Off-grid from zero · lesson 4 of 12

Loads before panels

Build an honest list of what runs on electricity, in watt-hours per day, because every other power number comes from it.

The most common solar mistake is buying the system first and discovering the loads second. A solar system is sized from the bottom up: what runs, for how long, on the worst day you intend to live through. Everything else (panels, battery, inverter, generator) is arithmetic on that list.

Be honest about the loads that matter most: refrigeration, a well pump, anything that heats with electricity. The nameplate reader will pull the real watts off a label; the solar calculator turns the list into a system. Do not skip the hours-per-day column; a fridge draws 100 W but runs 10 hours, while a "1,000 W" microwave actually pulls about 1,500 W from the rating plate and runs 20 minutes.

Numbers to carry
  • A lean cabin runs 2,000 to 4,000 Wh a day; add a well pump and a freezer and it is 4,000 to 8,000.
  • Worst-month sun in the north is 2.5 to 3 hours; sizing on the annual average under-builds the array by a third.
  • System losses are about 25 percent between the panel rating and the battery.
Read
Use
Do before the next lesson
  • Walk the house (or the list of what you will own) with the nameplate reader and build the load list.
  • In the solar calculator, enter your ZIP so the worst-month sun hours are real, not a guess.
  • Write down the single biggest surge load and the largest set of loads that can run at once; together they size the inverter and generator later.
← Lesson 3 Lesson 5: The battery, the inverter, and the voltage →
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