Last July my AC bill hit one hundred eighty dollars. For a two-bedroom apartment. I called my dad, who has Opinions about household expenses, and he asked one question: “What is on your windows?”

Cheap blinds. That was the answer. I spent forty dollars fixing it and the next month’s bill dropped by sixty bucks.

Windows Are Radiators in Summer

Sunlight hitting your windows becomes heat inside your home. About thirty percent of unwanted heat gain comes through windows. Your AC has to fight that heat constantly. Blinds help. Curtains help more. Blackout curtains with a thermal backing help the most.

What to Buy

Look for curtains labeled “thermal insulated” or “blackout.” These have a foam or rubber backing layer that does two things: blocks light and creates a thermal barrier. The price difference between basic blackout and thermal blackout is about five dollars per panel. Get the thermal kind.

I bought two panels for each south-facing and west-facing window. North and east windows get morning light but not the punishing afternoon heat, so I did those later. Start with the windows that get direct afternoon sun.

Installation That Maximizes the Effect

1. Mount the curtain rod four to six inches above the window frame and extend it past the sides. This blocks light that spills around the edges.
2. Curtains should kiss the floor or hover a half-inch above it. Too short and heat leaks out the bottom.
3. Close them before the sun hits the window, not after the room is already hot. Prevention, not response.

Does It Actually Cut Bills in Half

Okay — “half” is aggressive. The Department of Energy says properly installed window coverings can reduce heat gain by up to seventy-seven percent for the covered windows. Your total AC bill depends on how many windows face the sun and how big they are. My bill dropped about a third. For forty dollars in curtains, that paid for itself in the first month.