OneCalcApp LogoOneCalcAppConvert. Calculate. Simplify.
All solar calculators
☀️

Solar PV System Size

Energy-based PV sizing from daily, monthly or annual consumption: required kWp, module count, installed DC capacity, inverter AC capacity and DC/AC ratio, performance ratio loss chain and expected daily, monthly and annual generation.

Design

Inputs (SI units)

Quick mode is for stall/customer enquiries. Detailed mode keeps the existing engineering calculator unchanged.

Choose only the information available from the customer.

Enter current-consumption bill amount only. Arrears/deposit/penalty amounts can make bill-to-units reverse estimation inaccurate.

100% targets annual energy replacement. Use a lower value when roof area or sanctioned load limits the plant.

Tamil Nadu quick-sizing default. Replace with a site-specific PVsyst/irradiation value when available.

Results

Recommended solar system
2 kW
Raw energy requirement 1.96 kWp, rounded to a practical 0.5 kW step
Installed DC with selected panels
2.2 kWp
4 × 550 Wp panels
Estimated consumption
250 units/month
3,000 units/year
Expected solar generation
281 units/month
9.2 units/day · 3,373 units/year
Target offset
100 %
Tamil Nadu domestic bill before solar
₹ 1,805 / 2 months
Energy-charge estimate for 500 units
Estimated bill after solar
₹ 0 / 2 months
Approx. 0 net import units after estimated solar generation
Estimated bill saving
₹ 1,805 / 2 months
Input basis
₹2,000 bi-monthly domestic bill
Estimate confidence
Medium
Accuracy improves if actual units and operating hours are available.
✓ PASS — Tamil Nadu quick mode is an enquiry/sales sizing tool. The existing detailed engineering mode remains available on the same page.
⚠ WARNING — Bill-based reverse sizing is less exact than using actual units because arrears, deposits, penalties and other bill components cannot be reconstructed from the rupee amount alone.
⚠ WARNING — HP-only sizing depends strongly on actual running hours, motor loading and efficiency. Use the motor nameplate and operating schedule whenever possible.

Solar PV decision support

PV design decision summary

This section organises the existing calculator output into design checks, governing criteria and comparison cases. Core solar formulas and the calculator's original results are unchanged.

Solar PV sizing & array decision support v3 · 2026.08

Calculated — review warnings

2 checks require review. The numerical result was calculated, but the warning conditions should be resolved or accepted before use.

1 pass2 review

Governing criterion

Energy demand ÷ site yield

2 kW

Daily energy demand, peak sun hours, performance ratio and system losses determine the required PV DC capacity; whole-module rounding then sets installed capacity.

Design checks

Solar engineering check 1

pass

✓ PASS — Tamil Nadu quick mode is an enquiry/sales sizing tool. The existing detailed engineering mode remains available on the same page.

Structured directly from this solar calculator's own runtime alert.

Solar engineering check 2

review

⚠ WARNING — Bill-based reverse sizing is less exact than using actual units because arrears, deposits, penalties and other bill components cannot be reconstructed from the rupee amount alone.

Structured directly from this solar calculator's own runtime alert.

Solar engineering check 3

review

⚠ WARNING — HP-only sizing depends strongly on actual running hours, motor loading and efficiency. Use the motor nameplate and operating schedule whenever possible.

Structured directly from this solar calculator's own runtime alert.

Next design actions

Engine recommendation 1

For the closest match, prefer actual bi-monthly units from the customer's TNPDCL/TNEB bill over the rupee amount.

Engine recommendation 2

If the customer only knows motor HP, ask running hours/day and operating days/month; otherwise use the displayed 4–8 h/day range rather than presenting one number as exact.

Engine recommendation 3

Replace the 4.2 kWh/kWp/day quick-yield assumption with site-specific PVsyst/irradiation data before issuing a final proposal.

Engine recommendation 4

Final PV capacity must also be checked against roof/land area, sanctioned load, phase, inverter limits, structural feasibility and TNPDCL interconnection rules.

Primary design output

2 kW

Raw energy requirement 1.96 kWp, rounded to a practical 0.5 kW step

Next electrical verification

Run String Sizing + cable/protection checks

Energy sizing selects kWp and preliminary inverter AC capacity; it does not prove the final Voc/Vmp/MPPT, current, cable, protection or earthing design.

Scenario comparison

Lower and higher cases are recalculated by the same solar calculator engine. They are comparison cases, not weather forecasts or guaranteed production values.

Lower daily energy demand

daily energy demand = 16

Recommended solar system
2 kW
Installed DC with selected panels
2.2 kWp
Estimated consumption
250 units/month
Expected solar generation
281 units/month

Current inputs

daily energy demand = 20

Current design case
Recommended solar system
2 kW
Installed DC with selected panels
2.2 kWp
Estimated consumption
250 units/month
Expected solar generation
281 units/month

Higher daily energy demand

daily energy demand = 24

Recommended solar system
2 kW
Installed DC with selected panels
2.2 kWp
Estimated consumption
250 units/month
Expected solar generation
281 units/month

Decision sensitivity

daily energy demand · neutral

For the same calculator engine, the lower case changes Recommended solar system by +0.0% and the higher case by +0.0%.

This is a deterministic input sensitivity check, not a statistical uncertainty or weather forecast.

Methodology & limit

Decision support is structured from the existing solar calculator engine and the current user inputs. PASS/FAIL is shown only where the engine or entered project criteria support an explicit check. Final PV design still requires site survey, exact module/inverter datasheets, structural/electrical design and applicable statutory approval.

Sensitivity Analysis

Effect of varying Peak sun hours / equivalent full sun hours by ±30% on Recommended solar system.

Input changeRecommended solar systemImpact
-30%2 kW0.0%
-20%2 kW0.0%
-10%2 kW0.0%
+0%2 kW0.0%
+10%2 kW0.0%
+20%2 kW0.0%
+30%2 kW0.0%

Engineering Recommendations

  • →For the closest match, prefer actual bi-monthly units from the customer's TNPDCL/TNEB bill over the rupee amount.
  • →If the customer only knows motor HP, ask running hours/day and operating days/month; otherwise use the displayed 4–8 h/day range rather than presenting one number as exact.
  • →Replace the 4.2 kWh/kWp/day quick-yield assumption with site-specific PVsyst/irradiation data before issuing a final proposal.
  • →Final PV capacity must also be checked against roof/land area, sanctioned load, phase, inverter limits, structural feasibility and TNPDCL interconnection rules.
  • →Engineering-grade preliminary calculation. Final design must be verified against project-specific site conditions, manufacturer datasheets, applicable standards and utility requirements.

Detailed Calculation Log

Quick basis: ₹2,000 bi-monthly domestic bill.
Bill-to-units conversion uses Tamil Nadu domestic LT I-A current-consumption energy slabs and excludes arrears, deposits, penalties and other one-off charges.
Consumption normalised to 250 kWh/month and 8.22 kWh/day.
Solar yield assumption 4.2 kWh/kWp/day; raw requirement 1.96 kWp; practical recommendation 2 kW.
Panel selection 4 × 550 Wp = 2.2 kWp installed.
Domestic current-consumption estimate: 500 units → ₹1,805; after estimated solar net import 0 units → ₹0.

Save & Load Project

Designs are stored privately in this browser — nothing is uploaded.

Engineering Formula

  • Quick bill mode: bill → Tamil Nadu domestic slab reverse calculation → units → daily energy → PV kWp
  • Quick unit mode: entered units → daily energy → PV kWp
  • Quick HP mode: kW_input = HP × 0.746 × load factor / motor efficiency; kWh = kW × hours × days
  • Quick kW mode: kWh = kW × hours × operating days
  • PV raw kWp = target daily solar energy / expected net daily yield
  • Practical recommendation = round raw kWp up to the next 0.5 kW
  • Estimated post-solar bill = Tamil Nadu domestic slab bill on remaining net imported units

This is a PVsyst-style sizing chain. The performance ratio is built multiplicatively from the individual loss mechanisms defined in IEC 61724-1 rather than assumed, so thermal derating, soiling, mismatch, shading, DC/AC wiring, inverter and transformer conversion, LID/IAM/spectral effects and plant availability each show up in the loss diagram. Required DC capacity is the daily energy demand divided by the energy one kilowatt-peak actually delivers per day (PSH × PR), multiplied by a system factor that accounts for battery round-trip losses on hybrid and off-grid plants. Module count, physical area, AC inverter rating from the DC/AC ratio, first-year and lifetime generation with linear degradation, specific yield, CUF, CO₂ abatement and a simple financial summary all follow from that capacity.

Step-by-step Calculation

  1. 1.Convert customer input to monthly energyReverse Tamil Nadu domestic slab bill → units250 kWh/month
  2. 2.Average daily energyMonthly units × 12 / 3658.22 kWh/day
  3. 3.Solar energy targetDaily energy × target offset8.22 kWh/day
  4. 4.Raw PV capacityTarget daily energy / net daily yield1.96 kWp
  5. 5.Practical recommended sizeRound up to next 0.5 kW2 kW
  6. 6.Panel countceil(recommended kW × 1000 / panel W)4 panels
  7. 7.Installed DCPanel count × panel W / 10002.2 kWp
  8. 8.Expected generationInstalled kWp × net yield9.2 kWh/day
  9. 9.Post-solar domestic billRecalculate TN slab bill after estimated net imported units₹ 0 / 2 months

How to use this calculator: Solar PV System Size

Energy-based PV sizing from daily, monthly or annual consumption: required kWp, module count, installed DC capacity, inverter AC capacity and DC/AC ratio, performance ratio loss chain and expected daily, monthly and annual generation. The example below is calculated by this page's real engine from the displayed inputs.

  1. 1Use daily kWh from a meter, or divide a representative monthly bill's electricity units by the billing days. For example, 900 kWh over 30 days is 30 kWh/day. Do not enter the bill amount as energy.
  2. 2Select grid-connected, off-grid or hybrid. Battery systems need extra PV energy because charging and discharging are not loss-free.
  3. 3Enter plane-of-array peak sun hours for the location and proposed roof orientation. Replace the default with site data when available.
  4. 4Check temperature, soiling, shading, cabling, inverter and availability assumptions. The calculator multiplies these losses once to derive performance ratio.
  5. 5Confirm the whole-module count, installed kWp, shade-free roof area, DC/AC ratio and recommended inverter capacity all fit the property.
  6. 6A qualified installer must confirm structural capacity, shadows, string voltage, cable size, protection, earthing and DISCOM requirements.

Input guide and example values

Use values from the same measurement basis and time period. Conditional fields appear only when the related option is selected.

InputExample valueWhy it matters
Calculator modeQuick Tamil Nadu solar sizingQuick mode is for stall/customer enquiries. Detailed mode keeps the existing engineering calculator unchanged.
What does the customer know?Electricity bill amount (₹)Choose only the information available from the customer.
Bill / units periodTNEB bi-monthly cycle (recommended)Select the option that matches the real installation or scenario.
Electricity bill amount2000 ₹Enter current-consumption bill amount only. Arrears/deposit/penalty amounts can make bill-to-units reverse estimation inaccurate.
Target electricity offset100 %100% targets annual energy replacement. Use a lower value when roof area or sanctioned load limits the plant.
Expected net solar yield4.2 kWh/kWp/dayTamil Nadu quick-sizing default. Replace with a site-specific PVsyst/irradiation value when available.
Panel wattage550 WpMeasured or known panel wattage used by the calculation engine.

Formula inputs & variables for Solar PV System Size

These are the named quantities used by this calculator. When the source formula does not define a mathematical symbol, OneCalcApp keeps the real input label instead of inventing one.

Variable / inputUnitMeaning in this calculation
Calculator mode—Quick mode is for stall/customer enquiries. Detailed mode keeps the existing engineering calculator unchanged.
What does the customer know?—Choose only the information available from the customer.
Bill / units period—Select the option that matches the real installation or scenario.
Electricity bill amount₹Enter current-consumption bill amount only. Arrears/deposit/penalty amounts can make bill-to-units reverse estimation inaccurate.
Consumed unitskWhMeasured or known consumed units used by the calculation engine.
Total motor HPHPIf there are multiple motors, enter the total HP that normally runs together.
Motor running hours per dayh/dayIf the customer does not know, 6 h/day is used as a visible assumption.
Motor operating days per monthdays/monthMeasured or known motor operating days per month used by the calculation engine.

Understanding the result

Read the main result together with supporting checks, assumptions, limits and intermediate values.

For a manual check, repeat the first equation, confirm the units and change one input at a time.

Common mistakes when using Solar PV System Size

  • Do not mix units for Electricity bill amount (₹), Target electricity offset (%), Expected net solar yield (kWh/kWp/day). A unit mismatch changes the input magnitude even when the typed number looks reasonable.
  • Do not leave Calculator mode on the default choice unless that choice matches the real scenario; the selected option can change the calculation path or factor.
  • Do not replace the displayed Quick bill mode: bill → Tamil Nadu domestic slab reverse calculation → units → daily energy → PV kWp relationship with a different convention without also changing the underlying assumptions; compare like-for-like methods when checking the result.
  • Do not treat Recommended solar system = 2 kW from the worked example as a universal answer. It belongs to the displayed example inputs and must be recalculated for the actual case.

Next logical calculator

Continue with 100 & 200 Amp Service Wire Size Calculator

100 & 200 Amp Service Wire Size Calculator covers the same practical workflow from a related calculation angle, making it a useful cross-check after Solar PV System Size.

Open 100 & 200 Amp Service Wire Size Calculator

Solar PV System Size — Technical Guide

How many solar panels do I need?

First convert the electricity you want solar to supply into daily kWh. Divide that demand by peak sun hours × performance ratio to get the required DC capacity, then divide the capacity in watts by the selected module wattage and round up. The answer therefore changes with energy use, location, shade, temperature, system losses and panel wattage; house size alone is not enough.

Worked example: a home using 1,000 kWh per month

1,000 kWh over 30 days is about 33.3 kWh/day. At 5 peak sun hours and a 78% performance ratio, the preliminary array is 33.3 ÷ (5 × 0.78) = 8.55 kWp. That is 22 × 400 W panels or 16 × 550 W panels after rounding up, before a site survey confirms shade, roof structure, setbacks and local utility limits.

100 W, 400 W and 550 W panel-count comparison

For the same 5 kWp target, the arithmetic count is 50 × 100 W, 13 × 400 W or 10 × 550 W. Rounding means the installed capacities become 5.00, 5.20 and 5.50 kWp. Higher-wattage modules reduce module quantity, but the final choice must also fit roof dimensions, string voltage, MPPT current and product availability.

Why location, roof area and inverter size change the answer

Lower solar resource or higher system loss requires more DC capacity for the same annual electricity demand. The calculator also checks whole-module installed kWp, estimated module area and inverter AC size from the selected DC/AC ratio. Treat the result as preliminary sizing and verify orientation, shade, structure, protection, earthing and interconnection before purchase.

Engineering Explanation

What is Peak Sun Hours (PSH)?+

PSH is the number of hours per day for which irradiance would have to equal 1 kW/m² to deliver the same daily energy the plane of array actually receives. A site with 5.2 kWh/m²/day has 5.2 peak sun hours — it is not the number of daylight hours.

How is the performance ratio derived here?+

In derived mode PR is the product of (1 − loss) for temperature, soiling, mismatch, shading, cabling, inverter, transformer and LID/IAM/spectral effects, multiplied by plant availability, exactly as the loss chain is built in IEC 61724-1 and PVsyst. Switch to manual mode to force a contractual PR figure.

What DC/AC ratio should I use?+

1.1–1.3 for most sunny sites. Oversizing the array relative to the inverter harvests more morning and evening energy, at the cost of clipping around noon. Above 1.35 clipping losses grow quickly unless the site is hazy, high-latitude or east-west oriented.

What is CUF and how does it differ from PR?+

Capacity utilisation factor is annual energy divided by nameplate DC capacity × 8760 h — it mixes resource quality and plant quality, so a good plant in a poor climate shows a low CUF. PR normalises by the irradiation actually received, so it measures plant quality alone.

How do I calculate PV system size?+

Divide the daily energy solar must supply by peak sun hours × performance ratio. Round the module count up, recompute the installed DC capacity from the whole modules, then derive inverter AC capacity from that installed capacity and your chosen DC/AC ratio.

How many solar panels do I need?+

Number of modules = required DC capacity in watts ÷ module wattage, always rounded up. 4.08 kWp with 550 Wp modules is 7.42 → 8 modules, giving 4.40 kWp installed.

How do I calculate inverter size?+

Theoretical AC capacity = installed DC capacity ÷ target DC/AC ratio, then build that from real inverter units. 3,025 kWp at a 1.20 ratio needs about 2,521 kW AC — for example 8 × 320 kW. A single 500 kW unit under 3 MWp would give a DC/AC ratio of 6, which is not a valid design.

How much energy does a 1 MW solar plant generate?+

About 1.4 GWh per year at 5 peak sun hours and a 0.78 performance ratio. Expect nearer 1.35 GWh in cloudy temperate climates and up to 1.8 GWh at high-irradiance desert sites.

Why is my result different from another calculator?+

Mostly the performance ratio and how losses are combined. Tools that ignore losses, or add loss percentages instead of multiplying loss factors, produce a different kWp from the same load and sun hours.

Does PV system size depend on location?+

Yes. The same load needs a larger array where peak sun hours are lower or temperatures higher. Actual generation depends on location, orientation, tilt, weather, temperature, shading and system losses.

Is this calculator suitable for detailed EPC design?+

No. It is preliminary, energy-based conceptual sizing. Detailed engineering needs an hourly site-specific simulation with measured meteorological data, a 3D shading scene and the exact module and inverter models.

How much area does the plant need?+

Module area is nameplate power divided by 1000 × module efficiency — a 550 Wp module at 21% is about 2.62 m². Rooftop installations add ~15% for walkways and setbacks; ground-mount arrays need roughly 2.4× module area for inter-row pitch that avoids winter shading.

How many solar panels are needed for 1,000 kWh per month?+

Using 5 peak sun hours and a 78% performance ratio, 1,000 kWh/month is about 33.3 kWh/day and needs roughly 8.55 kWp before rounding. That is 22 × 400 W panels or 16 × 550 W panels. The exact result changes with local irradiation and losses.

Engineering Notes

  • •Specific yield (kWh/kWp) is the fairest way to compare plants across climates.
  • •CUF below 15% usually signals shading, poor tilt or a badly matched inverter.
  • •The loss diagram shows where energy disappears between the array plane and the meter.

Design Assumptions

  • • PSH is plane-of-array irradiation for the tilt and azimuth actually built.
  • • Loss factors are annual averages; monthly split uses a typical clear-sky low-latitude distribution.
  • • Rooftop area factor 1.15 for walkways and setbacks; ground mount 2.4 for inter-row pitch.
  • • Battery round-trip efficiency 90% (hybrid) and 85% with 80% DoD (off-grid).
  • • Degradation is linear; year-1 LID is already inside the loss chain.
  • • Financials ignore escalation, O&M and financing cost — use the LCOE calculator for a bankable figure.

Engineering Tips

  • ◆Keep the DC/AC ratio between 1.1 and 1.3 for most climates; up to 1.4 only in diffuse or high-latitude sites.
  • ◆Take PSH from NASA POWER, Meteonorm or Solargis for the exact coordinates, not a state average.
  • ◆Off-grid arrays must be sized on the worst irradiation month — reduce PSH to that month's value.
  • ◆Cross-check the derived PR against measured data from a nearby plant before financial close.

Warnings

  • ▲PR above 0.85 is not achievable in hot climates without tracking and very low soiling.
  • ▲Verify roof structural capacity and wind uplift (IS 875 Part 3 / ASCE 7) before installation.
  • ▲Utility sanctioned load and net-metering caps may limit the exportable AC capacity.

Standards & References

IEC 61724-1 (PV system performance monitoring)IEC 62548 (PV array design)IEC 61215 / IEC 61730 (module qualification & safety)IS 14286 (crystalline silicon PV modules)IEC 60364-7-712 (PV electrical installations)

Related Solar Calculators

Solar Panel Count Calculator

Calculate the whole number of solar panels needed for a target kWp using the actual module wattage, then check installed DC capacity and module area.

Inverter Sizing

Recommend inverter AC rating from PV DC and DC/AC ratio.

Solar PV String Sizing Calculator

Calculate PV modules per string, string voltage, MPPT operating range, temperature-adjusted voltage, parallel strings and inverter compatibility.

Performance Ratio (PR)

Measured plant PR and specific yield per IEC 61724-1.

Battery Bank Size

Bank Ah + count for autonomy-driven off-grid storage.

DC Cable Sizing

Copper/Aluminium cable size from voltage drop and thermal derating.

Related Engineering Articles

Deeper reading on the engineering behind this calculation.

PV System Size Calculator: How to Calculate the Right Solar PV System Size

The full PV sizing chain — load, peak sun hours, performance ratio, required DC capacity, module count, inverter capacity and DC/AC ratio — with worked equations.

Read Article →

How Many Solar Panels Do I Need? Complete PV Module Sizing Guide

Turn your electricity bill into a panel count: load, required DC capacity, module wattage, inverter sizing, plus illustrative examples from 3 kW to 1 MW.

Read Article →

Solar PV Performance Ratio (PR): Complete Guide to PV System Losses

What performance ratio means, every loss term from temperature to availability, how the losses multiply, and how PR differs from efficiency and capacity factor.

Read Article →

How to Size a Solar PV System Correctly

A step-by-step engineering walkthrough covering load assessment, peak sun hours, performance ratio and module selection.

Read Article →