This page documents exactly how the numbers on Green Energy Calculators are produced, so you can check them, reproduce them, or swap in your own inputs. Policy last checked: September 29, 2026.
What federal tax rules do the calculators use for 2026?
For systems a homeowner buys and installs in 2026, the federal credit is $0. The 30% Residential Clean Energy Credit (IRC §25D) ended for expenditures made after December 31, 2025, under the One Big Beautiful Bill Act (Public Law 119-21, signed July 4, 2025). The IRS treats an expenditure as made when installation is completed. Source: IRS FAQs on the OBBB changes to §§25C, 25D, 25E, 30C, 30D.
| Federal incentive | What it covered | 2026 status |
|---|---|---|
| §25D Residential Clean Energy Credit | Owned solar, batteries, geothermal, small wind | Ended for installs after Dec 31, 2025 |
| §25C Energy Efficient Home Improvement Credit | Heat pumps, insulation, windows, doors, audits | Ended after Dec 31, 2025 |
| §30D / §25E clean vehicle credits | New EVs (up to $7,500) / used EVs (up to $4,000) | Ended for vehicles acquired after Sep 30, 2025 |
| §30C refueling property credit | Home EV chargers | Ended for chargers placed in service after Jun 30, 2026 |
| §48E / §45Y | Third-party-owned solar (leases, PPAs), business and farm solar | Available for projects that began construction by Jul 4, 2026 or are placed in service by Dec 31, 2027 |
Lease and PPA calculators note that the system owner may pass part of the §48E value to you through a lower price; we never assume a specific pass-through.
How do you size a solar system?
System size (kW DC) = annual electricity use (kWh) ÷ (peak sun hours × 365 × 0.82)
- Annual use is the sizing input. It comes from your bill: monthly bill ÷ electricity rate × 12, or better, the kWh on your utility statements.
- Square footage is only a fallback proxy when usage is unknown: about 0.45 kWh per square foot per month. EIA reports the average US residential customer used about 899 kWh a month in 2022; we assume that corresponds to a ~2,000 sq ft home. Real homes vary widely with climate, heating fuel, appliances and occupants, so sq-ft pages show a 70% / 100% / 130% usage table and tell you to use your own kWh.
- Peak sun hours are approximate state averages based on NREL solar resource data (NSRDB); they are not address-specific. Use NREL PVWatts for your roof.
- 0.82 derate covers inverter, wiring, soiling, temperature and mismatch losses.
- Panels (400 W) = kW ÷ 0.4, rounded up unless within 0.1 panel of a whole number. National pages give one headline count at 4.5 peak sun hours plus one range: the same usage at 6.0 (sunny Southwest) and 4.0 (cloudy Northwest/Northeast) peak sun hours. State pages use ±10% production for roof orientation and shading.
How do you estimate cost?
We use $3.00 per watt installed before incentives — a blended 2026 US price. Competitive marketplace quotes are often lower (EnergySage reported about $2.60/W in mid-2026), while full-market medians from installer surveys are higher. A typical 8 kW system is therefore about $24,000. Your installer’s quote is the best input; every calculator lets you change it.
How do you estimate savings and payback?
- Year-1 savings = your monthly bill × 12 × a bill-offset share set by your state’s net-metering rules (or, on state pages, annual production × rate × the same share).
- Simple payback = installed cost ÷ year-1 savings.
- 25-year net savings = the sum of yearly savings with 3% annual electricity-price increases and 0.5% annual panel degradation, minus installed cost.
| Net-metering category | Share of bill/production value counted | Example states |
|---|---|---|
| Full retail net metering | 87% (fixed charges remain) | MA, NJ, NY, CO, MD, MN |
| Reduced export credit / net billing | 70% | AZ, NV, NC, IL, MI, UT, ID |
| Low export credit (battery usually needed) | 60% | CA (NEM 3.0), HI |
| No statewide net metering | 55% | AL, GA, LA, MS, TN, TX, SD |
| US blended average | 75% | — |
These shares are editorial estimates of how much value a right-sized system delivers under each regime, not tariff calculations. Export rules vary by utility; each state page names its rule and links to DSIRE for verification.
Where do electricity rates come from?
State average residential retail prices from the U.S. Energy Information Administration. The US average used for national pages is 18.19¢/kWh — source: EIA Electric Power Monthly, Table 5.6.B (year-to-date through July 2026), residential average retail price (July 2026). Monthly prices run higher in summer (EIA’s July 2026 US residential average was 18.31¢/kWh), so annual estimates use the year-to-date average. Rates are stored in one data file (data/model.json and data/states.json) and refreshed from EIA’s Electric Power Monthly when new data is published.
What don’t the estimates include?
- State or utility rebates and performance payments (listed on each state page, not subtracted automatically)
- Financing costs (see the solar loan calculator)
- Time-of-use rate structures, demand charges and minimum bills
- Battery value, which depends heavily on the utility tariff (see the battery storage calculator)
- Roof replacement, tree removal or main-panel upgrades
How can I report an error?
Email greenenergycalc@gmail.com with the page URL and the source for the correct figure. We fix confirmed errors and update the page’s “updated” date.
Estimates only — not tax, legal or financial advice.