Method: how the numbers here are produced
Every calculator on this site shows its formula, and every figure with a source lists where it came from and when it was read. This page explains the whole chain, including the parts that are estimates rather than measurements.
In one paragraph
The arithmetic is exact and the inputs are estimates. Appliance draw is a category typical value, sun hours are a location average, and depth of discharge is a planning assumption. That is why the site calls its output a scale rather than a measurement, and why every page names the assumption it is standing on.
What this site does
It answers one question: how much system does a US household need. Calculators return a number with the formula visible; guides explain the trade-offs and attribute each figure.
It deliberately does not do three things. It does not sell hardware, so no number is shaped by a margin. It does not print product prices, because a stale price is worse than no price. And it does not publish claims that would need hands-on measurement, because this site does not buy or test equipment.
Where the figures come from
Each type of figure has a stated source class, and the class decides how much trust it deserves. Anything marked as a category typical value is a planning figure, not a reading from your appliance.
| Figure | Source class | How to treat it |
|---|---|---|
| Appliance running and starting watts | Category typical value | A scale, not a reading; check your own label |
| Peak sun hours | Location average | Use a local dataset for your own address |
| System efficiency and depth of discharge | Planning assumption | Stated on each page that uses it |
| Certification requirements | Standards body | A conditional fact, not a warranty of compliance |
| Product capacity and output | Manufacturer specification | Verified before publishing; brand-locked figures change |
Sources are the Department of Energy, EIA, NREL, ENERGY STAR, UL Solutions and manufacturer sheets; each page lists what it used and when.
How the calculators are built
Every calculator on the site is a single plain JavaScript function with no server behind it. That has one practical consequence for you: the formula runs in your browser, so nothing you type is transmitted anywhere.
The formulas are stated on the pages rather than implied. Daily energy is running watts multiplied by hours. Nameplate storage is usable energy divided by depth of discharge. Array size is daily energy divided by sun hours and efficiency. Charge controller current is array watts divided by battery voltage with a margin. Runtime is usable energy divided by load.
Each function is tested against hand-calculated cases before publication, including zero input, out-of-range input and a load no single product can cover. If a published number disagrees with the calculator, the page is what gets fixed.
Where the assumptions can be wrong
Being explicit about this is more useful than a confidence claim. These are the four that move results the most.
| Assumption | Where it fails | What to do instead |
|---|---|---|
| Appliance draw is a typical value | Old or unusually large appliances | Read the energy label or a plug-in meter |
| Sun hours are a location average | Shade, unusual orientation, unusual year | Use a local dataset and check the roof |
| Depth of discharge is a planning figure | Chemistry and age of the pack differ | Use the manufacturer's usable-energy figure |
| Duty cycle is the category norm | Hot rooms, frequent door openings | Expect the worst case, not the average |
Two of those deserve emphasis. A fridge in a 90 °F garage cycles far more than the category figure assumes, and a battery that has lost a fifth of its capacity still carries its original label. Both push the real number below the calculated one, and neither is visible in a specification sheet.
How corrections are published
When a figure changes, the page changes and the changelog records what changed and why. Corrections are not made silently, because a silent edit makes every other number on the site impossible to audit.
Each page carries two dates: published and last updated. The updated date moves only when the content changes materially, not when a stylesheet or a navigation element is touched. That is a small discipline, but it is the difference between a date that means something and a date that is decoration.
A useful error report names the page, the figure and a source for the correct value. That is enough to check whether the fault is in the data or the formula, and to publish the correction.
What this site cannot tell you
It cannot tell you whether your wiring is adequate, whether your panel can take another circuit, or whether your utility will approve an interconnection. Those are questions for a licensed electrician and for the authority having jurisdiction, and a calculator that pretended otherwise would be dangerous.
It cannot tell you what an appliance actually draws in your house, because that depends on the unit, the voltage and your habits. It gives the scale, so a professional conversation can be specific.
It cannot replace a site assessment: shade, roof angle, snow and the state of your service are local facts no page can see. Use these numbers to ask better questions, then have the local work verified.
The full list of how money is earned is on the disclosure page, and what the site does and does not collect is on the privacy page. To start with a number, the load calculator turns a list of appliances into the daily energy figure that everything else depends on.
Questions
Do you test the products you list?
No. This site does not buy or test hardware, and it does not publish claims that would require hands-on measurement. Product figures are taken from manufacturer specification sheets and public certification records, with the source and the date it was read recorded next to each value.
How accurate are the calculators?
The arithmetic is exact and every formula is stated on the page that uses it. The inputs are estimates: appliance draw is a category typical value, sun hours are an average, and depth of discharge is a planning assumption. Treat the output as a scale, not a measurement of your house.
What happens when a number on the site turns out to be wrong?
The page is corrected and the change is recorded in the public changelog with the reason. Retractions are public rather than silent, because a silent edit makes every other number on the site impossible to audit.
Why does the site not show prices?
Because a printed price is wrong within days and a stale price is worse than none. Products link to the retailer so the current price is the one you see, and no page on this site states a product price.
Who writes this site?
It is an independent publication about US home energy sizing. It is not a utility, an installer or a manufacturer, and it does not sell installation services. The limits that come with that role are listed on this page rather than hidden. Who publishes it is on the about page.
- US Energy Information Administration, Units and calculators explained — the unit conventions every formula here follows. Read 2026-11-07.
- National Renewable Energy Laboratory, PVWatts Calculator — the sun-hour source class used for every array figure. Read 2026-11-07.
- US Department of Energy, Solar integration: inverters and grid services basics — inverters shut down after a large or long disruption. Read 2026-11-07.
Nothing on this site is professional electrical advice. Fixed wiring, battery banks, changeover equipment and any 240 V circuit need a licensed electrician and, in most places, a permit. Verify requirements with the authority having jurisdiction before buying or installing.
Figures on this site are planning values, not measurements of your home. Where a page cites a source, that source governs; where it states an assumption, the assumption is yours to check.