A remote wellsite RTU that runs out of power is worse than useless - it goes dark, stops reporting, and often stops controlling the well. The way to make sure that never happens is not to guess at a panel size but to do the arithmetic first: add up everything the site draws, figure out how much sun it will get, and confirm generation beats consumption with margin. That arithmetic is the solar power budget. This guide explains how to tally the load, match it to a panel and battery, and why the calculation comes before the hardware.
RTU Solar Power Budget in one line: A solar power budget for an RTU is the load-versus-generation calculation for a solar-powered wellsite: you add up the average power draw of the RTU, radio, and transmitters, convert it to daily energy, then size the solar panel and battery so that on the site's available sun-hours the panel replaces what the load consumes with margin to spare. It is the sizing worksheet you complete before choosing a panel and battery.
The load side starts with the electronics themselves. A modern RTU draws on the order of milliwatts to a few watts depending on how hard it is working, and it does not draw evenly - it may sleep low and wake to poll, so what matters is average power over a day, not just the peak. Add the communications device, which is often the largest single draw because a radio or cellular modem spikes when it transmits, then averages down between transmissions. The more often the site reports, the higher that average climbs, so report rate is a real design lever on the power budget.
Then come the field instruments. Each loop-powered pressure or temperature transmitter draws its own current whenever it is energized, and a site with several transmitters can add up to more than the controller itself. Anything actuated - a solenoid that strokes a valve, a heater relay - contributes too, at least during the moments it is energized. The right way to tally all this is to list every device, its average draw, and how much of the day it is active, then sum to a total average power. Multiplying that average power by twenty-four hours gives the daily energy the site consumes, which is the number the generation side has to beat.
A solar panel's rating is its output under full sun, but a site does not get full sun all day. The useful figure is the location's insolation, usually expressed as peak sun-hours: the equivalent number of hours of full-rated sunlight the site receives on an average day, which folds in latitude, season, and typical weather. A panel rated at a given wattage, at a site with a given number of sun-hours, produces roughly wattage times sun-hours of energy per day, less real-world losses in the charge controller, wiring, and battery round-trip. That daily energy figure is what you compare against the daily load.
The comparison must be sized for the worst realistic day, not the average one. Winter, low sun angle, and short days shrink sun-hours right when you can least afford a shortfall, so a budget that balances in summer can starve in December. Sound practice is to size generation against the poorest month the site must survive and to add margin on top, so the panel comfortably out-produces the load rather than just matching it. Under-sizing shows up as a slow brownout: the battery never fully recharges, sags a little further each cloudy stretch, and eventually drops the site. Building in generation margin is what keeps the average day from becoming the failure case when a bad week arrives.
It is tempting to pick a panel and battery that look about right and hope, but the power budget exists precisely to remove the hoping. Doing the load tally and the sun-hour math before buying hardware tells you the minimum panel wattage and battery capacity the site needs, and it exposes the trade-offs while they are still cheap to change - reduce the report rate, choose a lower-draw modem, or accept a larger panel. A brownout discovered after installation is a truck roll to a remote location and a stretch of lost data or lost control; the same problem caught on a worksheet is a line-item edit.
The power budget also connects directly to how the site is operated day to day. With a cloud SCADA platform such as Merobix, the RTU already reports its battery voltage and solar charge state as tags, so the assumptions baked into the budget can be checked against reality from a browser. If a site's battery is trending lower each morning than the budget predicted, an engineer sees it in the trend long before the site browns out and can act - dial back a report rate, schedule a panel cleaning, or flag a failing battery. The budget sets the design, and the telemetry confirms the design is holding, which together are what let an operator trust an unmanned solar site to stay up.
Usually the communications device. A radio or cellular modem spikes hard when it transmits, and the more often the site reports, the higher its average draw climbs, so report rate is a major lever on the budget. The RTU itself is typically small - milliwatts to a few watts on average - and loop-powered transmitters and any actuated devices add the rest. Tallying each device's average draw and duty cycle reveals where the energy goes.
A panel's daily energy is roughly its rated wattage times the site's peak sun-hours, minus real-world losses, so fewer sun-hours means less energy per day. Because sun-hours drop in winter and in poor weather, you must size generation against the worst month the site has to survive, not the annual average. Building the budget around the poorest realistic sun-hours, with margin on top, is what prevents a site that balances in summer from starving in winter.
Because fixing an under-sized site after installation means a truck roll to a remote location and lost data or control, while catching it on a worksheet is a simple edit. The budget tells you the minimum panel wattage and battery capacity you actually need and exposes cheap trade-offs - a slower report rate, a lower-draw modem, or a bigger panel - before any hardware is bought. It replaces guessing with arithmetic you can defend.
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