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Off-Grid Solar PV System Sizing Calculator

Size an off-grid photovoltaic system from your daily energy use. The calculator gives the array power in kWp and the number of panels, the battery bank in Ah for the days of autonomy you want, and an estimate of the charge-controller current.

For estimation and education. Not a substitute for a licensed engineer, the applicable codes (IEC 60364, NEC/NFPA 70, BS 7671, local regulations) or manufacturer data. Read the disclaimer.

Energy and sun

Wh/day

Sum of watts × hours for each appliance

h/day

kWh/m²/day for your site and tilt

Typical 0.75–0.80

Panels

W
A

Datasheet; leave empty to skip

Battery bank

days

0 = no battery

Lead-acid 0.5, LiFePO4 0.8

V

Battery discharge × inverter

Result

Solar PV sizing — Engineerly. Estimate for education only; verify against applicable codes and manufacturer data.

Array size needed
1.333kWp
Panels
4× 400 W
Battery bank
463Ah @ 48 V
Controller PV current
26.25A
Installed array
1.6kWp
Strings
2 × 2 panels
Expected daily yield
6,000Wh/day
Battery nominal energy
22.22kWh
MPPT output current (approx.)
33.33A

Working, with your numbers

  1. 1. Array power needed
    P_array = E / (PSH × derate)
    P = 5,000 Wh / (5 h × 0.75)
    = 1,333.3 Wp
  2. 2. Number of panels (round up)
    N = ⌈P_array / P_panel⌉
    N = ⌈1,333.33 / 400 W⌉
    = 4 panels
  3. 3. Battery capacity
    C = E × days / (V × DoD × η)
    C = 5,000 Wh × 2 / (48 V × 0.5 × 0.9)
    = 462.96 Ah
  4. 4. Controller PV input current
    I = 1.25 × Isc × strings
    I = 1.25 × 10.5 A × 2
    = 26.25 A
  5. 5. MPPT output (charging) current
    I_out ≈ P_array / V_batt
    I = 1,600 W / 48 V
    = 33.33 A

Assumptions

  • Peak sun hours = daily irradiation in kWh/m² on the panel plane (equivalent hours at 1000 W/m²).
  • Overall derate 0.75 covers temperature, soiling, mismatch, wiring, controller and battery charging losses.
  • Panel rating is at STC (1000 W/m², 25 °C cell temperature).
  • Battery sized for 2 day(s) with no sun, 50 % usable DoD and 90 % battery-to-load efficiency.
  • Controller PV current uses the 1.25 × Isc factor applied to PV source circuits in NEC 690.8.
Daily energy: 5000 Wh/day
Peak sun hours: 5 h
Derate: 0.75
Panel: 400 W, Isc 10.5 A, 2 per string
Autonomy: 2 days
Battery: 48 V, DoD 0.5, η 0.9

Check before you rely on this

  • Use the peak sun hours of the worst month you need to cover (often winter), not the annual average, for an off-grid system.
  • String voltage must stay within the controller/inverter limits at the coldest expected temperature (Voc rises in the cold). This tool does not check string voltage.
  • Conductors and overcurrent protection on the PV side follow NEC Article 690 / IEC 60364-7-712 and the equipment instructions.
  • Estimation and education only. Not a substitute for a licensed engineer, the applicable code (IEC 60364, NEC/NFPA 70, BS 7671 or local regulations) or manufacturer data.

How to use this calculator

  1. Add up daily energy: for each appliance, watts × hours per day, then sum (Wh/day).
  2. Enter peak sun hours for your site, tilt and worst design month. This equals the daily irradiation in kWh/m² on the panel plane.
  3. Keep the default derate of 0.75 unless you have a loss budget. 0.75–0.80 is typical for off-grid systems.
  4. Enter the panel rating, Isc and panels per string, then the battery autonomy days, DoD, bank voltage and efficiency.

How it works: the formula

P_array = E_day / (PSH × derate) [Wp] N_panels = ⌈P_array / P_panel⌉ (rounded up to whole strings) C_batt = E_day × days / (V_batt × DoD × η) [Ah] I_controller,PV ≥ 1.25 × Isc × strings I_MPPT,out ≈ P_array / V_batt

A peak sun hour is one hour of irradiance at 1000 W/m², the condition under which panels are rated (STC). If your site receives 5 kWh/m² per day, a 1 kWp array produces about 5 kWh before losses. The derate factor accounts for higher cell temperature, dust, mismatch, wiring, charge control and battery round-trip losses.

Variables and units

SymbolMeaningUnit
E_dayDaily energy demand at the loadWh/day
PSHPeak sun hours (kWh/m²/day on the array plane)h
derateOverall system performance factor—
P_panelPanel rating at STCW
DoD, ηUsable depth of discharge; battery-to-load efficiency—
IscPanel short-circuit current at STCA

Worked example

5000 Wh/day, 5 peak sun hours, derate 0.75, 400 W panels (Isc 10.5 A) two per string, 2 days autonomy on a 48 V bank at 50 % DoD and 90 % efficiency:

P_array = 5000 / (5 × 0.75) = 1333 Wp → 4 panels × 400 W = 1.6 kWp C_batt = 5000 × 2 / (48 × 0.5 × 0.9) = 463 Ah at 48 V I_PV = 1.25 × 10.5 × 2 strings = 26.3 A I_MPPT,out ≈ 1600 / 48 = 33.3 A

Assumptions

  • Energy balance on a daily basis for the design month.
  • The derate covers all system losses. Battery efficiency here applies to energy drawn during autonomy days.
  • Panels are rated at STC.

Limitations

  • String voltage must stay within controller and inverter limits at the coldest temperature, because Voc rises as temperature falls. This is not checked here.
  • Shading, orientation and seasonal variation need a proper irradiation dataset or simulation (for example PVGIS or NREL's PVWatts).
  • Battery Ah does not include ageing, temperature or the extra capacity needed to recover after cloudy spells.
  • Wiring and protection follow NEC Article 690 or IEC 60364-7-712 and the equipment instructions.

References

Standards and texts that cover this topic. Use the edition adopted in your jurisdiction.

  • IEC 60364-7-712, Low-voltage electrical installations — Requirements for special installations — Solar photovoltaic (PV) power supply systems.
  • NFPA 70, NEC Article 690 (Solar Photovoltaic Systems), including 690.8 on the 125 % factor for PV source circuit current.
  • IEEE Std 1562, Guide for Array and Battery Sizing in Stand-Alone Photovoltaic (PV) Systems.
  • IEC 61215 / IEC 61730 — PV module design qualification and safety; STC defined in IEC 60904-3 reference spectrum.

Frequently asked questions

Where do I get peak sun hours?

From an irradiation database such as the European Commission's PVGIS or NREL's tools. Use monthly averages on your panel tilt. For off-grid design, take the worst month you need the system to cover.

Why 1.25 × Isc for the controller?

On bright, cold days panels can produce more current than at STC, and electrical codes treat PV current as continuous. NEC 690.8 sets maximum PV circuit current at 125 % of the summed Isc, and controllers are commonly chosen on the same basis. Always check the controller's own input limits.

Is a 0.75 derate too pessimistic?

For off-grid systems with batteries it is realistic. Temperature losses alone are often 8–15 % in warm climates, and battery round-trip losses add more. Grid-tied systems without batteries often reach 0.80–0.85.

How many days of autonomy do I need?

Two to three days is common for homes. Critical loads in cloudy climates may need five or more, often with a generator as backup. More autonomy means a larger battery and a larger array to recharge it.

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How these formulas were checked: methodology and testing.