Solar PV System Sizing Basics

In plain English

A solar panel’s rating, such as 550 W, is the power it gives under bright laboratory light. On a real roof it gives less: the sun is not overhead all day, panels get hot and dusty, and some energy is lost in cables and the inverter. Sizing a solar system means finding how many panels are needed to produce the energy your loads use in a day, after all these losses.

For a system without a grid connection (off-grid), batteries store energy for the night and for cloudy days, so they must be sized too.

Technical definition

Panel ratings are given at standard test conditions (STC): 1000 W/m² of sunlight and a cell temperature of 25 °C. The array size is stated in kilowatts-peak (kWp). Peak sun hours (PSH) express a day’s sunshine as the number of hours at 1000 W/m² that would give the same energy; a site with 5 kWh/m² per day on the panels has 5 PSH. The performance ratio (PR) is the ratio of the energy actually delivered to the energy the array would give at its STC rating; it combines temperature, soiling, wiring, mismatch and inverter losses and is typically about 0.75–0.85 for a grid-connected system.

How it works and where it is used

The basic method has four steps:

1. Add up the daily energy used by the loads (kWh/day), from appliance power × hours of use.
2. Find the peak sun hours for the site, for the design month (often the month with the least sunshine for off-grid systems).
3. Estimate the performance ratio by multiplying the efficiencies of each loss: for example 0.90 for heat, 0.97 for dust, 0.98 for wiring, 0.98 for mismatch and 0.96 for the inverter. Off-grid systems also lose energy in the battery.
4. Divide the daily energy by PSH × PR to get the array size, and round up to a whole number of panels.

For batteries, start from the energy the loads take from the battery each day (including inverter losses for AC loads), multiply by the days of autonomy wanted, and divide by the allowed depth of discharge. The inverter must handle the largest power used at one time, plus motor starting surges.

Formula

PR = (1 − L_temp)(1 − L_soil)(1 − L_wire)(1 − L_mis) × η_inv

P_array (kWp) = E_day ÷ (PSH × PR)     Battery energy = E_batt,day × days ÷ DoD

L = loss fractions, η_inv = inverter efficiency, E_day = daily energy at the loads (kWh), PSH = peak sun hours (h/day), E_batt,day = daily energy leaving the battery, DoD = allowed depth of discharge (fraction).

Worked example

Example 1 — grid-connected array. A home uses 10 kWh per day; the site has 5 peak sun hours.

• PR = 0.90 × 0.97 × 0.98 × 0.98 × 0.96 = 0.805
• Array = 10 ÷ (5 × 0.805) = 2.48 kWp
• With 550 W panels: 2485 ÷ 550 = 4.5 → 5 panels (2.75 kWp)
• Expected output = 2.75 × 5 × 0.805 = about 11.1 kWh per day on an average day of that month

Example 2 — off-grid battery bank. AC loads use 4 kWh per day and DC loads 0.5 kWh; inverter efficiency 93%; 2 days of autonomy; 80% depth of discharge; 48 V system with 12 V, 100 Ah batteries.

• Energy from the battery per day = 4 ÷ 0.93 + 0.5 = 4.80 kWh
• Nominal battery energy = 4.80 × 2 ÷ 0.80 = 12.0 kWh → 12 003 Wh ÷ 48 V = 250 Ah
• Layout: 4 batteries in series for 48 V, 3 strings in parallel → 12 batteries (14.4 kWh rated, 11.5 kWh usable)

In the workshop and in practice

These steps give a first design for quotations and feasibility checks. A full design also checks the string voltage against the inverter limits at the coldest temperature, the roof area and orientation, shading, cable sizes and protective devices, and local grid-connection rules. Monitoring the real performance ratio after installation shows whether panels need cleaning or something has failed.

Common mistakes and misconceptions

• Using the panel rating as daily output. A 550 W panel does not give 550 W × 12 h; use peak sun hours and losses.
• Designing off-grid systems for the yearly average. Use the worst month, or the batteries will run flat in winter.
• Ignoring battery losses and depth of discharge. Taking a battery to empty every day shortens its life sharply.
• Sizing the inverter only on average power. Motors, pumps and compressors need several times their running power to start.

Related calculators

• 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.
• Solar Panel Output Estimator — Estimated DC and AC output of solar panels at a given sun and temperature, or their daily energy from peak sun hours.
• Solar Inverter Sizing Calculator — Inverter size in kW and kVA with motor surge and DC current, the DC/AC ratio, and a cold-weather string voltage check.
• Battery Bank Sizing Calculator — Battery capacity (Ah and kWh) and the series/parallel layout for your loads and days of autonomy.

Related topics

• Power and Energy: kW and kWh — Power is how fast energy is used; energy is power multiplied by time.

References

• IEC 61724-1: Photovoltaic system performance — Monitoring (performance ratio)
• NREL PVWatts Calculator documentation (system losses)
• IEC 62548: Photovoltaic (PV) arrays — Design requirements