Battery bank for AC sizing

Battery bank for AC sizing explained

Air conditioning is the hungriest appliance in mobile and off grid power. To size a battery bank for AC you translate cooling needs into energy and then match that with battery capacity, system voltage, inverter capability, and charging. The core math starts with watt hours. Multiply the air conditioner’s running watts by the hours you expect to run it each day. Then divide by inverter efficiency to account for conversion losses and adjust for the real duty cycle, which depends on climate, insulation, and thermostat setpoint.

A quick rule of thumb helps set expectations. Typical rooftop or compact mini split units for vans draw about 900 to 1600 watts while running, though initial compressor start can spike several times higher without a soft start device. If you plan to cool for four hours at 1200 watts, that is 4800 watt hours. With a realistic inverter efficiency of 90 percent, you should budget about 5330 watt hours from the battery. If your lithium iron phosphate bank has 85 percent usable capacity, total nominal capacity should be near 6270 watt hours to deliver that energy comfortably.

Do not ignore duty cycle. An air conditioner does not run at full tilt all the time. In mild evenings it might run half the time. In desert heat it might run almost continuously. Improving insulation, shading, and ventilation lowers duty cycle, which is often cheaper than adding more batteries.

Key terms decoded

• Running watts and surge: Running watts reflect steady draw. Surge is the brief compressor start current. Use a soft start to tame the surge so your inverter does not trip.
• Watt hours and amp hours: Energy for planning is watt hours. Amp hours depend on system voltage, since watt hours equal volts times amp hours. Higher voltage reduces current and cable losses.
• Usable capacity: Lithium iron phosphate commonly allows around 80 to 90 percent usable capacity. Lead acid allows far less, which makes it a poor choice for heavy AC loads.

AC load, inverter, and duty cycle

Pick an inverter that handles both continuous watts and start up surge. A 2000 watt inverter can run many compact units but confirm the start profile or add a soft start module. Plan on 6 to 10 percent conversion loss through the inverter. Duty cycle depends on thermal load. Use white exterior paint, window coverings, roof insulation, and airflow to reduce compressor run time. A better thermal shell is free energy every day.

Worked example

Assume a 12000 BTU unit drawing 1000 watts once settled, with five hours of runtime on a hot afternoon, and a soft start device installed.

• Energy need: 1000 watts times 5 hours equals 5000 watt hours.
• Inverter loss: 5000 divided by 0.9 equals 5556 watt hours from the battery.
• Battery sizing at 48 volts: 5556 watt hours divided by 48 equals 116 amp hours usable.
• Nominal capacity with 85 percent usable: 116 divided by 0.85 equals 136 amp hours at 48 volts.

You could meet this with a 48 volt, 150 amp hour lithium iron phosphate bank. The same energy at 12 volts would be roughly 444 amp hours nominal, which means heavier cabling and higher current. For AC heavy builds, 24 volt or 48 volt architecture is often more efficient.

Designing a van power system for air conditioning

Start by defining the mission. How many hours of cooling do you need without driving or plugging into shore power. What climates do you visit. How many consecutive hot days. With those answers, select battery voltage, chemistry, inverter size, and charging inputs that meet the target energy budget.

Battery chemistry matters. Lithium iron phosphate offers high cycle life, stable voltage, and deep usable capacity that suits compressor loads. Keep batteries within their preferred temperature window. Very high or low temperatures reduce available capacity, so consider thermal management of the battery space.

Distribute charging across solar, alternator, and shore power. Solar contributes during peak sun but may not carry AC on its own. Alternator charging through a quality DC to DC charger can add meaningful watt hours while you drive. Shore power is the reset button at campgrounds or overnight stops. Together they cut the time required to recover the bank.

Inverter and voltage selection

Choose system voltage to match your peak loads. At 24 volt or 48 volt, current is lower for the same power which reduces voltage drop and heat. Select an inverter with a continuous rating above your steady AC draw and a surge rating that covers compressor start. A soft start reduces the surge, widens inverter options, and improves reliability. Place the inverter close to the battery bank and size cables for low voltage drop.

Practical steps, calculators, and common mistakes

Follow a simple sequence to size the battery bank for AC with confidence.

1. Measure real power. Use a clamp meter or smart plug to log watts while the compressor runs. Avoid relying only on nameplate numbers.
2. Estimate daily hours by climate. Create two scenarios, one conservative and one worst case.
3. Calculate watt hours and add inverter losses. Divide by 0.9 unless you have measured efficiency data.
4. Convert to amp hours at your chosen voltage. Then divide by usable capacity to get nominal size.
5. Verify inverter surge and wire sizing. Confirm that cables and fuses meet current demands.
6. Plan charging. Add solar wattage, alternator charge rates, and shore options to rebuild the bank in a reasonable window.

Common pitfalls include undersizing the inverter for surge, choosing low voltage for high power loads, expecting solar alone to carry cooling, overlooking heat soak in poorly insulated vans, and ignoring the battery temperature effects on capacity. Prevent these by pairing energy math with thermal improvements such as reflective window covers, insulated roof panels, and cross ventilation.

A short field test beats assumptions. On a warm day, run the AC with a fully charged bank and log current over several hours. Note duty cycle and cabin temperature. Adjust setpoint, airflow, and shading to see how they influence energy draw. Fine tuning comfort settings can save hundreds of watt hours.

If you decide a custom system is the right path, explore our van platforms and build services. See our core offerings at recreational vans, dig into our custom van builds, or review mainstream van platforms if you want a finance friendly starting point.

Strong design brings everything together. A properly sized battery bank, an inverter with headroom, thoughtful wiring, and a charging plan that fits your travel rhythm will keep cool air flowing without constant worry. When the math and the hardware agree, the cabin stays calm even when the sun is relentless.

Final note on safety and longevity. Use proper overcurrent protection, secure battery mounts, ventilation clearances around inverters, and manufacturer recommended charge parameters. Keep connections tight and monitor system performance with a shunt based battery monitor so you know exactly how many watt hours you have left.

Ready to turn this plan into a reliable system. Our team builds complete power and climate packages that match your route, weather, and comfort goals. Visit our recreational vans page to see how we configure AC ready energy systems, and explore custom van builds to tailor power, cooling, and storage to your travel style. If you are considering a platform that finances, compare options at mainstream van platforms. Once you are ready, reach out and we will spec the battery bank for AC sizing, install it, and hand you the keys with confidence.