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Energy, electricity and mechanics

Off-grid photovoltaic solar installation

Panel power and battery capacity needed based on daily consumption and days of autonomy.

Wh/día

Sum of the energy of all appliances on an average day.

h

Peak sun hours (PSH) of the worst month at the location; equal to the daily irradiation in kWh/m² on the panel.

%

Combined losses from wiring, charge controller, battery and inverter. Typical: 75-85 %.

Days without sun that the battery bank must cover without recharging.

%

Lead-acid: 50 %. Lithium (LiFePO₄): 80 %.

Result

Panel power
— Wp
Battery capacity
— Ah
Stored energy
— Wh
Approximate charge controller current
— A

How it is calculated

Daily energy adjusted for losses: E = daily consumption ÷ efficiency. Panel power: Wp = E ÷ peak sun hours.

Energy the bank must be able to store: E_bat = E × days of autonomy ÷ depth of discharge. Capacity in amp-hours: Ah = E_bat ÷ bank voltage.

Approximate short-circuit current of the panels for sizing the charge controller: I ≈ (Wp ÷ bank voltage) × 1.25, with the usual 25 % margin from installation guides.

Keep in mind

  • Peak sun hours vary a lot with the season: always use the figure for the worst month so you are not short in winter.
  • The real depth of discharge shortens or extends battery life: discharging beyond the allowable value greatly reduces its cycle count.
  • The DC wiring between panels, charge controller and batteries must be sized separately (see "Voltage drop in a cable") because it works with high currents and low voltages.

See also

More tools in “Energy, electricity and mechanics”