Chennai Solar Calculator (TNPDCL)

Rooftop solar payback for Chennai and Tamil Nadu on TNPDCL's bi-monthly slabs, with the state's free units taken into account.

Your home

Average monthly electricity bill

₹

₹100₹50K

About 359 units a month on the TNPDCL tariff. Use a year's average if your bill swings with the seasons.

Roof type

RCC flat roof

RCC, elevated structure

Metal sheet roof

Fine-tune

System size

Sized to cover a year of your consumption, at 5.4 peak sun hours a day in Tamil Nadu.

Usable shadow-free roof area (optional)

sq ft

About 100 sq ft per kW. Leave at 0 for no limit.

Installed cost per kW, GST included

₹

Typical for an on-grid system from an empanelled vendor. Replace it with your quote.

Yearly rise in electricity prices

0%15%

Indian residential tariffs have historically risen 4-7% a year.

Panel output lost each year

0%2%

Monocrystalline panels typically lose about 0.5% a year.

How you pay

Upfront

Solar loan

Pays for itself in2.7 yearsthen ₹15.67 L ahead after 25 years

A 3 kW system makes about 379 units a month, 100% of your use. Your bill drops from ₹3,000 to about ₹0 in year one.

Installed cost: ₹1,80,000
PM Surya Ghar subsidy: − ₹78,000
Net cost to you: ₹1,02,000
Saving in year one: ₹36,710
Return (IRR): 40.4% a year; over 25 years, before tax
CO₂ avoided: 77 tonnes; over 25 years

Subsidy43%

You pay57%
Central subsidy43%

Your money over 25 yearsStill to recoverAhead

Year-by-year cash flowShow

Year	Units made	Bills without solar	Bills with solar	Upkeep and EMIs	Running total
1	4,553	₹36,097	₹0	₹0	-₹65,290
2	4,530	₹37,902	₹0	₹0	-₹26,833
3	4,508	₹39,797	₹0	₹0	₹13,464
4	4,485	₹41,787	₹0	₹0	₹55,693
5	4,463	₹43,876	₹0	₹0	₹99,956
6	4,440	₹46,070	₹0	₹1,800	₹1,44,557
7	4,418	₹48,374	₹0	₹1,800	₹1,91,406
8	4,396	₹50,792	₹0	₹1,800	₹2,40,619
9	4,374	₹53,332	₹0	₹1,800	₹2,92,316
10	4,352	₹55,999	₹0	₹1,800	₹3,46,625
11	4,330	₹58,799	₹0	₹1,800	₹4,03,679
12	4,309	₹61,738	₹0	₹22,800	₹4,42,620
13	4,287	₹64,825	₹0	₹1,800	₹5,05,645
14	4,266	₹68,067	₹0	₹1,800	₹5,71,912
15	4,244	₹71,470	₹0	₹1,800	₹6,41,582
16	4,223	₹75,043	₹0	₹1,800	₹7,14,825
17	4,202	₹78,796	₹0	₹1,800	₹7,91,821
18	4,181	₹82,735	₹0	₹1,800	₹8,72,756
19	4,160	₹86,872	₹0	₹1,800	₹9,57,829
20	4,139	₹91,216	₹0	₹1,800	₹10,47,244
21	4,119	₹95,777	₹0	₹1,800	₹11,41,221
22	4,098	₹1,00,565	₹0	₹1,800	₹12,39,986
23	4,078	₹1,05,594	₹0	₹1,800	₹13,43,780
24	4,057	₹1,10,873	₹0	₹1,800	₹14,52,854
25	4,037	₹1,16,417	₹0	₹1,800	₹15,67,471

How it works

The bill you enter is worked backwards through your DISCOM's slabs, fixed charges, surcharges and duty to the units you use. The panels' yearly output is set against that use under net metering, and each month is re-billed on what is left. Because the units solar removes come off your most expensive slab first, the saving per unit is often well above the average rate on your bill. Tariffs rise by the escalation rate each year while the panels slowly lose output; maintenance, an inverter replacement in year 12 and any loan repayments are taken off, and the running total gives the payback year.

Saving₍y₎ = [Bill(U) − Bill(U − G₍y₎/12)] × 12 × (1 + e)^(y−1),  G₍y₎ = kW × H × 365 × PR × (1 − d)^(y−1)

U: Units you use a month, worked out from your bill
G₍y₎: Units the panels make in year y
H: Peak sun hours a day in your state
PR: Performance ratio: the share of rated output that reaches the meter, 0.77 here
e: Yearly rise in electricity prices
d: Yearly loss in panel output

Worked example

A Chennai home on TNPDCL with a ₹3,000 monthly bill and a flat RCC roof, assuming prices rise 5% a year.

1Working the bill back through the TNPDCL slabs, charges and duty gives about 359 units a month
2At 5.4 peak sun hours, 3 kW makes about 379 units a month, 100% of that use
33 kW at ₹60,000 per kW costs ₹1,80,000; PM Surya Ghar pays ₹78,000, leaving ₹1,02,000
4The first year saves ₹36,710, and the saving grows as tariffs rise

The system pays for itself in about 2.7 years and leaves the household about ₹15,67,471 ahead over 25 years.

Frequently asked questions

How do Tamil Nadu's free units affect solar savings?

Homes get the first 200 units of each two-month bill free if they stay within 500 units, and 100 otherwise. Solar that brings a home under that line recovers the extra free units, which the calculator includes. Since those free units cost nothing already, solar mainly saves on the upper slabs, which reach ₹12.15 a unit.

Why does the calculator show monthly slabs when TNPDCL bills every two months?

To keep one monthly bill field for every state. TNPDCL's two-month bands are halved to monthly equivalents, which gives the same result for a steady household. Enter half of your bi-monthly bill.

How does net metering settle on TNPDCL?

Net metering for domestic consumers, billed every two months, with surplus settled yearly at the TNERC rate.

How much is the PM Surya Ghar subsidy?

₹30,000 per kW for the first 2 kW and ₹18,000 for the third, so ₹30,000 for 1 kW, ₹60,000 for 2 kW and ₹78,000 for 3 kW or more. It is paid into your bank account after the DISCOM inspects and commissions the system, and only for installations by an empanelled vendor using ALMM-listed panels, applied for through pmsuryaghar.gov.in.

Why is my saving per unit higher than my average rate?

Residential tariffs are telescopic: the first units are cheap and each slab above costs more. Solar takes units off the top of your bill, so it removes the dearest ones first. A home in a ₹10-plus slab saves that rate on every unit the panels cover, even if the bill averages out lower.

Why not simply fit the biggest system my roof will hold?

The central subsidy stops rising at 3 kW, and under net metering surplus units are usually bought back at the regulator's average power purchase cost, often ₹2-3 a unit, far below what you pay for grid power. Sizing to your own consumption earns the full retail rate on every unit. The calculator suggests that size; override it to see what a bigger system does to the return.