Solar panel size calculator

Enter the energy you use in a day and the peak sun hours where you are. You get the panel wattage, how many panels to buy, and a chart of what they make at every level of sun against what you use.

Inputs

Watts × hours for everything you run, added up. The battery bank calculator uses the same number.

4–5 is a reasonable baseline (Battle Born). It is about 2.5–3 in winter and 5–6 in summer, so size for the worst month you will use power.

Losses from heat, dirt, shading, wiring and the charge controller. 80% is the same as adding 25% to the panels; PVWatts assumes 86%.

W

The watt rating on the panel you plan to buy.

Solar panels you need

750W

To make 2,400 Wh a day on 4.0 peak sun hours at 80% system efficiency.

These panels cover your day from 3.8 sun hours up. In winter (about 2.5 hours) you would need 1,200 W, or a generator or shore power to fill the gap.

Panels to buy
4 × 200 W
Made on your sun hours
2,560 Wh
In winter (2.5 h)
1,200 W
In summer (5 h)
600 W
What your panels make against what you use
01,0002,0003,0004,0005,0006,00002468Peak sun hours (hours a day)Energy per day (Wh)4.0 sun hours2,560 Wh
  • 4 × 200 W panels
  • Your daily use

Making 2,400 Wh a day on 4.0 peak sun hours at 80% efficiency takes about 750 W of panels, which is 4 panels of 200 W. The chart plots the energy those panels make at each number of sun hours against your daily use: they break even at 3.8 sun hours. These panels cover your day from 3.8 sun hours up. In winter (about 2.5 hours) you would need 1,200 W, or a generator or shore power to fill the gap.

Safety first

Solar panels make voltage whenever there is light, and a long string can reach dangerous levels.

  • Fuse every circuit, including the wire from the panels to the charge controller and from the controller to the battery.
  • Follow the panel and charge controller instructions for maximum voltage and current. Do not connect more panels in series than the controller accepts.
  • Follow your local electrical code and the standard for your vehicle or boat, and use wire rated for sun and weather outdoors.
  • Have a qualified person check the installation before you connect the panels to the battery.

How it works

array watts       = daily energy (Wh) ÷ (peak sun hours × system efficiency)
energy per day    = array watts × peak sun hours × system efficiency
panels to buy     = array watts ÷ panel watts, rounded up
  • Peak sun hours is the day’s sunshine as hours at 1,000 W/m². Find yours for each month in a solar map or a tool such as NREL’s PVWatts.
  • System efficiency is 80% by default. It covers heat, dirt, shading, wiring and the charge controller.
  • The chart plots the energy your panels would make at every level of sun, from 0 to 8 hours, against your daily use. Where the line crosses the dashed one is the break-even sun; below it the panels fall short.

Worked example

A van uses 2,400 Wh a day and you camp where there are about 4 peak sun hours.

  • 2,400 ÷ (4 × 0.8) = 750 W of panels.
  • With 200 W panels that is 4 panels, 800 W in all. They make 800 × 4 × 0.8 = 2,560 Wh a day.
  • Those four panels break even at 2,400 ÷ (800 × 0.8) = 3.75 sun hours. In winter, at about 2.5 sun hours, the same day needs 1,200 W, which is six panels.

Tips from the road

  • Decide how you travel. A summer-only trip can use the summer column; a full-time setup has to cover December.
  • Shade beats everything. A shadow across one panel can cut a whole string. Keep roof vents and antennas clear of the panels.
  • Panels are the cheap part. If you have room, a fifth panel costs far less than a generator, and it makes the bank happier on cloudy days.

FAQ

How many watts of solar panels do I need?

Divide your daily watt-hours by the peak sun hours where you are, then add the losses. For 2,400 Wh a day on 4 sun hours that is 600 W with no losses, and 750 W with the 80% system efficiency the calculator uses. Battle Born suggests adding 20–30% to the plain answer.

What is a peak sun hour?

One hour of sunlight at 1,000 watts per square metre. Five peak sun hours does not mean five hours of daylight; it means the day's sun added up to the energy of five hours at full strength. A 100 W panel makes 100 Wh in one peak sun hour before losses.

What number of sun hours should I use?

Battle Born calls 4 to 5 a reasonable baseline. Skyenergi gives roughly 2.5–3 in winter and 5–6 in summer. If you need power all year, use the winter figure; if you only travel in summer, the summer one. The chips below the result show the array for both.

Why does the calculator use 80% efficiency?

Panels run hot, get dirty and shaded, and lose some energy in the wiring and charge controller. Battle Born adds 20–30% to the panels for this, which is an efficiency of 77–83%, so 80% sits in the middle. NREL's PVWatts assumes 14% losses (86%). Enter your own figure if you know it.

Do I need a bigger battery if I add panels?

Not for the energy, but the two work together. Battle Born suggests about 200 W of panels for each 100 Ah of lithium battery as a starting point. The battery bank calculator shows how the panels and bank behave over a day.

Sources

  1. How to Size Solar Panels for a Deep Cycle Battery System, Battle Born Batteries. Daily watt-hours ÷ peak sun hours = solar watts (2,400 Wh ÷ 4 h = 600 W), then add 20–30% for real-world losses; 4 to 5 peak sun hours is a reasonable baseline.
  2. How to Size a Victron Off-Grid Power System, Alchemy Industrial. Array watts = daily consumption ÷ peak sun hours ÷ system efficiency; 4,150 Wh ÷ 4.5 h ÷ 0.85 = 1,085 W.
  3. Sizing Solar Panels, a Campervan and Off-Grid Guide, Skyenergi. Peak sun hours are roughly 2.5–3 in winter and 5–6 in summer; size for the worst month you will camp in.
  4. Using PVWatts for Preliminary Solar Production Estimates, ExpertCE. Describes NREL's PVWatts, which defaults to 14% system losses.

Formula and sources last checked September 30, 2026. How we test formulas.

  • Off-grid solar system builderList your loads and size the whole off-grid system at once, with battery amp-hours, solar watts, inverter size, main cable gauge and a 24-hour charge chart.
  • Battery bank size calculatorHow many amp-hours of battery you need for an RV, van, boat or cabin, from your daily watt-hours and days of autonomy, with a 24-hour state-of-charge chart.
  • Amp-hour and battery runtime calculatorHow long a battery runs a load. Enter amp-hours, voltage, battery type and watts. Covers LiFePO4 and AGM, inverter losses and lead-acid discharge-rate loss.
  • DC wire size and voltage drop calculatorFind the AWG wire size for a 12, 24 or 48 V DC circuit from current and distance. Checks voltage drop and ampacity with NEC and Coast Guard tables.
  • Solar charge controller size calculatorWhat size solar charge controller you need, MPPT or PWM: its amp rating, the voltage it must take on a cold morning and the PV fuse, from your panels.
  • Series and parallel wiring calculatorWire batteries or solar panels in series, parallel or both and see the voltage, capacity and current you get, with a wiring diagram that updates as you type.

Safety note: Estimates only. Equipment ratings vary by manufacturer, so always follow the manufacturer’s instructions and ratings, and ask a qualified professional when safety is at stake.