Before you buy backup power,
do the one calculation
the spec sheet won't.
A fridge labeled 150 W doesn't draw 150 W. A 300 W power station can't start it at all. And the watt-hours on the box aren't the watt-hours you get. We do the honest math — duty cycles, startup surges, inverter losses — so you buy once, and buy right.
Three numbers everyone gets wrong
What a “150 W” fridge actually averages. Compressors cycle — they run about a third of every hour. Ignore duty cycle and you'll buy three times the battery you need.
What that same fridge demands for a split second at startup. If the station's surge capacity can't cover the spike, the motor never starts — the number-one reason these units get returned.
What survives the trip from battery to outlet. The BMS holds back ~5% and the inverter takes ~13% converting DC to AC. Nobody prints this on the box; every runtime depends on it.
How this site works
Tell the calculator what you'd actually plug in. It computes your average draw, your peak surge, and the smallest station class that startsand sustains all of it — conservatively. Telling you 20 hours when you have 16 is the one mistake that actually hurts, so we always round against ourselves.