Solar, lithium, and 400 watts of regret

The spreadsheet said 400 W of panel and 200 Ah of lithium would carry a three-day cloudy stretch with margin. The spreadsheet had never met a 12 V compressor fridge in a metal box in direct sun.

Where the model broke

• Fridge duty cycle: modelled at 35%, measured at 61% on a hot day
• Panel output: 400 W nominal, ~270 W realistic at a flat roof angle
• Inverter idle draw I had simply forgotten to include at all
• Lithium charge tapering above 90%, which the datasheet mentions and I did not read

Every electrical system is fine until the second consecutive cloudy day.

The fix

Another 200 W of panel, better fridge insulation, and a shunt-based battery monitor so I am reading reality instead of a percentage guess. Total cost of the correction: about what I would have spent doing it properly the first time.