TAHA Engineering Toolsby TAHA Knowledge Lab

Energy & Renewables

Solar PV System Sizing Calculator

Solar array size (kWp) and number of panels for a daily energy need, from peak sun hours, losses and a design margin.

Inputs

A solar array of N panels feeding an inverter and loads, with the sun giving peak sun hours.PSH h/dayN panels = P ÷ P(panel)Inverter(and battery)LoadsE kWh/dayP = E ÷ (PSH × PR)PR = product of (1 − loss) terms
The array must deliver the daily energy E. Each kWp of panels gives about PSH kWh a day at standard conditions; the performance ratio PR covers temperature, dust, wiring and inverter losses (and battery losses off-grid). P = E ÷ (PSH × PR). Diagram not to scale.

Energy at the appliances (AC side) per day. The Electricity Cost calculator gives kWh per day for each appliance.

Daily solar energy on the panels in kWh/m², i.e. hours of full 1000 W/m² sun. Use local data (for example PVGIS or NASA POWER) for your tilt and the worst month you design for. 5.0 is only an example.

Extra capacity for uncertainty, ageing or future loads. 0% means no margin.

Result

Required array size (with margin)
2.485kWp
Required array size without margin
2.485kWp
Number of panels
5panels
Installed array size
2.75kWp
Estimated AC energy per day
11.0673
Estimated AC energy per month (average)
336.629kWh
Estimated AC energy per year
4,039.55kWh
Performance ratio used
0.8049
Working
PR = 0.80489 P = E × (1 + margin) ÷ (PSH × PR) = 10 × 1 ÷ (5 × 0.80489) = 2.4848 kWp Panels = ⌈2,484.8 W ÷ 550 W⌉ = 5 → 2.75 kWp Estimated AC energy = 2.75 × 5 × 0.80489 = 11.067 kWh/day

About 2.485 kWp of panels is needed: 5 panels of the rating entered (2.75 kWp), giving an estimated 11.0673 kWh per day.

  • Estimated output for the sun hours and losses entered, not a guaranteed generation figure. Real output changes with weather, orientation, shading and ageing.

Educational estimate — check the assumptions below.

About this calculator

This calculator estimates the size of a solar array for a given daily energy use. It divides the daily energy by the peak sun hours — the daily solar energy on the panels expressed as hours of full 1000 W/m² sun — and by the performance ratio, which accounts for temperature, dust, wiring, mismatch, the inverter and, for an off-grid system, the battery.

The results are an estimate for the values you enter, not guaranteed generation. For a site-specific yield, use an hourly simulation tool such as PVGIS or NREL PVWatts.

Formula

PR = (1 − Ltemp)(1 − Lsoil)(1 − Lwire)(1 − Lmis)(1 − Lother) × ηinv [× ηbatt off-grid]

Parray = Eday × (1 + m) ÷ (PSH × PR)    N = ⌈Parray ÷ Ppanel⌉    Eest = N × Ppanel × PSH × PR

Eday = daily energy at the loads (kWh), PSH = peak sun hours (h/day), m = design margin, Ppanel = panel rating at standard test conditions (STC). For comparison, NREL’s PVWatts uses a default total system loss of 14% plus a 96% nominal inverter efficiency.

Worked example

10 kWh/day, 5 peak sun hours, losses 10% temperature, 3% soiling, 2% wiring, 2% mismatch, inverter 96% (grid-tied), margin 10%, 550 W panels:

  • PR = 0.90 × 0.97 × 0.98 × 0.98 × 0.96 = 0.8049
  • P = 10 × 1.10 ÷ (5 × 0.8049) = 2.733 kWp (2.485 kWp without the margin)
  • ⌈2733 ÷ 550⌉ = 5 panels = 2.75 kWp → about 11.07 kWh/day estimated.

Assumptions and limitations

  • Estimated output only, not guaranteed: weather, shading, orientation, dust and ageing change real generation.
  • Peak sun hours must be for the panel tilt and orientation and for the month you design for. No location default is built in.
  • Losses are multiplied together. Shading, inter-row spacing, snow and degradation are not modelled unless you include them in “other losses”.
  • For an off-grid system the battery must be sized separately (Battery Bank Sizing), and the array must also recharge it after cloudy days.

References

  • Dobos, A. P. (2014), PVWatts Version 5 Manual, NREL/TP-6A20-62641 (default system losses 14%, nominal inverter efficiency 96%).
  • IEC 61724-1 — Photovoltaic system performance monitoring (definition of performance ratio).