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 - Panel Size, Daily kWh and Battery Estimate

Agarapu Ramesh — Editor and content reviewer
Editorially reviewed Reviewed by Agarapu Ramesh, science educator (chemistry). LinkedIn
Last reviewed: May 2026 | Standard electrical engineering formulas

Solar sizing starts with daily energy, not panel count. Take the kWh you need per day, divide by peak sun hours, then add losses. If you skip the loss factor, the system looks cheaper on paper and weaker on the roof.

Formula at a glance

  • array W = daily Wh / peak sun hours / derate factor
  • daily Wh = load W x hours
  • battery Wh = daily Wh x backup days / usable DoD

Field note: A 400 W panel rarely gives 400 W all afternoon. Heat and angle take their cut. Plan for that and the system behaves.

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kWh/day
h/day
W
%
Days
%
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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.

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Battery Backup Energy Consumption kW to Amps

How to use the Solar Panel Calculator

Use this as a first solar sizing pass, then adjust for local sun hours, roof angle, heat, dust and inverter losses. Panels do not produce their sticker wattage all afternoon.

Worked example

Example: 5 kWh per day with 5 sun hours and 75% system derate needs about 1,333 W of panels before rounding up.

Practical checks before you trust the number

  • Use local peak sun hours, not a national average.
  • Inverters, dust, heat and cable losses eat production.
  • Battery sizing needs usable Wh, not nameplate Wh alone.

Common mistake

A 400 W panel rarely gives 400 W all afternoon. Heat and angle take their cut. Plan for that and the system behaves.

Sources and 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.