A rooftop array that is too small leaves you buying grid power, while an oversized one wastes money and roof space. A few honest numbers about your load, your sunshine and your losses give a design that works every day.
Every good solar design starts with the energy you actually use and the sunshine your site actually receives. From those two numbers, derating factors turn a rough guess into a reliable array, battery and inverter size.
What Is Solar PV System Sizing?
Solar PV system sizing is the process of calculating how many kilowatt peak of panels, how much battery storage and what inverter rating a site needs to meet its daily energy demand. It begins with a load list in kWh per day, as shown in electrical energy consumption calculation.
The inverter converts DC from panels or batteries into AC for the loads, as explained in power inverter working principle. Its efficiency and rating enter the calculation just like panel losses.
Peak Sun Hours Across Indian Regions
Peak sun hours, or PSH, is the daily solar energy on a panel expressed as hours at 1000 W per square metre. A site receiving 5 kWh per square metre per day therefore has 5 peak sun hours.
MNRE states that most parts of India receive about 4 to 7 kWh per square metre per day, with about 300 sunny days a year. That is excellent for solar, but the number varies strongly by region and season.
| Region | Typical Annual PSH | Design Note |
|---|---|---|
| Rajasthan, Gujarat, Ladakh | 5.5 to 6.5 | High yield, plan for dust and heat |
| Maharashtra, Karnataka, Telangana | 5.0 to 5.8 | Monsoon months drop to about 3.5 |
| Delhi, UP, Punjab plains | 4.8 to 5.5 | Winter fog and smog lower output |
| Kerala, coastal Tamil Nadu | 4.5 to 5.3 | Long monsoon, salt corrosion |
| Northeast, West Bengal | 4.0 to 4.8 | Cloud cover, size with margin |
These are typical ranges, so use NASA POWER or PVGIS site data. For off grid solar PV system sizing, always use the worst month, not the annual average.
A 1 kWp rooftop array in most of India produces roughly 4 to 4.5 kWh per day on average. That is why the Delhi solar guidelines cap incentive generation at 1500 kWh per kWp per year.
7 Proven Solar PV System Sizing Steps
Step one decides everything else, so measure rather than guess. A month of electricity bills gives the real average, and a clamp meter or energy meter confirms individual loads, as in generator sizing for industrial load.
Reduce the load before you size anything, because every kWh saved per day removes about 250 Wp of panels and a large slice of battery. Swap old fans, tube lights and refrigerators for efficient BEE star rated models first.
Derating Factors That Shrink Your Output
Firgelli notes that module power falls by about 0.4 to 0.5 percent for every degree Celsius above 25 °C. On an Indian roof in April, cell temperature easily reaches 60 °C or more, so heat alone can cost 15 percent.
Cell heating above 25 °C cuts voltage and power.
Dust, bird droppings and pollen block light.
Conversion loss in the inverter or charge controller.
Resistive loss in DC and AC cables.
Firgelli also reports that soiling can reduce output by 3 to 8 percent, and that quality inverters reach 93 to 96 percent efficiency at optimal loading. Cable loss is kept small by following voltage drop calculation limits of about 1.5 to 2 percent on the DC side.
Multiplying typical values such as 0.88 × 0.95 × 0.96 × 0.97 × 0.98 gives about 0.76. Most solar PV system sizing work therefore uses an overall derating factor between 0.75 and 0.80.
Solar PV System Sizing Formulas
Number of panels = Array Wp ÷ Panel Wp, rounded up
Battery Ah = kWh × Autonomy days × 1000 ÷ (Battery V × DoD)
Example, off grid home in Pune:
Daily load 10 kWh, PSH 5, derating 0.75
Array = 10 ÷ (5 × 0.75) = 2.67 kWp
Panels = 2667 ÷ 540 = 4.94, so 5 panels of 540 Wp
Battery = 10 × 1 × 1000 ÷ (48 × 0.8) = 260.4 Ah
Result: 2.67 kWp array, 5 panels, 260 Ah battery at 48 V
For battery backup on a shorter scale, the same energy logic appears in battery backup time calculation. A UPS or inverter must also be rated in kVA, as shown in UPS capacity kVA calculation.
Solar Array and Battery Calculator
Battery Sizing for Off Grid Homes
Depth of discharge, or DoD, is how much of the rated capacity you allow the battery to use each day. Firgelli points out that flooded lead acid batteries rated for 1200 cycles at 50 percent DoD may give only about 400 cycles at 80 percent DoD.
That is why lead acid banks are usually sized at 50 percent DoD, while lithium iron phosphate banks are commonly used at 80 to 90 percent. The same 10 kWh load therefore needs a much larger lead acid bank in Ah.
Autonomy of one to two days suits most Indian homes with a grid backup, while remote telecom or instrument sites may need three to five days. Understand the ageing mechanisms in causes of battery failure before you trim the margin.
Doubling the battery voltage from 24 V to 48 V halves the current for the same power. Resistive cable loss depends on current squared, so that loss drops to one quarter.
Inverter and Charge Controller Sizing
An off grid inverter must supply the largest simultaneous load continuously, with about 25 percent margin. Motors such as pumps and refrigerator compressors draw three to six times their running current at start, so check the surge rating as well.
On grid inverters are sized against the array instead, and many designs use a DC to AC ratio of 1.1 to 1.3 because panels rarely reach full rating. Battery side current and cable selection follow cable size for load current.
A PWM charge controller needs panel voltage matched to the battery, while an MPPT controller uses a DC converter to track the best operating point. The converter stage is similar to the one in boost converter working principle.
On Grid vs Off Grid vs Hybrid
| Feature | On Grid | Off Grid | Hybrid |
|---|---|---|---|
| Battery | None | Essential | Partial backup |
| Works in power cut | No, shuts down for safety | Yes | Yes, for backed loads |
| Sizing basis | Annual kWh and roof area | Worst month kWh and autonomy | Critical load kWh plus bill saving |
On grid systems use net metering, where the utility meter records both import and export and bills only the difference. Rules for capacity limits and settlement periods are set by each State Electricity Regulatory Commission.
MNRE runs the grid connected rooftop programme, and residential users can apply for central financial assistance under PM Surya Ghar Muft Bijli Yojana through the national rooftop portal. Check the current subsidy slabs on the portal, since they are revised from time to time.
For an on grid rooftop, size the array to your yearly consumption and not above your sanctioned load limit. Exported surplus is often credited at a low rate, so a bigger array does not always pay back.
String Sizing at Low Temperature
Panel open circuit voltage rises as temperature falls, so the coldest morning gives the highest string voltage. A typical 540 Wp module has Voc near 49.5 V and a temperature coefficient of about 0.27 percent per °C, with voltage increasing as the cell cools.
Example: Voc 49.5 V, coefficient 0.0027 per °C, Tmin 5 °C
Voc max = 49.5 × (1 + 0.0027 × 20) = 52.17 V
Inverter maximum DC input 1000 V
Maximum modules per string = 1000 ÷ 52.17 = 19.2, so 19
Also check the minimum string voltage at the hottest cell temperature against the lowest MPPT voltage. If the string falls below that window on a hot afternoon, the inverter loses tracking and output collapses.
Protection, Earthing and Installation
Every array needs DC isolators, string fuses where three or more strings run in parallel, and surge protection on both DC and AC sides. The device types are compared in surge protector types and MCB vs MCCB vs ELCB vs RCCB.
Module frames and mounting structures must be bonded to a proper earth pit with low resistance. Verify it using earthing resistance calculation, and add a lightning arrester on exposed rooftops.
- Load list signed off in kWh per day with day and night split.
- Inverter surge rating checked against motor loads.
- Cold weather Voc per string below inverter maximum.
- DC SPD, isolator and string fuses installed and labelled.
Common Mistakes and Troubleshooting
- Meets load in the worst month.
- Avoids wasted panels and batteries.
- Longer battery life at correct DoD.
- Safe string voltage on cold days.
- Depends on accurate load data.
- PSH varies year to year.
- Needs local temperature records.
- Tariff and subsidy rules keep changing.
Solar Plant Guidelines PDF
The Delhi guidelines state that a minimum vacant roof area of about 10 to 12 square metres is needed for each kWp. Use that figure to check early whether the roof can hold the array you calculated.
Off Grid Sizing Video
Solar PV System Sizing FAQ
It is the calculation of array kWp, battery capacity and inverter rating from the measured daily energy demand of a building. The result depends on the site sunshine, the season chosen for design and the total system losses.
A correct size meets the load in the design month without wasting money on idle panels. It also keeps battery depth of discharge and string voltage inside safe limits.
Most Indian sites fall between about 4.5 and 6 peak sun hours on an annual average basis. Western desert regions such as Rajasthan are highest, while the cloudy northeast is lower.
For off grid systems, use the worst month value, which often falls in the monsoon. Local data from PVGIS or NASA POWER is always better than a single national average figure.
An overall factor of 0.75 to 0.80 is common for rooftop systems in India. It combines temperature, dust, inverter, cable and module mismatch losses into one simple multiplier.
Hot and dusty sites with poor cleaning should use the lower end of this range. Well ventilated, regularly washed arrays on cooler sites can justify the higher end.
Multiply daily kWh by the days of autonomy and by 1000 to get watt hours of storage. Divide that figure by battery voltage times the allowed depth of discharge as a fraction.
For 10 kWh, one day, 48 V and 80 percent DoD, the result is about 260 Ah. A lead acid bank limited to 50 percent DoD would need roughly 417 Ah instead.
An off grid inverter should carry the largest simultaneous load with about 25 percent spare margin. It must also handle the starting surge of pumps, compressors and other motor loads without tripping.
On grid inverters are matched to the array rating rather than to the load. A DC to AC ratio of about 1.1 to 1.3 is widely used for Indian rooftops.
Open circuit voltage of a panel rises as the cells get colder. The coldest clear morning of the year therefore produces the highest string voltage on the inverter input.
If that value exceeds the inverter maximum, the inverter can be damaged or will refuse to start. Always use the local record low temperature, not the 25 °C datasheet value.
Yes, net metering for rooftop solar is offered under state electricity regulations across India. A bidirectional meter records import and export, and you pay only for the net energy used.
Capacity limits, banking periods and settlement rules differ from state to state. Check with your DISCOM and the national rooftop portal before finalising the solar PV system sizing.
Related Articles
- Electrical Energy Consumption Calculation
- Calculate Battery Backup Time
- Power Inverter Working Principle
- Cable Size for Load Current
- Surge Protector Types and Working
External References
- Guidelines for Installing Solar Photovoltaic Power Plant, Government of NCT of Delhi
- Solar PV System Sizing Interactive Calculator, FIRGELLI
- Photovoltaic System, Wikipedia
What We Learn Today
- Solar PV system sizing starts with a measured daily kWh load and the peak sun hours of the site, using the worst month for off grid designs.
- Array kWp equals daily energy divided by peak sun hours times a derating factor of about 0.75, then panels are rounded up.
- Battery Ah depends on autonomy, voltage and depth of discharge, while strings must be checked for Voc at the coldest temperature.