Solar Energy Savings Calculator: A Bihar Homeowner Guide
Use your last three electricity bills to calculate exactly what rooftop solar saves you each month
Learn how to calculate your real residential solar savings unit by unit using your own SBPDCL or NBPDCL bills. This step-by-step tutorial helps Bihar homeowners find a trustworthy monthly savings number and realistic payback period.
TL;DR
Your bill will never be â¹0 - Fixed charges (â¹100 to â¹300/month) apply regardless of solar generation, and nighttime consumption still draws from the grid.
Use your last 3 bills, not a generic calculator - Map your actual consumption, tariff slabs, and solar generation month by month to get a savings number you can trust.
Net metering status is the #1 hidden problem - If your bi-directional meter isn't active, every unit you export to the grid is wasted with zero credit on your bill.
Solar saves you the most expensive units first - Because Bihar uses a slab tariff, solar offsets your highest-rate consumption, making per-unit savings higher than a simple average would suggest.
Realistic payback in Bihar is 4 to 7 years with subsidy - After that, your panels produce essentially free electricity for 20+ more years. Shift heavy appliance use to daytime hours to accelerate payback further.
What You'll Achieve: A Trustworthy Savings Number, Not a Guess
If you installed rooftop solar panels and your bijli bill still isn't zero, you're not alone. Most Bihar homeowners feel confused when the electricity bill keeps showing charges even after going solar. The truth is, a zero bill was never guaranteed. What you need is a reliable solar energy savings calculator approach built on your own consumption data, not someone else's promises.
By the end of this tutorial, you will have calculated exactly how many units your solar system offsets each month, how much money you're actually saving, and what your realistic solar system payback period looks like. You'll also understand why your bill still shows charges and what you can do about it.
Your success criteria: a single, trustworthy number (in rupees per month) that reflects your real residential solar savings, verified against your last three electricity bills.
Prerequisites: What You Need Before Starting
Gather these items before you begin. Missing even one will make your calculation incomplete.
Your last 3 electricity bills (SBPDCL or NBPDCL bills from South or North Bihar Power Distribution Company)
Your solar system's capacity in kW (check your installation agreement or inverter display)
Net metering status (confirmed active, pending, or not applied)
A calculator or smartphone (no special app needed)
Roof area knowledge (approximate shadow-free space in square feet)
Time estimate: 45 to 60 minutes for the full exercise. The main blocker is not having your bills handy, so collect those first.
Why This Bill-by-Bill Method Works Better Than Generic Calculators
Most online solar calculators ask for your location and a single monthly bill amount, then spit out a projected savings number. As the City of Calgary's solar tool notes, these are informational starting points, not final answers. They don't account for your actual seasonal usage swings, your specific tariff slab, or whether your net meter is even active.
This method is different. You'll work backwards from real bills, compare pre-solar and post-solar consumption, and isolate exactly where your money is going. It's harder than plugging numbers into a website, but the answer you get is one you can trust. Think of this as a self-assessment, not a sales pitch.
Step 1: Record Your Monthly Consumption from Three Bills
Take your three most recent electricity bills. For each bill, find and write down these numbers:
Billing period (e.g., March 2025)
Total units consumed (listed as kWh or "units" on the bill)
Total amount charged (in â¹, including all taxes and surcharges)
Units exported (if net metering is active, this appears as a separate line)
Checkpoint: You should have a simple table with three rows (one per month) and four columns. If "units exported" is missing or shows zero, your net meter may not be active yet. This is critical and we'll address it in Step 5.
Common issue: Some SBPDCL bills show "adjusted units" instead of raw consumption. Use the adjusted figure, as that's what you're billed for.
Step 2: Estimate Your Solar System's Monthly Generation
A commonly used sizing rule in Indian solar guidance is that 1 kW of solar generates about 1,350 to 1,400 units per year. We'll use 1,350 units to keep the estimate conservative for Bihar's climate (dust, humidity, and cloudy monsoon months reduce output).
Formula:
Monthly generation = (System capacity in kW à 1,350) ÷ 12
Example: If you have a 3 kW system:
Monthly generation = (3 à 1,350) ÷ 12 = 337 units/month (average)
Important: This is an annual average. In summer (April to June), expect 15 to 20% more. In monsoon (July to September), expect 20 to 30% less. Write down your estimated monthly generation number.
Checkpoint: Your number should be between 110 units (for 1 kW) and 560 units (for 5 kW). If it's outside this range, recheck your system capacity.
Step 3: Calculate Your Self-Consumption vs. Export Split
This is the step most people skip, and it's the reason they're confused about their bill. Not all solar units reduce your bill equally. The split matters.
Self-consumed units are the solar units your home uses directly during the day (when the sun is shining and your appliances are running). These units directly reduce your bill.
Exported units are surplus solar units pushed back to the grid through your net meter. These earn you a credit, but usually at a lower rate than what you pay for grid electricity.
How to estimate the split:
If your family is home during the day (AC, fans, fridge, TV running): assume 60 to 70% self-consumption
If the house is mostly empty during the day (everyone at work/school): assume 30 to 40% self-consumption
Self-consumed units = Monthly generation à self-consumption percentage
Exported units = Monthly generation - Self-consumed units
Example (3 kW system, 60% self-consumption):
Self-consumed = 337 Ã 0.60 = 202 units
Exported = 337 - 202 = 135 units
Step 4: Map Your Tariff Slabs to Find Real Rupee Savings
Bihar's domestic electricity tariff uses a slab structure. The more you consume, the higher the per-unit rate. This means solar savings are not a flat "â¹X per unit." Your savings depend on which slab your consumption falls in.
Look at your bill for the tariff breakdown, or check your DISCOM's (SBPDCL/NBPDCL) current tariff order. A typical Bihar domestic slab structure looks something like this:
0 to 100 units: approximately â¹4.50 to â¹5.00/unit
101 to 200 units: approximately â¹5.50 to â¹6.00/unit
201 to 300 units: approximately â¹6.50 to â¹7.00/unit
300+ units: approximately â¹7.50 to â¹8.50/unit
Key insight: Solar shaves units off the top slab first. If you consume 400 units without solar and your system offsets 200 units, you drop from the 400-unit slab to the 200-unit slab. Your savings come from the most expensive units, not the cheapest ones.
Calculate your savings:
Savings from self-consumed units = Sum of (units offset à rate of each slab eliminated)
Credit from exported units = Exported units à net metering credit rate
The net metering credit rate in Bihar is typically the same as the applicable tariff rate for adjustment in subsequent billing cycles, but verify this on your bill or with your DISCOM.
Checkpoint: Your monthly rupee savings should be higher than a simple "units à average rate" calculation. If it's lower, you may be using the wrong slab rates.
Step 5: Check if Net Metering Is Actually Active on Your Connection
This is the single biggest reason your bill isn't what you expected. Many Bihar homeowners have solar panels installed but net metering not yet activated. Without an active bi-directional meter, every unit your system exports to the grid is simply lost. You get no credit for it.
How to verify:
Check your bill for a line item showing "exported units" or "solar adjustment"
Look at your meter: a net meter displays both import (grid to home) and export (home to grid) readings
If neither appears, your net metering application is likely still pending
If net metering is not active, your actual savings are limited to only the self-consumed units. The exported units are wasted. This alone can explain why your bill is higher than expected.
What to do: Follow up with your DISCOM immediately. The solar subsidy approval process includes net metering activation, and delays here are common. Push for a timeline.
Step 6: Account for Fixed Charges That Solar Cannot Eliminate
Even if your solar system generates every unit your home needs, your electricity bill will never be â¹0. Here's why:
Fixed/demand charges: A flat monthly fee (â¹50 to â¹150 depending on your connection load) that has nothing to do with consumption
Meter rent: A small monthly charge for the meter itself
Electricity duty and surcharges: Government levies applied as a percentage or flat fee
Minimum billing: Some DISCOMs enforce a minimum bill amount regardless of consumption
Add up these fixed charges from your bill. This is the floor below which your bill will never drop. For most Bihar households, this floor is between â¹100 and â¹300 per month.
Checkpoint: Subtract fixed charges from your total bill. The remaining amount is what solar can actually reduce. If you've been comparing your total bill to "zero," this explains the gap.
Step 7: Build Your Monthly Savings Summary
Now pull everything together into a single calculation. Use this template for each of your three billing months:
Month: [e.g., March 2025]
A. Total bill without solar (estimate from pre-solar bills or current consumption + solar generation): â¹____
B. Solar units generated: ____ units
C. Self-consumed solar units (B Ã self-consumption %): ____ units
D. Exported solar units (B - C): ____ units
E. Savings from self-consumed units (C Ã applicable slab rate): â¹____
F. Credit from exported units (D Ã net metering rate): â¹____
G. Total monthly savings (E + F): â¹____
H. Fixed charges (cannot be reduced): â¹____
I. Actual bill after solar (A - G): â¹____
Do this for all three months. Average the "Total monthly savings" (G) across the three months. This averaged number is your trustworthy monthly savings figure.
Step 8: Calculate Your Solar System Payback Period
Now that you have a real monthly savings number, calculating your solar system payback period is straightforward.
Net system cost = Total installation cost - Subsidy received
Payback period (months) = Net system cost ÷ Average monthly savings
Payback period (years) = Payback period (months) ÷ 12
Example: If your 3 kW system cost â¹1,80,000 and you received â¹78,000 under the PM Surya Ghar Muft Bijli Yojana, your net cost is â¹1,02,000. If your average monthly savings are â¹1,800:
Payback = 1,02,000 ÷ 1,800 = 56.7 months â 4.7 years
For most Bihar households with subsidy, the payback period typically falls between 4 and 7 years. After that, your solar savings are essentially free for the remaining 20+ years of panel life.
If you haven't yet applied for or received your subsidy, check the real cost breakdown of solar panels to understand what affects your net cost beyond just panel pricing.
Configuration and Customization: Adjust for Your Household
The calculations above use conservative defaults. Here's where you might adjust:
Generation estimate: If your panels face due south with zero shading, use 1,400 units/kW/year instead of 1,350. If you have partial shading (trees, water tanks, neighboring buildings), drop to 1,200 or lower. Remember, roughly 80 sq. ft. of shadow-free roof space per kW is the standard requirement.
Self-consumption percentage: If you've added a solar battery or shifted heavy appliances (washing machine, iron, water pump) to daytime use, increase self-consumption to 75 to 85%.
Tariff rates: Use the exact rates from your latest bill, not the approximations listed above. Bihar tariffs are revised periodically.
Seasonal variation: For a more precise annual estimate, calculate each month separately rather than using a flat average. Summer months will show higher savings; monsoon months will show lower.
Safe defaults vs. must-change: The generation estimate and self-consumption percentage are safe to use as defaults for a rough calculation. The tariff slab rates are a must-change: always use your actual bill rates.
Verification: How to Confirm Your Numbers Are Right
Your calculation is only useful if it matches reality. Here's how to verify:
Compare your calculated "Actual bill after solar" (line I) with your real post-solar bill. They should be within 10 to 15% of each other. If the gap is larger, recheck your self-consumption estimate or tariff slab mapping.
Check your inverter's generation data. Most inverters (or their companion apps) log daily and monthly generation. Compare this with your Step 2 estimate. If the inverter shows significantly less generation, your panels may need cleaning or have a shading issue.
Verify exported units on your bill match your estimate. If your bill shows zero exports but your inverter shows surplus generation, net metering is not working.
Edge case: If you recently changed your connected load (added an AC or water heater), your pre-solar baseline is no longer valid. Use the last three months of post-solar bills and add back the estimated solar generation to reconstruct your "without solar" consumption.
Common Errors and Fixes
"My bill is almost the same as before solar"
Symptom: Post-solar bill shows little to no reduction. Cause: Net metering is not active, so exported units earn no credit. Or, your consumption has increased (new AC, more family members). Fix: Verify net metering status. Compare total consumption (grid + solar) to pre-solar consumption to isolate whether the issue is generation or usage growth.
"My inverter shows good generation but my bill doesn't reflect it"
Symptom: Inverter app shows 300+ units generated, but bill reduction is only 100 units worth. Cause: Most generation is happening when nobody is home (low self-consumption). Units are being exported, but net metering credit is delayed or not applied. Fix: Shift heavy loads to daytime. Check if your DISCOM applies credits in the same billing cycle or carries them forward.
"My calculated payback period seems too long (8+ years)"
Symptom: Payback exceeds 8 years even with subsidy. Cause: Low monthly consumption (under 200 units) means lower slab rates and less rupee savings per unit offset. Or subsidy was not fully received. Fix: Confirm subsidy disbursement status. For low-consumption homes, a smaller system (1 to 2 kW) may actually offer a better payback ratio than an oversized one.
"I don't know my pre-solar consumption to compare"
Symptom: No baseline data available. Cause: Didn't save old bills. Fix: Add your current grid consumption (from your bill) and your inverter's self-consumed generation. This reconstructed total is your approximate "without solar" consumption. Alternatively, request consumption history from your DISCOM office.
"The DISCOM website shows a different tariff than my bill"
Symptom: Online tariff schedule doesn't match bill charges. Cause: Additional surcharges, fuel adjustment charges, or a different consumer category applied. Fix: Always use the per-unit breakdown printed on your actual bill, not the published base tariff.
Next Steps: What to Do After You Have Your Number
Now that you know your real monthly savings and payback period, here's how to build on this work:
Optimize self-consumption: Run your washing machine, water pump, and iron during peak solar hours (10 AM to 3 PM). This shifts more units from "exported" to "self-consumed" at higher slab rates, increasing your rupee savings.
Track monthly: Repeat this calculation quarterly. As tariffs rise (and they do), your savings increase automatically. Keep a simple spreadsheet.
Explore battery storage: If your self-consumption is below 50%, a solar battery can capture daytime surplus for evening use, further reducing grid dependence.
If you're still in the planning stage and haven't installed solar yet, teams like Ghar Ghar Solar can walk you through a Bihar-specific assessment using your actual bill data, helping you size the system correctly before you commit. Getting the system size right upfront is the single best way to ensure your payback period matches expectations.
For homeowners who have installed but are stuck waiting on approvals, revisit the step-by-step guide to navigating solar subsidy approvals to push your net metering activation forward.
Frequently Asked Questions
What is the PM Surya Ghar Muft Bijli Yojana and how does it work?
The PM Surya Ghar Muft Bijli Yojana is a central government scheme that provides subsidies for residential rooftop solar installations. For systems up to 3 kW, homeowners can receive substantial subsidies that reduce the upfront cost significantly. The subsidy is disbursed after installation and inspection by your DISCOM. You can check eligibility and apply through the national rooftop solar portal.
How much subsidy can I get for rooftop solar in Bihar?
Under the current scheme, households can receive up to â¹30,000 per kW for systems up to 2 kW, and â¹18,000 per kW for the portion between 2 kW and 3 kW. For a 3 kW system, this works out to approximately â¹78,000 in total subsidy. The exact amount depends on your system size and whether all documentation and inspections are completed.
How does net metering work for households with solar panels?
Net metering uses a bi-directional meter that tracks both the electricity you draw from the grid (import) and the surplus solar electricity you push back to the grid (export). At billing time, your exported units are adjusted against your imported units. You only pay for the net difference. If your exports exceed imports in a billing cycle, the credit typically carries forward to the next cycle.
Why is my electricity bill not zero even after installing solar panels?
Three main reasons: First, your solar system may not generate enough to cover 100% of your consumption, especially at night and during monsoon months. Second, fixed charges (meter rent, demand charges, electricity duty) apply regardless of consumption. Third, if net metering is not yet active, you're getting no credit for exported units. Use the step-by-step method in this guide to identify which factor is affecting your bill most.
How can I increase my residential solar savings without adding more panels?
The most effective strategy is to increase daytime self-consumption. Run heavy appliances like washing machines, water pumps, and irons between 10 AM and 3 PM when solar generation peaks. This converts exported units (lower value) into self-consumed units (higher value, since they offset top-slab grid rates). Keeping panels clean and free of bird droppings also maintains generation efficiency.
What is a realistic solar system payback period for Bihar homeowners?
With the government subsidy applied, most Bihar households see a payback period between 4 and 7 years, depending on system size, monthly consumption, and self-consumption ratio. Homes consuming 300+ units per month tend to see faster payback because they offset higher tariff slabs. After the payback period, the system generates essentially free electricity for 18 to 21 more years.