How we calculate this

A short, plain-English account of the math, the sun data, the November 2026 audit, and how those numbers compare with real vans and RVs. These are planning estimates, not electrical advice.

Last checked: November 2026.

The idea in one minute

We add up what you run in a day, in watt-hours. We size the battery so it can cover that use for two days with no charging, using 80% of a LiFePO4 bank or 50% of an AGM bank. We size solar for the weakest month you said you travel, using NASA monthly sun averages for a flat roof at the place you picked, after typical real-world losses. Alternator or shore charging, if you enter it, is subtracted before solar is sized.

That is safer than a single “sun hours” guess. A yearly average hides winter. In Hailey, Idaho, a flat panel gets about 7.7 peak sun hours in July and about 1.5 in December. Sizing to the average would leave a year-round traveler short for months.

Sun data

The 74 locations use NASA POWER all-sky surface shortwave irradiance (ALLSKY_SFC_SW_DWN), monthly climatology from January 2001 through December 2025, on a horizontal surface. That matches most RV and van panels, which sit flat on a roof. A tilted portable panel will do better in winter than these figures. NASA’s solar grid is about 1° (roughly 100 km), so a mountain valley or a coast can differ; you can enter custom sun hours if you have a local number.

Losses we count

With the defaults and an MPPT controller plus lithium batteries, about 76% of the panel’s rated output × sun hours reaches the battery. The defaults are 10% for hot panels, 3% dirt, 0% shade (raise this if you park under trees), 3% wiring, 3% mismatch and aging, 3% for the MPPT, and 5% for lithium charging (15% for AGM). PWM controllers are much less efficient on typical panels, and we use 75% for those.

The November 2026 accuracy audit

The math was independently audited in November 2026: a separate Python script rebuilt the energy, battery, solar, inverter, fuse and cable math and compared it with the site. After a few fixes — the important one being that a kettle or hair dryer marked “one at a time” had been left out of inverter sizing — the site matched the hand calculation on every output in 11 scenarios. Sun figures were re-fetched from the NASA POWER API for seven cities and matched the bundled data. Cable sizing uses the ABYC 90 °C copper table and a 3% voltage-drop limit.

We do not claim the calculator is a substitute for an installer or a manufacturer’s manual. Results use monthly averages; a run of cloudy days is why the battery has a reserve. Fuse and cable suggestions are a starting point, and the manual wins when it disagrees.

Cross-check against real rigs

We then compared the presets and recommendations with documented factory vans, popular kits and owner-reported DIY builds (for example a 400 W / 200 Ah weekender, FarOutRide’s measured 360 W / 200 Ah van, Victron-style 800 W / 400 Ah systems, and fifth-wheel banks around 300–400 Ah with 800–1,000 W of solar).

The recommendations line up with those common builds when you enter the same season and charging the owners actually use. They look “high” mainly when a factory Class B or C van with a large lithium bank and only 100–250 W of solar is compared with a solar-only, two-day, worst-month plan — those vans rely on a second alternator. One real gap was fixed: the Class C preset had assumed a propane fridge and omitted fridge energy. Most new Class C rigs use an 8–10 cu ft 12 V compressor fridge, which we now include (remove it if yours still runs on propane).

What this is not

It is not a promise that your rig will perform exactly as shown. It is not a design for roof air conditioning as the main load, all-electric water heating, or other uncommon setups, though you can add those loads yourself. It is not electrical, engineering or legal advice. If you are not confident with high-current DC wiring, hire a qualified RV or marine electrician. See our Terms of Use.

Open the calculator Peak sun hours by place