How to Size RV Solar Panels and Check an MPPT Controller
Part 5 of 8 in the RV Solar Planning Series.
An RV roof is not a blank rectangle. Vents, air conditioners, antennas, curved edges, service access and shadows divide it into usable zones. Array planning has to satisfy both the energy target and the roof.
Estimate the array target
A useful first pass divides daily watt-hours by conservative equivalent peak-sun hours, then adds allowance for system loss. Solar resource changes by location, season, weather, shade, tilt and temperature, so document the assumption instead of calling one number “average.” NREL’s PVWatts tool can help explore solar resource, although its model is intended for grid-connected PV and RV conditions still require judgment.
Draw the roof to scale
Measure overall usable areas and every obstruction. Leave manufacturer-required clearance around roof equipment and room for inspection or service. Consider which objects cast shadows as the sun moves. A panel that technically fits can still be a poor placement.
Record each panel’s electrical data
From the panel label or datasheet, record open-circuit voltage (Voc), short-circuit current (Isc), voltage at maximum power (Vmp) and current at maximum power (Imp). The series or parallel arrangement changes array voltage and current.
Check the MPPT limits
Verify the controller’s maximum PV open-circuit voltage, maximum PV short-circuit current, operating voltage requirements and rated battery-charge output for the selected bank voltage. Controller specifications vary by model and battery voltage.
Cold raises panel open-circuit voltage. Calculate or verify the array Voc at the coldest expected panel temperature using manufacturer data and keep it below the controller’s absolute maximum. Victron’s MPPT manuals specifically direct users to account for cold-condition Voc and stay under the charger limit.
Use MPPT for a new plan
An MPPT controller provides the flexibility and conversion needed for modern array planning. A legacy PWM system can be documented as existing equipment, but a new system should be designed around a compatible MPPT controller rather than allowing an old controller to dictate the entire investment.
The roof, panel and controller pages in the Gram & Pap RV Solar Planning and Setup Guide keep the physical layout beside Voc, Isc, Vmp, Imp, string configuration and controller limits.
Make the physical and electrical plans agree
The best array on a calculator is useless if it blocks a vent or exceeds a controller limit. The best-looking roof layout is equally unhelpful if it cannot supply the planned energy. Both drawings must describe the same system.
Safety note: This article is a planning aid. RV electrical systems and equipment requirements vary. Follow current manufacturer manuals and applicable codes, and use a qualified RV electrician when the work or requirements are uncertain.
