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Grid Bill Offset

Monthly bill offset from a solar system.

Financial

Inputs (SI units)

Total energy the site consumes from the load in a typical month.

Total PV energy produced in the same month.

Share of PV generation used directly on-site rather than exported; depends on load-generation coincidence.

Retail rate charged per kWh imported from the grid.

Rate credited per kWh of surplus PV exported to the grid (0 if exports are not compensated).

Flat monthly charge unaffected by solar generation.

Charge based on peak kW demand — solar rarely reduces coincident peak demand fully; entered separately from energy charges.

Percentage of the demand charge solar is expected to shave (0 if solar does not coincide with the peak-demand window).

Results

Monthly savings
5,400
Bill without solar minus bill with solar
Annual savings
64,800
Remaining monthly bill
2,850
Solar offset of load
83.3 %
Self-consumed energy
525 kWh/mo
Exported energy
225 kWh/mo
✓ PASS — Bill with solar is non-negative or export credits are capped appropriately.
✓ PASS — Solar offset percentage is within a sane 0–100% band.

Engineering Recommendations

  • →This model never multiplies generation × tariff directly — savings are built from self-consumption, export credit, remaining energy charge, fixed charge and demand charge separately.
  • →Demand charges are rarely eliminated by solar unless the plant reliably shaves the coincident peak; keep the demand-offset input conservative.
  • →Engineering-grade preliminary calculation. Final design must be verified against project-specific site conditions, manufacturer datasheets, applicable standards and utility requirements.

Detailed Calculation Log

Component | Without solar | With solar
Energy charge | 8,100 | 3,375
Fixed charge | 150 | 150
Demand charge | 0 | 0
Export credit | 0 | -675
Total bill | 8,250 | 2,850

Save & Load Project

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

Engineering Formula

  • Consumption offset = min(gen×self%, bill/tariff)
  • Savings = offset × tariff
  • Export credit = gen × (1 − self%) × feed

Behind-the-meter savings equal displaced tariff units. Surplus feeding the grid earns the feed-in rate. Net saving is the sum.

Step-by-step Calculation

  1. 1.Self-consumed PV energymin(Gen × self%, Load)525 kWh
  2. 2.Export energyGen − Self-consumed225 kWh
  3. 3.Grid import reduced= Self-consumed525 kWh
  4. 4.Remaining grid importLoad − import reduced375 kWh
  5. 5.Energy charge avoidedImport reduced × tariff4,725
  6. 6.Export creditExport × export rate675
  7. 7.Demand charge avoidedDemand charge × offset%0
  8. 8.Bill without solarLoad×tariff + fixed + demand8,250
  9. 9.Bill with solarRemaining energy + fixed + remaining demand − export credit2,850

How to use this calculator: Grid Bill Offset

Monthly bill offset from a solar system. The example below is calculated by this page's real engine from the displayed inputs.

  1. 1Confirm that the Grid Bill Offset matches the quantity or design check you need.
  2. 2Enter Monthly load consumption, Monthly PV generation, and Self-consumption fraction of PV generation using the units printed beside each field.
  3. 3Check every value before calculating, especially decimal points and measurement units.
  4. 4Calculate, then follow the substituted equations in the worked example and compare the result with any stated limit.
  5. 5Read the assumptions, warnings and cited references before using the result for a financial, medical or engineering decision.

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
Monthly load consumption900 kWh/moTotal energy the site consumes from the load in a typical month.
Monthly PV generation750 kWh/moTotal PV energy produced in the same month.
Self-consumption fraction of PV generation70 %Share of PV generation used directly on-site rather than exported; depends on load-generation coincidence.
Energy charge (import tariff)9 currency/kWhRetail rate charged per kWh imported from the grid.
Export credit rate3 currency/kWhRate credited per kWh of surplus PV exported to the grid (0 if exports are not compensated).
Fixed / service charge150 currency/moFlat monthly charge unaffected by solar generation.
Demand charge0 currency/moCharge based on peak kW demand — solar rarely reduces coincident peak demand fully; entered separately from energy charges.
Demand charge reduction from solar0 %Percentage of the demand charge solar is expected to shave (0 if solar does not coincide with the peak-demand window).

Formula inputs & variables for Grid Bill Offset

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
Monthly load consumptionkWh/moTotal energy the site consumes from the load in a typical month.
Monthly PV generationkWh/moTotal PV energy produced in the same month.
Self-consumption fraction of PV generation%Share of PV generation used directly on-site rather than exported; depends on load-generation coincidence.
Energy charge (import tariff)currency/kWhRetail rate charged per kWh imported from the grid.
Export credit ratecurrency/kWhRate credited per kWh of surplus PV exported to the grid (0 if exports are not compensated).
Fixed / service chargecurrency/moFlat monthly charge unaffected by solar generation.
Demand chargecurrency/moCharge based on peak kW demand — solar rarely reduces coincident peak demand fully; entered separately from energy charges.
Demand charge reduction from solar%Percentage of the demand charge solar is expected to shave (0 if solar does not coincide with the peak-demand window).

How the Grid Bill Offset works

The Grid Bill Offset uses Monthly load consumption, Monthly PV generation, Self-consumption fraction of PV generation, Energy charge (import tariff), Export credit rate, Fixed / service charge, Demand charge, and Demand charge reduction from solar to calculate Monthly savings, Annual savings, Remaining monthly bill, Solar offset of load, Self-consumed energy, and Exported energy. Its engine applies Consumption offset = min(gen×self%, bill/tariff); the worked values below come from that same live calculation rather than a separately typed example.

With Monthly load consumption 900 kWh/mo, Monthly PV generation 750 kWh/mo, Self-consumption fraction of PV generation 70 %, Energy charge (import tariff) 9 currency/kWh, Export credit rate 3 currency/kWh, Fixed / service charge 150 currency/mo, Demand charge 0 currency/mo, and Demand charge reduction from solar 0 %, the main worked-example result is Monthly savings = 5,400.

How each Grid Bill Offset input is used

Monthly load consumption

The Grid Bill Offset worked example uses Monthly load consumption = 900 kWh/mo. This value is passed directly into the calculation, with an allowed minimum 0.01. Total energy the site consumes from the load in a typical month.

Monthly PV generation

The Grid Bill Offset worked example uses Monthly PV generation = 750 kWh/mo. This value is passed directly into the calculation, with an allowed minimum 0. Total PV energy produced in the same month.

Self-consumption fraction of PV generation

The Grid Bill Offset worked example uses Self-consumption fraction of PV generation = 70 %. This value is passed directly into the calculation, with an allowed minimum 0 and maximum 100. Share of PV generation used directly on-site rather than exported; depends on load-generation coincidence.

Energy charge (import tariff)

The Grid Bill Offset worked example uses Energy charge (import tariff) = 9 currency/kWh. This value is passed directly into the calculation, with an allowed minimum 0.01. Retail rate charged per kWh imported from the grid.

Export credit rate

The Grid Bill Offset worked example uses Export credit rate = 3 currency/kWh. This value is passed directly into the calculation, with an allowed minimum 0. Rate credited per kWh of surplus PV exported to the grid (0 if exports are not compensated).

Fixed / service charge

The Grid Bill Offset worked example uses Fixed / service charge = 150 currency/mo. This value is passed directly into the calculation, with an allowed minimum 0. Flat monthly charge unaffected by solar generation.

Demand charge

The Grid Bill Offset worked example uses Demand charge = 0 currency/mo. This value is passed directly into the calculation, with an allowed minimum 0. Charge based on peak kW demand — solar rarely reduces coincident peak demand fully; entered separately from energy charges.

Demand charge reduction from solar

The Grid Bill Offset worked example uses Demand charge reduction from solar = 0 %. This value is passed directly into the calculation, with an allowed minimum 0 and maximum 100. Percentage of the demand charge solar is expected to shave (0 if solar does not coincide with the peak-demand window).

Grid Bill Offset formulas and result interpretation

Formula 1: relationship used

In the Grid Bill Offset, Consumption offset = min(gen×self%, bill/tariff). The quantities in this relationship come from the named inputs or from an earlier calculation step shown in the worked example.

Formula 2: relationship used

In the Grid Bill Offset, Savings = offset × tariff. The quantities in this relationship come from the named inputs or from an earlier calculation step shown in the worked example.

Formula 3: relationship used

In the Grid Bill Offset, Export credit = gen × (1 − self%) × feed. The quantities in this relationship come from the named inputs or from an earlier calculation step shown in the worked example.

Monthly savings

For the displayed Grid Bill Offset worked example, Monthly savings is 5,400. Bill without solar minus bill with solar Verify Monthly load consumption, Monthly PV generation, and Self-consumption fraction of PV generation and their units before relying on this output.

Annual savings

For the displayed Grid Bill Offset worked example, Annual savings is 64,800. Verify Monthly load consumption, Monthly PV generation, and Self-consumption fraction of PV generation and their units before relying on this output.

Remaining monthly bill

For the displayed Grid Bill Offset worked example, Remaining monthly bill is 2,850. Verify Monthly load consumption, Monthly PV generation, and Self-consumption fraction of PV generation and their units before relying on this output.

Solar offset of load

For the displayed Grid Bill Offset worked example, Solar offset of load is 83.3 %. Verify Monthly load consumption, Monthly PV generation, and Self-consumption fraction of PV generation and their units before relying on this output.

Self-consumed energy

For the displayed Grid Bill Offset worked example, Self-consumed energy is 525 kWh/mo. Verify Monthly load consumption, Monthly PV generation, and Self-consumption fraction of PV generation and their units before relying on this output.

Exported energy

For the displayed Grid Bill Offset worked example, Exported energy is 225 kWh/mo. Verify Monthly load consumption, Monthly PV generation, and Self-consumption fraction of PV generation and their units before relying on this output.

Grid Bill Offset accuracy, checks and limitations

  • Grid Bill Offset units check: confirm Monthly load consumption (kWh/mo), Monthly PV generation (kWh/mo), Self-consumption fraction of PV generation (%), Energy charge (import tariff) (currency/kWh), Export credit rate (currency/kWh), Fixed / service charge (currency/mo), Demand charge (currency/mo), and Demand charge reduction from solar (%) before calculating.
  • Grid Bill Offset result check: compare Monthly savings, Annual savings, Remaining monthly bill, Solar offset of load, Self-consumed energy, and Exported energy with the substituted formula steps and the displayed rounding precision.
  • Grid Bill Offset: This is an estimate, not a lender offer. Verify fees, taxes, rate changes, eligibility and repayment terms in the official product documents.

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 Grid Bill Offset

  • Do not mix units for Monthly load consumption (kWh/mo), Monthly PV generation (kWh/mo), Self-consumption fraction of PV generation (%). A unit mismatch changes the input magnitude even when the typed number looks reasonable.
  • Do not replace the displayed Consumption offset = min(gen×self%, bill/tariff) relationship with a different convention without also changing the underlying assumptions; compare like-for-like methods when checking the result.
  • Do not treat Monthly savings = 5,400 from the worked example as a universal answer. It belongs to the displayed example inputs and must be recalculated for the actual case.

When the Grid Bill Offset is useful

Grid Bill Offset is designed for cases where Monthly load consumption, Monthly PV generation, Self-consumption fraction of PV generation, Energy charge (import tariff) are known and you need Monthly savings, Annual savings, Remaining monthly bill. The page keeps the live calculator, calculation method and worked example together so the result can be checked instead of treated as a black-box number.

Use the calculator for the scope described by its inputs and notes. The displayed method is Consumption offset = min(gen×self%, bill/tariff). If the real project or decision needs factors that are not represented here, treat the result as an estimate and add the missing checks separately.

Monthly load consumption and Monthly PV generation: what changes the answer

The worked example uses Monthly load consumption = 900 kWh/mo, Monthly PV generation = 750 kWh/mo, Self-consumption fraction of PV generation = 70 %, Energy charge (import tariff) = 9 currency/kWh. With those values, Monthly savings is 5,400. Changing an input should be interpreted according to that field's unit, range, option and hint rather than by the number alone.

For this calculator, the main input roles are: Monthly load consumption (kWh/mo): Total energy the site consumes from the load in a typical month. Monthly PV generation (kWh/mo): Total PV energy produced in the same month. Self-consumption fraction of PV generation (%): Share of PV generation used directly on-site rather than exported; depends on load-generation coincidence. Energy charge (import tariff) (currency/kWh): Retail rate charged per kWh imported from the grid.

How to sanity-check a Grid Bill Offset result

Start by confirming the entered values and units, then compare the substituted working with the displayed formula or calculation steps. Pay particular attention to Monthly savings, because it is the first worked-example output shown by the live engine.

Finally, compare the result with the assumptions, warnings and related calculators on this page. A nearby calculator can be useful as a cross-check when it measures the same workflow from a different input or output direction.

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Standards, source trail and limitations

References show the method used. Check the current local edition, amendments and project specification before a regulated decision.

Design Assumptions

  • • Uniform tariff (no slabs).
  • • No demand charges.

Engineering Tips

  • ◆Move heavy loads to daylight for higher self-consumption.
  • ◆TOU tariffs typically improve payback by 10–15%.

Warnings

  • ▲Net metering caps vary by state — check DISCOM policy.

Standards & References

CERC / SERC net-metering regulations

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