How to Size a PV System for Your Boat or RV

In general, sizing any PV system depends on many variables. You want to ask yourself the following questions:

  • What is your anticipated daily energy requirement?
  • Will the system be off-grid (with batteries) or grid-tied (with or without batteries?
  • For an off-grid system, how much autonomy (days without Sun) do you prefer?
  • Will there be a backup energy source?
  • How much sunlight is available at your location, and does it vary seasonally
  • Will the array tilt be adjustable, or will the array azimuth track the Sun?
  • How much maintenance will the system need?
  • Are there ways improve efficiency and reduce loads?

To size a system for a boat or RV, let’s focus on four application-specific variables: daily energy requirement, autonomy, environment, and space.

How Daily Energy Requirements Influence Other Variables

Your daily energy requirement (DER) greatly influences the other variables. The DER is often expressed as Ampere-hours (Ah). Although Ah is technically not a correct expression for electrical energy, the assumption is that the DC electrical system is operating at either 12V or 24V (nominal), so you can infer a valid energy requirement. For example, 83 Ah x 12 V = ~1 kWh. And, deep-cycle battery capacity is typically expressed as Ah.

There are two methods for determining your DER: a hard way and an easy way. The hard way requires conducting a detailed energy audit to determine the daily net energy requirement for each and every house-battery-powered electrical load, then adding up all the values. Don’t overlook “small” or hidden loads. For example, an LP gas detector that draws “only” 75 mA will consume 1.8 Ah per day, and the radio and fridge may have standby power requirements.

Any AC load calculations must include inverter losses. You may be able to leave out major AC loads such as an A/C or a residential fridge if they’re not to be operated from the batteries. And, the DER may vary seasonally. For example, the furnace fan runs in the winter and not during the summer, and lights operate longer during short winter days.

The easy way to determine your DER is to use your battery bank as a gauge. If your two fully charged Group 27 batteries (210 Ah total) last for two days in moderate temperatures before dropping to 50% capacity, then you’re consuming 52.5 Ah per day. Note that battery operating efficiency isn’t 100%. Losses during charging and discharging may be ~85%, so 62 Ah may need to be generated in order to recharge the battery bank after supplying only 52.5 Ah to the loads.

How Operational Factors Determine PV System Autonomy

Autonomy is the number of days your system will meet typical DER without requiring recharging from an external source (shore power, generator, and/or vehicle alternator). Do you want to live quietly “off-grid” for two days or a week, or are you okay with running a generator for some period of time every day?

The operational environment includes many factors. Will the PV array be exposed to the sun or be shaded? Is the array laying directly on the roof, or will there be an air gap between the modules and the roof to help cool the array? A tilt-able array is an important feature in the winter when the sun rests lower in the sky. Will it be tiled up while parked and down while traveling?

How Weather & Space Requirements Affect Sizing a PV System

Geography and weather are other important environmental factors. Will you be using the system in the sunny Southwest or in the rainy Northwest? Solar radiation data and models show how much sunlight is available in various locations at different times of the year for assorted PV module orientations. Ambient temperature affects module performance (cold ambient temperature equals higher module voltage), and temperature also affects battery performance.

Space is usually at a premium in mobile installations, but it is crucial to consider space for batteries and PV modules. Ask yourself, is there enough accessible space to safely store batteries? Is there room on the roof to install PV modules while working around the refrigerator vent, fan vents, and antennas?

3 Examples of Sizing a PV System

Example No 1: 52.5 Ah net DER for 5 days, with 2 consecutive days autonomy
• Gross DER = 52.5 Ah (net) / 85% battery charging efficiency = 62 Ah
• Total load for 2 days autonomy + 3 days to recharge: 62 (Gross DER) x 5 = 310 Ah
• Environment: Tilted array with 6 hours equivalent full sun x 3 days: 18 hours sun
310 ah / 18 hrs = 17.2 A x 14 V nominal voltage / 85% module efficiency = ~285 W STC PV array
• System size and space: 3x 95 W PV modules (285 W STC total) modules and 2x Group 27 batteries (105 Ah ea x 2 = 210 Ah; 52.5 Ah net DER x 2 days = 105 Ah; 105 Ah = 50% discharge)

Example No. 2: 52.5 Ah net DER for 5 days with average daily full sun
• Gross DER = 52.5 Ah (net) / 85% battery charging efficiency = 62 Ah
• Total load for 5 days: 62 (Gross DER) x 5 = 310 Ah
• Environment: tilted array with 6 hours equivalent full sun x 5 days: 30 hours sun
310 Ah / 30 hrs = 10.3 A x 14 V nominal voltage / 85% module efficiency = ~170 W STC PV array
• System size and Space: 2x 85 W PV modules (170 W STC total) modules and 1x Group 27 batteries (50% discharge per day) or 2x Group 27 batteries (25% discharge per day)

Example No. 3: 88 Ah net DER for 5 days with average daily full sun
• Gross DER = 87.5 Ah (net) / 85% battery charging efficiency = 103 Ah
• Total load for 5 days: 103 (Gross DER) x 5 = 515 Ah
• Environment: tilted array with 6 hours equivalent full sun x 5 days: 30 hours sun
516 Ah / 30 hrs = 17.2 A x 14 V nominal voltage / 85% module efficiency = ~285 W STC PV array
• System size and space: 3x 95 W PV modules (285 W STC total) modules and 2x Group 27 batteries (42% discharge per day)

PV System Design Experts

Despite similarities in battery bank and PV array sizes in the examples above, energy use and environmental factors will affect system performance.

Here are two handy sizing tools from MidNite Solar to get you started: the MidNite Classic Sizing Tool and the MidNite Kid Sizing Tool

Or give us a call at 615-285-1734, and we’ll be happy to help you review your PV system design. Visit www.inverterservicecenter.com.


About the Author – Jim Goodnight – also known as “crewzer” — is a retired solar industry application engineer, product manager, and forum moderator and has previously written for Home Power and Solar Professional magazines.