Solar changes the timing and shape of an electric bill
A solar array produces electricity when sunlight reaches the modules. The household may use some of that power instantly and export surplus electricity to the grid. At other times, the home may still import power. The utility bill reflects the tariff and how imports, exports and fixed charges are treated—not just the annual production number.
Self-consumption and exported electricity
When solar generation coincides with household demand, it can reduce electricity imported from the utility. Surplus generation may be exported, stored in a battery or curtailed depending on system design and utility rules. Export compensation may differ from the retail price paid for imported electricity. Do not assume each exported kWh offsets one retail-priced kWh.
Fixed charges and other bill components
Customer charges, minimum bills, demand charges, taxes, riders and other fees may remain after solar is installed. Some charges are based on usage; others are not. A solar proposal should show which parts of your bill the system is expected to reduce and which are likely to remain.
Time-of-use tariffs
Under a time-of-use tariff, electricity prices vary by time period. Solar generation may align with some high-price periods, but not necessarily all of them. Battery storage can change when power is used or exported, but it also adds equipment cost, conversion losses, maintenance and possible replacement expense.
Electric-bill input worksheet
Before using an annual calculator, identify which parts of your bill actually change when one fewer kWh is imported or one extra kWh is exported. The categories below are common examples; your tariff may use different names or rules.
| Bill component | Question to ask | Calculator treatment |
|---|---|---|
| Usage / energy charge | What marginal cents-per-kWh charge is avoided when consumption falls? | May be used as an import-rate input if the tariff supports that interpretation |
| Fixed customer charge | Does this remain regardless of energy use? | Not modeled; do not count it as solar savings |
| Demand charge | Is the bill based on peak kW in a defined interval? | Not modeled; annual kWh alone cannot calculate it |
| Time-of-use periods | What are peak, off-peak and shoulder prices and time windows? | Not modeled in this annual-share calculator |
| Export credit | How are exported kWh credited, and do the rules vary by time or month? | Enter the applicable credit only as a simplified annual scenario |
| Taxes, riders and minimum bills | Which charges remain or vary with imported energy? | Not modeled unless they are explicitly included in a visitor-entered marginal rate |
If your tariff has tiered, seasonal or time-varying prices, ask the utility or installer for a month-by-month model rather than forcing it into one annual rate.
Data to gather before estimating savings
- 12 months of utility bills, including kWh usage and rate-plan details.
- The current tariff and interconnection/export compensation rules from the utility.
- A production estimate based on actual address, roof orientation, tilt, shading and local weather data.
- The installed cash price and separate costs for storage, electrical upgrades or roof work.
- Financing terms, incentives and tax eligibility confirmed from current official sources and qualified advisers.
Why a calculator can only provide a first pass
A simple calculator can compare scenarios but cannot recreate the utility's billing engine or a site-specific solar model unless it has accurate inputs and validated data. Use calculator outputs to ask better questions, not as a promise of savings. Ask the installer to show monthly imports, exports, bills and assumptions—not only annual offset.
Official information matters
Check the current tariff and interconnection rules with your serving utility or state utility commission. Tax rules and incentive programs can change and depend on timing and eligibility; verify them with official government sources and a qualified tax professional. This guide does not determine eligibility or provide tax advice.
Primary-source resources
- US Department of Energy: Will I Save Money with Solar Energy?
- US Department of Energy: Homeowner's Guide to Going Solar
- NLR PVWatts Calculator — use a location-specific model and check its assumptions, losses and expected variability; results are estimates rather than guarantees.
- US Department of Energy: Evaluating Your Utility Rate Options — explains energy, demand and fixed charges.
Important source limitation: The DOE Homeowner’s Guide is linked here for roof suitability, project planning and utility/export considerations. Its live financing section still contains historical 26%/22% federal tax-credit figures and an old 2024 expiry statement. Do not use that section to determine current tax eligibility. For the current Section 25D termination and installation-timing explanation, consult the IRS FAQ for the 2025 law change.
Pre-publication requirement: Before this guide is published, assign an accountable author, verify technical statements against current primary sources, record any review that actually occurred and its date, and document material corrections. No author or technical reviewer has been assigned to this draft.