A solar-powered wellsite is only as reliable as its ability to ride through the days when the sun does not show up. Days of autonomy is the number that captures exactly that: how long the site can keep running on its battery alone, with no charging, before the battery is drained to the point where you must stop pulling from it. It is the metric that decides how big the battery bank has to be. This guide explains what days of autonomy means, how the depth-of-discharge floor bounds it, and why unmanned wellheads are typically sized for five to seven days.
Days of Autonomy in one line: Days of autonomy is how many days a solar SCADA site can run its load on battery alone, with zero solar input, before the battery reaches its safe depth-of-discharge floor. It is set by dividing the battery's usable capacity by the site's daily energy draw, and it is the reserve that carries the site through cloudy, snowy, or short-day stretches. Unmanned wellheads are commonly sized for five to seven days.
Days of autonomy answers a simple survival question: if the panel produced nothing starting today, how many days would the site keep running before the battery ran too low to use? It is computed from two figures already known from the power budget - the battery's usable energy capacity and the site's daily energy consumption. Divide the usable capacity by the daily draw and the quotient is the days of autonomy. A site that consumes a small daily energy and carries a large battery has many days of reserve; a heavy load on a small battery has few.
The word usable is doing real work in that sentence, because you cannot drain a battery to empty. Every battery chemistry has a limit on how deeply it should be discharged before its life shortens dramatically or it is damaged, expressed as a depth of discharge. A lead-acid bank is typically only allowed to give up a fraction of its nameplate capacity per cycle, so its usable energy for autonomy purposes is well below its rated amp-hours; lithium chemistries tolerate deeper discharge and offer more usable capacity per nameplate. Days of autonomy is therefore measured against the safe floor, not the empty point - the battery has to still be above its depth-of-discharge limit when the sun finally returns.
The target number of days is a judgment about weather and consequences. A site could be sized for a single day of autonomy and would work fine on any sunny stretch, but the first multi-day storm, snow cover on the panel, or run of heavy overcast would drop it. Sizing for more days buys resilience against exactly those events, at the cost of a bigger, heavier, more expensive battery bank. Somewhere around five to seven days is a common landing point for unmanned wellheads because it covers the typical bad-weather stretch a remote site sees without oversizing the battery to the point of absurdity.
The right figure shifts with the climate and the criticality of the site. A location with long, dark, snowy winters may warrant more reserve than a sunny desert; a well whose loss of monitoring or control carries higher consequence justifies a larger margin than one that is easy to reach and forgiving to lose for a day. The autonomy figure also has to be reconciled with recharge time: the battery not only has to survive the sunless stretch but recharge afterward before the next one, which is a check on making it enormous. Five to seven days is a starting rule of thumb, and the final number is tuned to the specific site's weather, access, and importance.
Days of autonomy is a design assumption, and like any assumption it deserves to be checked against what the site actually does. Batteries age and lose usable capacity, panels get dusty or snow-covered, and a load can creep up as instrumentation is added, all of which quietly erode the reserve the site was designed for. A battery that started with a week of autonomy can drift to two or three days after a few years without anyone noticing until a cloudy stretch drops the site. The reserve is real only as long as the battery still holds the capacity the design counted on.
With a cloud SCADA platform such as Merobix, the battery voltage and charge state are already reported as tags, so autonomy stops being a paper number and becomes something an engineer can watch. Trending battery voltage across cloudy days shows how the reserve is really behaving - how far it sags on the third overcast morning, how quickly it recovers when the sun returns - and a bank that no longer rides through the weather it once handled reveals itself in the trend before it fails outright. Seeing that across a whole field from a browser lets a team replace an aging battery on a schedule rather than after an outage, which is the difference between designed-for autonomy and autonomy you can actually count on.
Divide the battery bank's usable energy capacity by the site's daily energy consumption. Usable capacity is not the full nameplate rating - it is the portion you can safely discharge before hitting the depth-of-discharge floor, which for lead-acid is only a fraction of nameplate and for lithium is a larger share. The result is the number of days the site can run on battery alone with no solar input before reaching that floor.
Because it covers the typical multi-day stretch of clouds, snow, or short winter days a remote site sees without making the battery bank impractically large. Fewer days risks dropping the site during the first real storm; many more days adds cost, weight, and recharge time for diminishing benefit. Five to seven days is a rule of thumb that gets tuned up for harsh-winter or high-consequence sites and down for sunny, easily accessed ones.
Yes - that is central to it. Days of autonomy is measured against the battery's safe depth-of-discharge floor, not the point where it is fully empty, because draining a battery too deeply shortens its life or damages it. That is why usable capacity, not nameplate capacity, goes into the calculation, and why lithium chemistries that tolerate deeper discharge yield more autonomy per nameplate than lead-acid.
Merobix reads your field devices into a cloud SCADA - the real thing behind these terms, live in days from any browser.