Buy the right gear, not just the popular gear
Short, honest guidance on how to size and choose everything we cover — plus a calculator for each.
Every product on GearSizer is sized before it’s recommended. These guides explain the one spec that actually matters for each category, the rules of thumb we use, and the mistakes to avoid — then hand you a calculator that does the math.
Home climate
What size air purifier do I need?
Air purifiers are rated by CADR (Clean Air Delivery Rate) in CFM. The AHAM “2/3 rule” says a purifier’s smoke CADR should be at least two-thirds of your room’s floor area — so a 300 sq ft room wants ~200 CFM. Taller ceilings need more, since there’s more air to move. Buy at or above the number the calculator gives you, and you’ll get roughly 4–5 air changes per hour.
What size dehumidifier do I need?
Dehumidifiers are sized in pints of water removed per day, based on room size and how damp the space is. A moderately damp 500 sq ft room needs about 10 pints/day; a wet basement of the same size can need 14–16. Add capacity for laundry, bathrooms, or lots of occupants. Note that DOE changed its test method in 2019, so modern pint ratings look lower than older ones for the same real-world performance.
What size space heater do I need?
A common rule is 10 watts of heating per square foot for a room with 8 ft ceilings and average insulation — so 150 sq ft wants about 1,500 watts, which is the most a standard 120 V outlet can supply. Poorly insulated or high-ceilinged rooms need more; well-sealed rooms need less. If the calculator asks for more than 1,500 watts, you’ll want two heaters or a 240 V unit.
What size air conditioner do I need?
Air conditioners are rated in BTUs. Start at roughly 20 BTU per square foot, then adjust: add ~10% for a very sunny room, subtract ~10% for heavy shade, add 600 BTU for each person beyond two, and add 4,000 BTU if it’s a kitchen. Oversizing is a real mistake — an AC that’s too big cools fast but leaves the room clammy because it never runs long enough to pull out humidity.
What size humidifier do I need?
Humidifiers are rated by how much moisture they add per day (gallons/day) and the area they cover. A rough guide is about one gallon per day for every ~350 sq ft in moderately dry air — more in winter or very dry climates. Too small and it can’t keep up; too large and you risk over-humidifying (aim for 30–50% indoor humidity). Evaporative and ultrasonic units run quietest; warm-mist adds a little heat.
What size ceiling fan do I need?
Ceiling fans are sized by blade span (diameter, in inches), matched to a room’s floor area. As a guide: rooms up to ~75 sq ft want a 29–36" fan, ~75–175 sq ft a 42–48" fan, ~175–300 sq ft a 52–56" fan, and larger rooms a 60"+ fan (or two fans). A fan that’s too small barely moves air; hang the blades 8–9 ft off the floor and at least 18" from the walls for the best airflow.
Power & off-grid
What size generator do I need?
Generators have two ratings: running (rated) watts for continuous load, and starting (surge) watts for the extra jolt motors need to spin up. Add the running watts of everything you’ll run at once, then add the single largest starting surge on top — a fridge or well pump can briefly draw 2–3× its running watts. Your generator’s starting-watt rating must clear that peak, or it’ll stall.
What size power station do I need?
Power stations are sized two ways: watt-hours (Wh) of battery capacity, and watts of output. Multiply each device’s wattage by how many hours you’ll run it, add them up, and pad ~20% for conversion losses — that’s your Wh. Then make sure the station’s continuous output rating (in watts) is higher than everything you’d run at the same time. LiFePO4 batteries last far longer than standard lithium if you’ll cycle it often.
What size UPS do I need?
A UPS is sized by the load it carries (watts) and how long it must hold that load (minutes). Add the wattage of everything plugged in — PC, monitor, modem, router — then decide whether you need 5 minutes to save and shut down or 30+ minutes to ride out short outages. More runtime means more battery (watt-hours). Buy a UPS whose watt rating comfortably exceeds your load; running near 100% shortens both runtime and battery life.
How many solar panels do I need?
Solar sizing comes down to the watt-hours (Wh) you use per day and how many “peak sun hours” your area gets. Divide your daily Wh by your sun hours, then add ~30% for real-world losses (panel angle, heat, clouds, charge-controller inefficiency) to get the wattage you need. Example: 600 Wh/day ÷ 4 sun hours ≈ 150 W, so a ~200 W panel is a safe pick. Fixed panels beat folding portable ones on cost-per-watt; match the panel’s voltage to your power station’s solar input.
Three rules that apply to everything
- Match the spec, don’t chase stars. A five-star unit that’s undersized still fails in your room.
- Don’t over-size. Bigger costs more — and for air conditioners it actually works worse, since short cycling leaves the room humid.
- Buy a little headroom, not a lot. Sizing up ~10–20% is smart insurance; beyond that you’re just paying for capacity you’ll never use.