Why Does My Electricity Bill Change Every Month Even When I Use the Same Units? (Fixed Charges vs FCA Explained)

The Four Parts of Your Electricity Bill

Almost every Indian electricity bill is really four line items added together, not one number:

Takeaway: only one of these four moves in direct proportion to the units you used — the other three follow their own rules.

Fixed Charge: The Part That Never Changes With Usage

The fixed (or demand) charge is billed on your sanctioned load— the maximum power draw approved for your connection — not on how much electricity you actually consumed that month. It appears identically on your bill even in a month of unusually low usage, because it's recovering the DISCOM's cost of keeping the connection and infrastructure ready for you, not the cost of the power itself.

As an illustration only (actual fixed-charge rates vary widely by state and connection category), one board's domestic tariff structures this as roughly ₹60/kW for the first 1 kW of sanctioned load and ₹70/kW for the remainder — a flat monthly amount that doesn't care whether you used 50 units or 500 that month.

Takeaway: your fixed charge is set by your connection's sanctioned load, not your meter reading — it's the same whether you're home all month or away.

Energy Charge: The Part Your Slab Calculator Estimates

The energy charge is the only component billed directly on your actual consumption. Most Indian DISCOMs bill domestic connections using telescopic slabs — tiered per-unit rates where each slab is priced separately, so only the units that fall inside a higher slab cost more, not your whole bill.

This is exactly the part our state-by-state electricity bill calculators are built to estimate precisely, using each DISCOM's own published slabs — for example MSEDCL (Maharashtra) or TNEB (Tamil Nadu). But a slab calculator estimates this one component — the fixed charge, the fuel adjustment and electricity duty sit on top of it, and aren't fully predictable in advance since the fuel adjustment changes monthly and isn't published far ahead.

Takeaway: if your calculator estimate looks close but not exact, that's expected — it's pricing the energy charge, not the other three line items.

“FCA, FAC, FPPCA, Power Purchase Cost Adjustment” — Same Thing, Different Names

Base tariffs are set periodically — often once a year — by each state's electricity regulatory commission. But the DISCOM's actual cost of buying power (coal, gas, market purchases) moves every month. Rather than reopen the whole tariff order monthly, regulators let DISCOMs pass this cost gap through as a separate line, reviewed and approved on a monthly or quarterly cycle depending on the state. Different states and DISCOMs simply call this line by different names:

This line isn't always positive. When a DISCOM's actual power-purchase cost comes in below what the tariff order assumed, the adjustment can turn negative and appear as a credit that reduces your bill — not just an add-on charge. Some states also cap how large this adjustment can get in a single cycle, as a safeguard against a very large pass-through landing on customers all at once.

Takeaway: FCA, FAC and FPPCA are the same mechanism wearing different labels — check the exact line on your own bill rather than assuming it matches a neighbour's state.

Electricity Duty: The Part That's Actually a Tax, Not a DISCOM Charge

Electricity duty is collected on your bill but paid to the state government, not kept by the DISCOM — so it's not something your DISCOM can waive or discount. It's usually charged as a percentage of your energy charge, and the rate is set per state and often per connection category.

The rate genuinely varies: Tamil Nadu's TNEB, for instance, charges 0% electricity duty on residential connections but 5% on commercial and industrial ones — the same DISCOM, two different duty rates depending on your connection category. That kind of category-specific split is exactly why a duty percentage from one state (or even one connection type) shouldn't be assumed to apply to another.

Takeaway: if a bill looks high specifically because of the duty line, that's a state tax rate applying to your category — not a DISCOM markup.

Worked Example: Same Units, Different Bill

Here's an illustrative two-month comparison showing how a bill can move even when consumption barely does:

Nothing about this consumer's usage habits changed. The entire difference came from a line that moves independently of consumption — which is exactly why comparing only your units, month to month, doesn't explain the whole bill.

Takeaway: before assuming a billing error, check whether the fuel/power-purchase adjustment line itself moved — that's the most common reason units stay flat but the total doesn't.

Can You Actually Reduce Any of This?

• ✓Energy charge — the only component you control day to day. Using fewer units, or staying below your next slab threshold, directly lowers this line. See how telescopic slabs work to understand exactly where your next threshold is.
• ✓Fixed charge — possible, but only if your sanctioned load is clearly oversized for your actual usage. You can request a review of your sanctioned load, but that process is handled directly through your DISCOM and depends on your local rules — not something a calculator can action for you.
• –Fuel/power-purchase adjustment and electricity duty — not controllable by you as a consumer. The first tracks the DISCOM's power-purchase cost; the second is a state tax rate. Neither responds to how carefully you use electricity.

Takeaway: real savings come from the energy charge — that's also the one part a slab calculator can help you plan around in advance.

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