How much battery and solar does your van need?

How this works

The calculator adds up your daily energy use in watt-hours, then sizes each part of the system from it. A watt-hour (Wh) is one watt used for one hour: a 10 W light on for 5 hours uses 50 Wh. An amp-hour (Ah) is how much a battery holds; at 12 V, 1 Ah is about 12 Wh.

The formulas

• Daily use (Wh) = Σ watts × hours for DC items + Σ watts × hours ÷ 0.90 for AC items. AC items go through the inverter, which loses about 10% as heat.
• Battery (Ah) = daily Wh × days without charging ÷ (system volts × usable share). We use 80% of a lithium (LiFePO₄) battery and 50% of an AGM battery, then round up to the next 10 Ah.
• Solar (W) = daily Wh ÷ (peak sun hours × 0.74), rounded up to the next 50 W. The 0.74 is how much of a panel’s label rating we expect to reach the battery: 0.86 × 0.90 × 0.95.
• Inverter (W) = the AC items that can run at the same time, added together, × 1.25, rounded up to a common inverter size. It is never smaller than your single biggest AC item.

With the example list (about 1,400 Wh a day at 12 V, two days without charging, Phoenix’s year-round average of 5.8 sun hours), that is about 300 Ah of lithium or 470 Ah of AGM, 350 W of solar and a 1,200 W inverter. These are example inputs, not a recommendation for your van.

Why the fridge uses its average, not its label

A 12 V compressor fridge switches on and off all day. The amps on its label are what it draws while the compressor runs, not over a day. Dometic lists 79 kWh a year for a CFX3 45 in testing, which is about 9 W on average, against 8.2 A (about 98 W) on the label. In hot weather, or if you open the lid often, it runs more. We start at 15 W for 24 hours. If your fridge’s manual lists Ah per day or kWh per year, use that instead: divide kWh per year by 8.76 to get average watts.

The losses we assume

• Inverter: 90% efficient. Victron lists 87–92% maximum efficiency for its Phoenix 12 V inverters, and real use is often a little lower. An inverter also uses power when it is on with nothing plugged in (8 W for Victron’s Phoenix 12/1200, which is about 190 Wh a day). Add the “Inverter left on” item if you leave it on.
• System losses: × 0.86. NREL’s PVWatts model uses 14% by default for dirt, shading, wiring, connections, mismatch, panel ageing and downtime.
• Heat: × 0.90. Panels make less power when hot, and flat roof panels get hot. PVWatts uses −0.47% per °C for standard panels; a panel about 21 °C above the 25 °C test temperature loses about 10%.
• Charging: × 0.95. Our allowance for the charge controller and the battery itself. Lead-acid batteries lose more than lithium when charging.
• Usable battery: 80% lithium, 50% AGM. Lithium makers such as Battle Born rate 100% as usable; we keep 20% in reserve for cold, ageing and the low-voltage cutoff. AGM makers such as Lifeline advise keeping average discharge to 50% or less for long life.
• Inverter margin: +25%. Our planning margin. Inverters can’t give their full rating when warm (Victron’s 12/1200 drops from 1,000 W at 25 °C to 900 W at 40 °C), and running flat out all the time is hard on them.

Surge: the moment things start

Motors and compressors, such as microwaves, blenders, AC fridges and power tools, can draw several times their running watts for a moment when they start. Inverters list a separate peak or surge rating for this; Victron’s Phoenix 12/1200, for example, lists 1,000 W continuous and 2,400 W peak. Check the peak rating covers your biggest starting load.

Peak sun hours and flat panels

Peak sun hours are the energy the sun delivers to a flat surface in a day, written as hours of full 1,000 W/m² sun. The place list uses NASA’s 25-year monthly averages for a horizontal surface, which matches panels lying flat on a van roof. Tilting panels toward the sun gets more in winter, but most van panels stay flat. Compare places and months in the sun hours by month tool.

What this does not cover

Charging from driving (a DC-DC charger or alternator), shore power and generators are not included, so this is a sun-only plan. Battery heating, very cold weather, shading from roof racks or trees and ageing panels can all reduce what you get. Have a qualified installer check the design, and follow your equipment’s manuals.

Sources

• NASA POWER: monthly all-sky surface irradiance on a horizontal surface (ALLSKY_SFC_SW_DWN), 2001–2025 average, retrieved 2026-11-01.
• NREL, PVWatts Version 5 Manual (2014): default 14% system losses and −0.47%/°C temperature coefficient.
• Victron Energy, Phoenix Inverter 180–1200 VA datasheet: efficiency, continuous, peak and zero-load power.
• Battle Born Batteries, 100 Ah LiFePO₄ datasheet: 100% rated depth of discharge.
• Lifeline Batteries, AGM FAQs: 50% depth of discharge for best cycle life.
• Dometic CFX3 operating manual, technical data: 79 kWh a year and 8.2 A rated current for the CFX3 45.
• Starlink Mini specification sheet (25–40 W average) and Starlink Standard specification sheet (75–100 W average).
• Webasto Air Top 2000 STC datasheet: 13–30 W electrical consumption.
• US Department of Energy, Estimating Appliance and Home Electronic Energy Use: watts × hours = daily energy, and reading the wattage label.

The other appliance numbers are typical starting values to edit, not measurements of your devices. The label on each device is the best source.

Can I print the result?

Yes. Use “Print or save as PDF” in the results box, then choose a printer or “Save as PDF”. Your list resets when you reload the page.