Numbers you can check, from sources we name.
Solar Calculator 4U is a set of 20 free solar, EV, and home electricity calculators. Every tool shows the formula it uses, cites the data behind its defaults, and states plainly where the estimate stops being reliable — so you can verify the answer instead of taking our word for it.
Who writes these calculators
Every tool on this site is built and maintained by the Solar Calculator 4U Team. Energy analysts and engineers documenting every formula, source, and limitation behind our calculators. Each calculator is written by whoever researched its formula, checked it against the primary source it cites, and reviewed it again whenever the underlying rates, incentives, or code references change. Each page carries a byline with its publication and last-reviewed dates, so you always know how current the numbers are.
Where our data comes from
We do not invent defaults or copy them from other calculators. Every figure traces back to a primary source we name on the page that uses it:
U.S. Energy Information Administration, Electric Power Monthly, Table 5.6.A — average residential retail price by state. Current vintage: May 2026.
DSIRE, the NC Clean Energy Technology Center database of state and utility programs, checked per state against the current program rules.
EPA eGRID 2024 subregion emission factors, used for CO2 avoided and carbon payback estimates.
NEC 690.8, 690.9, and 706.30 continuous-duty and overcurrent rules; ASCE 7 methodology for snow load.
State-average residential benchmarks before incentives, cross-checked against published EnergySage and NREL cost data.
PVWatts-style derate factors for system losses, with panel degradation applied year by year over the 25-year horizon.
The state rate and incentive table was last reviewed in full during August 2026. We re-check it at least annually, and sooner whenever a federal or state policy change lands.
How we keep results accurate
Accuracy is a process, not a claim. Before a calculator goes live, and again at every review, it has to clear the same four checks:
The formula is published on the page, so you can reproduce the result by hand
Every default traces to a named primary source with a stated vintage
A full worked example runs the numbers end to end
Limitations and appropriate-use notes are stated on the page, not buried
Where a model simplifies — a flat rate instead of time-of-use pricing, a two-segment charging curve instead of a real taper, representative factors instead of full site-specific engineering values — we say so directly on the calculator that does it. You can read the complete method for every tool on our methodology page.
Found something wrong? Please tell us.
We would rather hear about a mistake than leave it live. If a number looks wrong, a rate is out of date, a formula does not match the source it cites, or a result contradicts your own utility bill or installer quote, email us and we will look into it.
We reply to every accuracy report within 2 working days.
It helps if you include the calculator's page address, the values you entered, the result you expected, and the source you are comparing against. Corrections that change a method or a default also move that page's last-updated date, so you can see exactly what changed and when.
The same address is the right one for general questions, correction requests, and press or partnership enquiries.
Why we created it
Solar and EV numbers are often hidden behind cluttered pages, sales quotes, or unclear assumptions — and a quote with no visible arithmetic is impossible to sanity-check. We build each tool around one well-defined question, with an explanation of the method, practical limitations, and no account requirement.
How the tools work
Inputs are processed in your browser. Each calculator uses a method appropriate to its subject, and its page explains the formula and the assumptions behind the result. Every default is a starting point meant to be replaced with your own numbers — your actual rate, your actual quote — which is where the estimate gets genuinely useful.
Privacy by design
Calculator inputs are processed locally in your browser. They are not submitted to an account or stored in our database, and we never ask for your address, phone number, or contact details to show you a result. General site usage may be measured through Google Analytics as explained in our Privacy Policy.
What we do not do
We do not sell leads, and we do not pass your details to installers — there are no details to pass on. Nothing on this site is sponsored placement, and no installer pays to change a default, a ranking, or a result.
Limitations
Solar Calculator 4U provides informational estimates, not professional advice. Results depend on assumptions that vary by home, utility, and equipment, and no calculator can see your roof, your tax position, or your utility's specific tariff. Verify results used for financial, tax, or purchasing decisions with a qualified installer, utility, or advisor.