Here is the method, a worked example and the things that make real output lower than the brochure.
The formula
Daily energy (kWh) = array size (kWp) x peak sun hours x performance ratio.
Array size is the total rated power of the solar panels on the trailer, in kilowatts peak. Peak sun hours is the number of hours of full sun equivalent at the site, which varies by location and season. Performance ratio covers the losses between the panels and the load: heat, dust, wiring, inverter and battery losses.
Peak sun hours by site and season
Sunny regions often see five to six peak sun hours a day in good months, while cloudy or high-latitude sites may see two or three in winter. Public solar resource tools give monthly figures for any location. Always size for the worst month in which the trailer must carry the load, not the annual average.
Performance ratio: where energy goes
On a mobile trailer, performance ratio is often in the range of 0.65 to 0.8. Panels lose output when hot. Dust on the panels can cost several percent, more on mining and construction sites, which is why soiling and washing matters. Energy that passes through the battery loses a little more. Panels that cannot be tilted or turned towards the sun, or that are shaded by nearby structures, produce less again.
Worked example
| Input | Value |
|---|---|
| Array size | 12 kWp |
| Peak sun hours, worst month | 4.5 |
| Performance ratio | 0.72 |
| Estimated daily energy | 12 x 4.5 x 0.72 = about 39 kWh |
If the load uses 30 kWh a day, this trailer covers it in the worst month with some margin. If the load uses 50 kWh, it does not, and the gap must come from a second trailer or a generator. These figures are illustrative; use the actual array size and your site’s solar data.
Batteries and night loads
Solar only produces in daylight, so any load at night runs from the battery. The usable battery capacity must cover the night load plus a margin, and the array must produce enough by day to run the daytime load and recharge the battery. If cloudy days are common, either more battery or a backup generator is needed. Our pages on solar trailer battery storage and night cover cover this in more depth.
Matching the trailer to the load
Start with a load profile: what runs, at what power, for how many hours, and when. Lighting towers, CCTV and telecom loads are small and steady and suit solar well. Pumps, workshops and site offices with air conditioning are larger and peakier and usually need a hybrid with a generator. See building a load profile for a simple template.
When a generator still belongs on site
A hybrid of solar, batteries and a small generator often gives the best result: solar does most of the work, the battery covers the night, and the generator runs only when the battery is low. Fuel use and engine hours fall sharply compared with a generator running all day, without risking an outage in a cloudy week.
Solar trailers from a power plant builder
USP&E builds solar, hybrid and thermal power plants and supplies mobile solar trailers for remote and temporary sites. With 150+ power projects in 45+ countries and 25,000 MW contracted since 2002, we size trailers against real loads. Read about our hybrid power systems.
Offices: Colorado Springs (1755 Telstar Dr, Suite 300), Knoxville (800 S Gay St, Suite 700), London (162-168 Regent Street), Johannesburg (22 Kildoon Rd, Bryanston), Cape Town (31 Brickfield Road, Woodstock) and China (No. 1 Ningzhou Road, Wuxi). Regions: USA | AFRICA | UK | UAE | CHINA. ISO 9001 and ISO 45001 certified.
Solar trailer output FAQ
How many kWh does a solar trailer produce per day?
Array size in kWp times peak sun hours times a performance ratio of about 0.65 to 0.8. A 12 kWp trailer with 4.5 sun hours gives roughly 39 kWh a day.
How do I find peak sun hours for my site?
Public solar resource tools give monthly peak sun hours for any location. Use the worst month in which the trailer must carry the load.
Can a solar trailer run loads at night?
Yes, from its battery. The battery must cover the night load, and the array must recharge it the next day.
Do I still need a generator with a solar trailer?
For small steady loads, often not. For larger loads or cloudy climates, a small generator in a hybrid gives reliable power with much less fuel.
Sizing a solar trailer for your site?
Send us your load and location, and we will estimate daily output and recommend a trailer or hybrid set-up.
Send drawings, load data or questions via the solar sizing team; the switchboard is +27 10 822 2324 and the inbox is info@uspeglobal.com. Generators for hybrid set-ups are in the USP&E inventory.