Formula used
Bill Offset (%) = Annual Bill Savings ÷ Original Annual Bill × 100 Usage Offset (%) = Solar Energy Used On-Site ÷ Annual Consumption × 100 Annual Savings = Original Annual Bill - New Annual Bill Remaining Bill = Import Cost + Standing Charges + Fixed Fees - Export Credit Simple Payback = Net System Cost ÷ First-Year Net Savings NPV = Σ Cash Flow in Year t ÷ (1 + Discount Rate)^t
The calculator separates financial offset from energy offset. Standing charges can remain after solar installation. Export credits can also change the final result.
How to use
- Enter your bill, usage, and electricity tariff.
- Add the solar system size or known production.
- Set self-consumption, export, and battery assumptions.
- Enter costs, incentives, and financing details.
- Use monthly values when precise data is available.
- Select calculate and review every result section.
Use recent bills for the best starting estimate. Compare results with and without a battery. Confirm final decisions using installer-specific production modelling.
Worked example
| Example input | Value | Purpose |
|---|---|---|
| Annual electricity use | 5,000 kWh | Sets household demand. |
| Import tariff | £0.28 per kWh | Prices avoided grid imports. |
| Standing charges | £220 yearly | Shows costs solar cannot remove. |
| Annual solar production | 4,500 kWh | Sets available solar energy. |
| Direct self-consumption | 60% | Estimates immediate solar use. |
| Export tariff | £0.15 per kWh | Prices exported electricity. |
Frequently asked questions
What does solar bill offset mean?
It measures the bill savings created by solar. It includes avoided imports and export credits. Fixed charges may prevent a complete offset.
Why is bill offset different from energy offset?
Energy offset compares solar use with consumption. Bill offset compares money saved with prior costs. Different tariff rates create different percentages.
Does the calculator include standing charges?
Yes, daily standing charges are included. Monthly fixed fees are also supported. These charges normally remain after solar installation.
How is solar production estimated?
Production uses system size and local yield. Orientation, tilt, losses, and age adjust it. Known annual production overrides this estimate.
How does battery storage change savings?
A battery stores surplus daytime solar. Stored energy can reduce later grid imports. Efficiency and cycling limits reduce usable output.
What is net metering?
Net metering credits exports at the import rate. Net billing uses a separate export tariff. Local schemes can apply additional rules.
Can I enter monthly data?
Yes, monthly consumption and production are optional. Entered values replace the seasonal default profiles. This improves results for seasonal households.
What does self-consumption mean?
Self-consumption is solar used on-site. Direct use and battery output both count. Higher self-consumption often increases import savings.
How is payback calculated?
Simple payback divides net cost by annual savings. The detailed projection includes future costs and financing. Actual returns can vary considerably.
Does the result include panel degradation?
Yes, annual panel degradation affects future generation. System age also adjusts current estimated output. Battery degradation is modelled separately.
Is this suitable for final investment decisions?
Use the result as an informed estimate. Production, tariffs, and equipment performance can change. Obtain site-specific quotes before making commitments.