Solar Panel Calculator sizes a PV system from your daily kWh load, peak sun hours, panel wattage, and battery autonomy with formulas, examples, FAQs, and references.

Solar Panel Calculator

What this calculator does

The Solar Panel Calculator estimates the required solar array size, number of panels, and battery bank capacity for an off-grid or hybrid solar power system based on your daily energy consumption.

Inputs explained

  • Daily energy consumption: The total energy your appliances use per day in kWh.
  • Peak sun hours: The equivalent hours of full direct sunlight your location receives per day.
  • Panel wattage: The rated power of a single solar panel you plan to use.
  • System efficiency / loss factor: Accounts for losses in wiring, dust, inverter, and temperature.
  • System voltage: The DC voltage of your battery bank (12V, 24V, or 48V).
  • Days of autonomy: How many days the battery must power the load without sun.
  • Depth of discharge (DoD): Maximum safe battery discharge (50% for lead-acid, 80% for lithium).
  • Battery type: Lead-acid or Li-ion.

How it works / Method

The system calculates total wattage required by dividing daily energy by peak sun hours and efficiency. It then determines battery capacity by multiplying daily Ah by days of autonomy and dividing by depth of discharge limits to protect battery health.

Formulas used

  • Required Array (W) = (Daily kWh × 1000) ÷ (Peak sun hours × Loss factor)
  • Number of panels = ceil(Required W ÷ Panel W)
  • Daily Ah = (Daily kWh × 1000) ÷ System V
  • Battery bank Ah = (Daily Ah × Days autonomy) ÷ (DoD/100)
  • Battery kWh = Bank Ah × System V ÷ 1000
  • Roof area = Number of panels × 2.0 m²
  • CO2 saved (kg) = Annual kWh × 0.82

Units: Energy in kWh, Power in W, Capacity in Ah, Area in m².

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Calculator Tool

Enter values to compute

kWh/day
h/day
W
%
Days
%
Results
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Formulas

Array W(kWh/day × 1000) / (Sun × Eff)
Battery Ah(Daily Ah × Days) / DoD
PanelsArray W / Panel W

Quick Reference: Peak Sun Hours (India)

RegionAvg h/day
Rajasthan5.5 - 6.0
Chennai5.2
Delhi5.0
Bangalore5.0
Mumbai4.8
Kolkata4.5

Efficiency Factor: Inverter 95% × Wiring 98% × Dirt 95% × Temp 90% ≈ 80% System Eff.

Related Calculators

Battery Backup Energy Consumption kW to Amps

Step-by-step example

Scenario: You need 5 kWh/day using 540W panels, 5 sun hours, 80% efficiency, 24V system, 1 day autonomy, 50% DoD lead-acid battery.

Formula: Array W = (Daily kWh × 1000) ÷ (Peak sun hours × Loss factor)

  1. Array W = (5 × 1000) / (5 × 0.8) = 5000 / 4 = 1,250 W.
  2. Number of panels = 1,250 W / 540 W = 2.31 → round up to 3 panels.
  3. Daily Ah at 24V = 5000 Wh / 24V = 208.3 Ah.
  4. Battery Bank Ah = (208.3 × 1) / 0.50 = 416.6 Ah.

Result: Array: ~1250W (3 x 540W). Battery: 416 Ah @ 24V.

Use cases

  • Sizing off-grid residential homes and cabins.
  • Planning solar backup for rural telecom towers.
  • Sizing agricultural solar water pumping systems.
  • Rooftop residential grid-tie estimates.
  • RV, camper, and boat power generation planning.

Assumptions & limitations

  • Peak sun hours represent an annual average; winter months will produce less power.
  • Calculations assume flat, unshaded roof space oriented optimally toward the sun.
  • Inverter surging (motor starts) is not calculated here; ensure your inverter handles peak kW loads.
  • Grid-tied systems do not require battery sizing and focus strictly on array wattage.
  • Always consult local electrical codes and a licensed installer before purchasing equipment.

Sources & references

Related calculators

Frequently Asked Questions

First find the system size. For 30 kWh per day in a sunny region with about 5 sun-hours, system size = 30 ÷ 5 = 6 kW DC. With derating for temperature, dirt, and inverter losses, target around 7 kW DC. If you pick 400 W panels, 7000 ÷ 400 = 18 panels. Rooftop space, shading, and tilt also matter. We always survey the roof azimuth and any nearby water tank shadows before locking the count.

Add up daily kWh from your bill, divide by sun-hours per day to get kW, then add roughly 25–30% margin for losses and future load growth. Example: a household uses 20 kWh per day in a 5 sun-hour area, so 20 ÷ 5 = 4 kW base, and after derating you want about 5–5.5 kW DC. Don't forget to include AC and water pump load if those are coming on solar later.

Battery sizing depends on backup hours and depth of discharge (DoD). Battery kWh = (daily load × backup days) ÷ DoD. For a 5 kWh per day load with one day backup at 80% DoD on lithium, you need 5 ÷ 0.8 = 6.25 kWh of usable battery. For lead-acid keep DoD at 50%, so the bank doubles. Always include inverter efficiency too, around 90 to 95%, otherwise the bank looks smaller than it really is.

Take daily kWh and divide by your local sun-hours to get the kW system rating. Multiply that kW by 1000 to get watts. Example: 15 kWh per day in a 5 sun-hour location = 3 kW = 3000 W of panels. Then add 20–30% headroom for soiling, temperature, and inverter losses. So order around 3.6 to 3.9 kW of panels. Sun-hours change by region, so check your district's data, not just a national average.

We typically apply a derating factor of 0.75 to 0.80 for residential rooftops in India. That covers panel temperature loss (around 10%), dust and soiling (5%), wiring and inverter loss (5–7%), and aging (around 0.5% per year). Example: 5 kW DC × 0.78 ≈ 3.9 kW of usable AC output at peak sun. Without derating, juniors often promise customers more energy than the system actually delivers, which leads to complaints in month two.

Divide the system size by the panel wattage. 5,000 W ÷ 400 W = 12.5, so round up to 13 panels. But also check string voltage and inverter MPPT range — 13 panels in a single string at around 40 V each gives 520 V open circuit, which fits most 5 kW inverters. If your inverter limits are different, split into two strings. Mechanical layout often forces 12 or 14 panels, so plan with the structural team early.

Yes, a good calculator estimates backup days based on battery bank capacity, daily load, and DoD. Off-grid backup days = (battery kWh × DoD) ÷ daily load kWh. For example, a 20 kWh lithium bank at 80% DoD covers a 4 kWh per day load for 4 days. Add a sunny-day recharge factor for realistic planning. We always recommend a small generator backup for monsoon weeks because no battery sizing alone covers a 7-day cloudy spell.