Our Methodology

How Solar & Battery Guide ranks sources, picks calculator defaults from EIA, NREL PVWatts and LBNL data, reviews pages and discloses its use of AI tools.

Last updated: October 18, 2026

This page explains where our facts come from, how we choose the default numbers in our calculators, how often we review pages and how we use AI tools. It complements our editorial policy.

Sourcing hierarchy

We rank sources in this order and use the highest-ranked source available for each claim:

Law and official guidance. Statutes (for example, Public Law 119-21 for the end of the 25D credit), IRS forms, instructions, notices and FAQs, and state statutes.
Regulators and tariffs. State utility commission decisions and the utility tariff sheets that actually set bill credits and rates.
Federal agencies and national labs. The US Department of Energy, NREL, EIA, Lawrence Berkeley National Laboratory, EPA, FTC and CFPB.
Standards bodies and official program pages. Organizations that set equipment and installer standards, and the official pages of state or utility incentive programs. We also use the DSIRE database to find programs, then confirm them on the program's own page.
Peer-reviewed or primary research. Studies with published methods and data.

We do not use other publishers, marketplaces or installer marketing as sources of fact. If two good sources disagree, we say so and explain which one we follow and why. If we cannot verify a claim, we leave it out.

Dates and "as of"

Rules for taxes, net metering and incentives change often. Time-sensitive facts carry an "as of" date, and each source in a guide's source list shows the month we checked it. For example, our federal credit coverage is written "as of October 2026" and links to the IRS's own pages.

How calculator defaults are chosen

Each calculator starts with defaults so it shows a meaningful result before you type anything. Defaults are reasonable starting points, never promises.

Input	Default	Source
Electricity rate	17.5¢/kWh national; state averages in presets	EIA retail price data (Electric Power Monthly, Table 5.6.B)
Yearly output per kW	1,350 kWh/kW national; higher in sunny states	NREL PVWatts
System losses	14%	NREL PVWatts default
Panel degradation	0.5% per year	NREL degradation research (median near 0.5%)
Installed price	$3.00 per watt	Documented example, sanity-checked against LBNL Tracking the Sun
Federal tax credit	None	IRS: 25D ended for installations completed after 2025
Export credit	100% of retail nationally; lower in net-billing states	State commission decisions and utility tariffs

A few principles guide those choices:

Prefer conservative over flattering. When data gives a range, we lean to the middle rather than the most favorable value.
Show the default and its source on the tool page so you can replace it.
Use your numbers when you have them. Your bill, your quotes and a PVWatts run for your address beat any national average.

EIA reported a US residential average of about 18.2¢ per kWh for January through July 2026 in its Electric Power Monthly (opens in a new tab), slightly above our 17.5¢ national default. We keep defaults rounded and review them on a schedule rather than chasing each monthly figure.

How the calculators compute

Each tool page writes out its formula and assumptions and includes a worked example using the default inputs, so you can reproduce the result by hand. Calculations run in your browser; nothing you enter is sent to us. Calculators use yearly or monthly averages, not hourly utility data, and they do not model shading or every tariff detail. Those limits are listed on each tool page.

Review and update cadence

Every guide and tool page is reviewed at least every six months.
Event-driven updates happen when a law, IRS guidance, commission decision or tariff we cover changes. These get priority over new content.
Calculator defaults are reviewed at least twice a year and when major datasets are released.
Links and sources are rechecked during each review; dead or moved sources are replaced with the official current version.

Each page shows when it was last updated and reviewed. Substantive changes are described on the page, as explained in our corrections policy.

Use of AI tools

Research and drafting on this site are assisted by AI tools. They help us summarize source documents, outline pages and produce first drafts. Every page is then checked by our editorial team against the primary sources it cites, and worked examples are recomputed with our own formulas. We do not use AI to invent facts, statistics, quotes, reviews or people. The editorial team is responsible for everything we publish.

Independence

Our methodology is not influenced by any installer, lender or manufacturer. We take no referral fees, and no company can pay to change a default, a ranking or a conclusion. See our about page for how the site is run.

Frequently asked questions

Why does the calculator use 17.5¢ per kWh by default?

It is a national starting point based on EIA residential price data; EIA reported a US average of about 18.2¢ per kWh for January through July 2026. State presets use state averages instead. Your own bill is always a better input, so divide your energy charges by the kWh you used.

Where does the 1,350 kWh per kW figure come from?

It is a national default for yearly output per kW of panels, in line with NREL PVWatts estimates for typical US locations. Sunny states such as Arizona get higher presets. For your exact address, run NREL PVWatts and enter its result.

How do you choose the installed cost default?

The cost tools start at $3.00 per watt as a documented example, which we sanity-check against Lawrence Berkeley National Laboratory's Tracking the Sun market data. Prices vary a lot by state, installer and equipment, so we recommend entering the price per watt from your own quotes.

Do you disclose AI use?

Yes. Research and drafting are assisted by AI tools, and every page is checked by our editorial team against the primary sources it cites before it is published. We do not use AI to invent facts, quotes or reviews.

How often do you update calculator defaults?

We review defaults at least twice a year and whenever a major source publishes new data, such as EIA's annual electricity price tables or a new LBNL report. Changes to defaults are recorded on the affected tool page.

On this page

25D credit (Residential Clean Energy Credit): A 30% federal tax credit homeowners could claim for solar and batteries they paid for. It ended for expenditures made after December 31, 2025; the IRS counts the expense when installation is completed.
48E credit: A federal business tax credit for clean electricity property. Companies that own leased or PPA systems on homes may claim it, subject to construction deadlines and sourcing rules. The homeowner does not claim it.
Avoided cost: What it would cost a utility to produce or buy the same power elsewhere. Some states pay rooftop exports at an avoided-cost rate, which is far below retail.
Dealer fee: A charge a solar lender takes from the loan, usually built into the amount you borrow so the advertised interest rate looks low. The CFPB has warned these fees can add tens of percent to the price.
Degradation: The slow loss of a panel’s output over time. NREL research puts the median around 0.5% per year, and most panel warranties guarantee roughly 80–90% of original output after 25 years.
Escalator: A yearly percentage increase written into a lease or PPA, often 0–3%. A 2.9% escalator raises the payment by about 100% over 25 years.
FEOC rules: Federal “foreign entity of concern” sourcing limits added in 2025 that a business must meet to claim 48E credits on projects starting construction in 2026 or later.
Interconnection: Your utility’s approval to connect a solar system to the grid. Most utilities require an application before installation and a final permission to operate (PTO) before you turn it on.
Inverter: The device that turns the direct current (DC) from panels or batteries into the alternating current (AC) your home uses. Types include string inverters, microinverters and hybrid inverters that also manage a battery.
Islanding (backup mode): When a solar-plus-battery system disconnects from the grid during an outage and keeps powering your home. Ordinary grid-tied solar without a battery shuts off in an outage for line-worker safety.
kW (kilowatt): A unit of power: how fast electricity is made or used at one moment. Solar system size is quoted in kW of panel capacity (DC). A 7 kW system has panels rated 7,000 watts in total.
kWh (kilowatt-hour): A unit of energy: using 1 kW for one hour. Your electric bill charges you per kWh, and solar output and battery capacity are measured in kWh.
Microinverter: A small inverter mounted under each panel. Shade on one panel affects only that panel, and each panel can be monitored separately. Usually costs a little more than one central string inverter.
NABCEP certification: A voluntary professional certification for solar installers from the North American Board of Certified Energy Practitioners. It is a useful signal, but you should also check state contractor licensing.
NEM 3.0: The common name for California’s Net Billing Tariff, which applies to new rooftop solar customers of PG&E, SCE and SDG&E who applied on or after April 15, 2023. Exports earn hourly values far below retail rates.
Net billing: A billing rule where exported solar earns a credit set by a separate, usually lower, export rate rather than the full retail price. California’s Net Billing Tariff (often called NEM 3.0) is the best-known example.
Net metering: A billing rule where each kWh your panels send to the grid earns a credit equal (or close) to the retail price you pay. Credits offset power you buy later, often rolling over month to month.
Offset: The share of your yearly electricity use that solar is designed to produce. A 100% offset aims to match a year of use; under low export credits a smaller offset can pay back faster.
Payback period: How many years of bill savings it takes to recover what you paid for a system. A shorter payback means a better investment, all else equal.
Peak sun hours: Daily sunlight expressed as the number of hours at full-strength sun (1,000 watts per square meter). Five peak sun hours means the day’s sunlight equals five hours of full sun.
PPA (power purchase agreement): A contract where a company owns the system and you pay for the electricity it produces at a set price per kWh, often rising each year.
PTO (permission to operate): The utility’s final go-ahead to switch on a grid-connected system. Running a system before PTO can violate your utility agreement.
Round-trip efficiency: The share of energy you get back out of a battery compared with what went in. About 90% is typical for home lithium batteries.
Solar lease: A contract where a company owns the panels on your roof and you pay a fixed monthly amount, often with a yearly increase (escalator), usually for 20–25 years.
Specific yield: Yearly energy a solar system produces per kW of capacity, in kWh/kW. It depends on sunshine, tilt, direction and shade. Sunny Southwest roofs often reach 1,600–1,800; cloudier northern roofs 1,100–1,300.
Time-of-use (TOU) rate: An electricity plan where the price per kWh changes by time of day, with the highest prices in late afternoon and evening. Batteries save money by covering those peak hours.
True-up: The annual settlement on a net metering or net billing account. Charges and credits from the past 12 months are totaled, and you pay any balance owed. Leftover credits may be paid out at a low rate or lost.
Usable capacity: The share of a battery’s energy you can actually draw. Spec sheets list it in kWh; compare usable, not total, capacity between batteries.