VoltBudget

Solar panel calculator

How many solar panels do you need?

Enter your daily energy use and the sun you get. The calculator sizes the panels and charge controller to refill your batteries each day.

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Pick the worst season you'll rely on solar.
Panels—
Solar needed—W
Array size—W
MPPT controller—A

Assumes 75% real-world efficiency for heat, shading, dirt and wiring losses. Flat-mounted roof panels often do a bit worse.

Gear that fits this result

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How the solar calculation works

A solar panel's rating is what it produces in lab conditions with full, direct sun. Real installations get less because panels heat up, sit at the wrong angle, collect dust, and lose some power in wiring and the charge controller. Planning on about 75% of the rated output gives you a realistic number.

Solar watts = Daily Wh ÷ (Peak sun hours × 0.75)

Peak sun hours are not daylight hours. They're the number of hours of full-strength sun your location gets, averaged over a day. A summer day in Arizona might deliver 6 or more; a December day in Seattle might deliver 1 or 2.

Worked example

You use 1,200Wh a day and travel in places with about 5 peak sun hours. 1,200 ÷ (5 × 0.75) = 320W. Two 200W panels give you 400W, a little headroom for cloudy days.

Sizing the charge controller

The charge controller sits between the panels and the battery. Its amp rating is about the current it delivers to the battery, so it depends on battery voltage, not panel voltage.

Controller amps = Array watts ÷ Battery voltage × 1.25

The 1.25 factor covers bright, cold days when panels briefly beat their rating. Also check that the controller's maximum input voltage is above your panels' combined open-circuit voltage, especially if you wire panels in series.

Series or parallel panels?

Wiring panels in series raises the voltage, which means less current, thinner wire, and better MPPT performance early and late in the day. Parallel wiring keeps working better when one panel is shaded, which matters on a van roof with vents and racks. Many installs use a mix.

Winter matters most. If you plan to rely on solar year-round, size for your darkest month and expect a surplus in summer. Otherwise plan to top up from your alternator or shore power.

Frequently asked questions

How many solar panels do I need to charge a 100Ah battery?

Refilling a 12V 100Ah lithium battery from 20% takes about 960Wh. With 5 peak sun hours and real-world losses, that's around 250–300W of solar, so two 150W panels or one 300W panel.

Is 200W of solar enough for a camper van?

200W produces roughly 600–750Wh on a good summer day. That covers a fridge, lights and phones, but not a laptop all day or any cooking appliances. Most full-time van setups use 300–600W.

Do I need an MPPT or PWM charge controller?

MPPT controllers cost more but harvest more energy, especially in cold weather, partial light, and with panels wired in series. For anything over about 200W, MPPT is usually worth it.

Do solar panels work on cloudy days?

Yes, but output can drop to 10–25% of normal under heavy cloud. That's why the calculator asks for your worst-season sun hours and why a battery bank with a day or two of storage helps.