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Sizing a Small Solar System
Work backwards from your loads, never forwards from a panel you happened to buy.
Step one: measure the load
Add up watt-hours per day for everything you intend to run. A device drawing 20 watts for 6 hours is 120 watt-hours. Measure real devices with a meter; nameplate ratings are near-useless.
Step two: size the battery
Divide daily watt-hours by battery voltage to get amp-hours. Then divide by the usable depth of discharge — 50% for lead-acid, 80% for LiFePO4. Then multiply by the number of days of autonomy you want to survive without sun.
Step three: size the panels
Divide daily watt-hours by your location's peak sun hours — typically 3 to 5, not the number of daylight hours. Then add 25% for charge controller inefficiency, wiring loss, panel soiling, and the fact that panels never hit their rated output in the real world.
Charge controllers
An MPPT controller extracts roughly 20-30% more energy than a PWM controller from the same panel, and the gap widens in cold weather and low light. PWM is only sensible on very small systems where the controller cost dominates.