A customer in Milton tried to cool his bedroom off a battery last August. He had a window AC unit, a 5kWh battery, and a heat wave. The unit slammed on at full power, cooled the room, clicked off, then slammed back on a few minutes later. Every time it kicked on it pulled close to 1,400W. By the time he went to bed the battery was half gone, and the grid was still up. He called us asking why his backup felt so weak.
His backup was fine. His air conditioner was the problem. A window unit is a blunt tool. It runs flat out or not at all, and that on-off hammering is brutal on a battery. Swap it for a mini-split and the same room, the same battery, and the same heat wave behave completely differently. So here is the plain-English version of what a mini-split is, why it pairs so well with solar, and how to size one without overthinking it.
What a mini-split actually is
A mini-split is a small heat pump in two pieces. One box hangs on the wall inside the room. The other sits outside on a pad or a bracket. A thin line set of copper and wire runs between them through a three-inch hole in the wall. No ductwork, no air handler in the attic, no tearing the house apart. That is why people call them ductless, and it is why one is realistic for a single bedroom, a shop, a converted garage, or an addition the central system never reached.
The "heat pump" part is the piece worth understanding. A mini-split does not make cold. It moves heat. In summer it pulls heat out of your room and dumps it outside. In winter it runs backward, pulling warmth out of the outdoor air (yes, even cool air has heat in it) and bringing it inside. One unit cools in July and heats in January. For a Florida bedroom you mostly care about the cooling, but the heating is there for the two cold weeks the panhandle gets.
Why it sips power instead of gulping it
Here is the part that matters for solar. A mini-split runs a variable-speed inverter compressor. Instead of the all-or-nothing cycling a window unit does, it ramps up to cool the room fast, then throttles way down to hold the temperature. Picture cruise control on the highway versus stomping the gas and brake over and over. The cruise-control version uses far less fuel to cover the same road.
So a 12,000 BTU mini-split (that is one ton, a good size for a bedroom up to roughly 500 square feet) might pull 800W to 1,000W for the first few minutes while it knocks the heat down. Once the room is comfortable it settles to something like 300W to 500W to keep it there. That low, steady draw is exactly what a battery and a solar array like to feed. Compare that to the window unit hammering 1,400W on and off all afternoon, and you can see why the same battery lasts so much longer.
The efficiency shows up on a spec sheet as SEER2. Higher is better. A builder-grade window unit might land around 11 or 12. A decent mini-split runs 18 to 22 or higher. That number is roughly how much cooling you get per watt, so a SEER2 of 20 against a SEER2 of 11 is close to twice the cooling for the same power. On solar, where every watt comes off your roof, that gap is real money and real battery runtime.
There is a bonus that lines up perfectly with panels. The hottest part of the day, when the room needs the most cooling, is also when the sun is highest and your array is making the most power. A mini-split running at midday is often running straight off the panels with the battery barely touched. The need and the supply peak at the same hour.
Sizing it without guessing
The rough rule is about 20 BTU per square foot for a sunny Florida room, then adjust. A 300 square foot bedroom lands near 9,000 BTU. A 500 square foot room with west-facing windows and a hot attic above it pushes toward 12,000 BTU. Bigger is not better here. An oversized unit cools too fast, shuts down before it pulls the humidity out, and you end up with a clammy cold room. Match the unit to the space and let it run long and slow, which is what it is built to do.
For the solar side, size the battery off the steady running draw rather than the startup spike. If your unit holds the room at 400W and you want it running six hours into the evening, that is about 2.4kWh out of the bank. Add your other loads and you have your number. We walk through that exact math in the battery-sizing guide.
Where to start
This is the build we put first for a reason. A solar mini-split kit is the simplest system we sell and the one most people get working on their own. A couple of panels, a charge controller, a battery, the mini-split, and one cool room running on sunlight. It is the cleanest way to see the whole trick work before you scale it to the rest of the house.
If you want one for a bedroom, a shop, or a room the central air never reached, the system builder walks you through panels, battery, and the right size unit in a few minutes, and you can change any number on the review screen. Or browse the mini-split kits once you know the square footage you are cooling.