no need to refrigerate the tea until retiring for the night.

* Good iced tea shouldn't cost more than 3 cents a glass.

The biggest disappointment came during experiments No. 10 and 11, when I tried herbal tea for the first time. The brand I purchased promised to be especially Nice Over Ice. The ingredients included hibiscus and chamomile flowers, spearmint and peppermint leaves, orange and lemon peel, rose hips and cinnamon.

Nice Over Ice contained no artificial coloring, no artificial flavoring, no preservatives and no caffeine. Unfortunately, it didn't taste like tea.

I tried herbal tea alone. The reddish liquid tasted medicinal with cinnamon overtones. I tried to combine it with orange pekoe and ended up dispatching the whole batch down the drain.

But success was doubly rewarding:

Experiment No. 9 suited me to a T and cost less than 20 cents for two quarts of iced tea since I bought a box of 100 regular-size plain tea bags for $1.99.

Experiment No. 15 I dubbed Hint of Mint, and the price was a little higher--about 25 cents for two quarts--because flavored tea is a bit more expensive.

Now, without further ado, let me present: THE BEST ICED TEA UNDER THE SUN (Makes 2 quarts) 2 quarts cold water 9 regular-size tea bags (9 orange pekoe or 8 orange pekoe and 1 mint) Sugar or sweetener and lemon juice (optional)

Fill wide-mouth jar with 1 quart cold tap water, immerse nine tea bags (tags removed), screw on lid and place under sun 4 1/2 hours. Remove lid, squeeze tea bags against inside of jar and discard. Stir in remaining quart cold water and let stand 5 minutes. Fill iced tea glass 3/4 full with ice cubes and pour in tea, leaving room for several more cubes. To serve, stir in sugar or sweetener and lemon juice. If you included a mint tea bag in the brew, omit the lemon juice.