How We Estimate | Power Preflight

The three formulas

All three calculators share one small library of formulas, so a watt-hour means the same thing everywhere on the site. Here they are, exactly as implemented.

1. Battery runtime

Used by the Battery Runtime Calculator. We first discount the battery's rated capacity for conversion losses and a safety reserve, then divide by your device's power draw:

usableWh      = batteryWh * (efficiency / 100) * (1 - reserve / 100)
runtimeHours  = usableWh / deviceWatts

Worked example: a 500Wh battery running a 50W device at the default 85% efficiency and 10% reserve gives 500 × 0.85 × 0.90 = 382.5 usable Wh, or 7.65 hours — about 7 hr 39 min.

2. Power station sizing

Used by the Power Station Sizing Calculator. This is the runtime formula run in reverse: given a device and how long you need it to run, what capacity should you shop for?

requiredWh    = (deviceWatts * desiredHours) / (efficiency / 100)
recommendedWh = requiredWh / (1 - reserve / 100)

The recommended figure is deliberately larger than the bare minimum, so a battery that ages or a device that draws a little more than its label won't sink the plan.

3. Solar recharge

Used by the Solar Recharge Calculator. Panels almost never produce their rated wattage in the field, so we estimate realistic daily energy first:

dailyWh        = solarWatts * sunHours * (efficiency / 100)
daysToRecharge = batteryWh / dailyWh

The calculator also shows the "lab-perfect" figure (battery Wh ÷ panel watts) for contrast, so you can see how much optimism a spec sheet contains.

Default assumptions

The defaults below are conservative, mainstream values. Every calculator lets you change them if you know your gear performs differently.

Calculator defaults and what they account for
Assumption	Default	What it accounts for
Conversion efficiency	85%	Energy lost turning stored DC power into the AC or USB power your device uses. Typical AC inverters land around 80–90%; DC-to-DC output usually does better.
Safety reserve	10%	Capacity we assume you won't touch, so the plan doesn't depend on draining the battery to exactly 0% — which is bad for longevity and leaves no margin.
Solar charging efficiency	70%	The gap between a panel's lab rating and field output: sun angle, temperature, partial shading, cable and charge-controller losses, and the battery's charge curve slowing near full.
Peak sun hours	4.5 (adjustable)	One peak sun hour = one hour of sunlight at full standard intensity (1,000W/m²). It is not the same as daylight hours — most of the continental US sees roughly 3–6 peak sun hours per day depending on season and location.

Why estimates differ from reality

A formula can't know your exact hardware, so real-world results will drift from ours. The usual reasons:

Real wattage varies. A "60W" laptop might idle at 15W and spike to 90W. Duty-cycled devices like fridges average far less than their running draw — but surge much higher for a moment at startup.
Batteries age. Capacity fades with charge cycles and time; a two-year-old battery may hold noticeably less than its label.
Temperature matters. Cold reduces usable capacity; heat hurts both output and battery life.
Manufacturer limits. Some units reserve capacity internally, throttle output, or rate capacity under favorable conditions.

This is why we round conservatively and always show the assumptions next to the result. Treat every output as a planning range, not a promise — and when a purchase depends on it, verify with the manufacturer's specs.

Where our wattage data comes from

The figures in the Device Wattage Library are compiled from manufacturer specification ranges and public spec sheets for mainstream devices in each category, then rounded to sensible planning values. They are running watts, not surge watts (we flag surge-prone devices separately), and they describe a typical device of that type — not your specific unit. The most reliable number is always the one on your own device's label or a watt-meter reading.

Our no-fake-reviews pledge

We do not publish ratings, scores, or "hands-on" claims for products we haven't used. If we ever personally test something, the page will say so clearly. Product entries on our comparison pages are category examples with specs to verify, and some links may earn Cynosure LLC a commission — disclosed on every page where it applies and explained in full on the Affiliate Disclosure. Commissions never change the formulas or the recommendations.

How often we update

Wattage figures and product-category specs are reviewed on a rolling basis, and every product entry displays a "last checked" date so you can judge freshness yourself. When a reader reports an error through the Contact page, corrections take priority over new content. Changes to the formulas or defaults themselves are rare, and this page is updated whenever they happen.

The fine print

Our estimates are planning tools with documented limits. The full legal version of that sentence lives in our Terms & Disclaimer; the operative part is below.

Calculations are estimates only. Real runtime depends on battery age, inverter efficiency, device behavior, temperature, surge loads, manufacturer limits, and actual measured wattage. Always verify product specifications before buying or relying on a setup.