Once you know how many watt-hours a remote site burns in a day, the next question is how big the battery bank has to be to survive the stretches when the sun does not deliver. That is a separate calculation from the daily load, and it is where several multipliers stack up: how many days the bank must run with little or no charging, how deeply you are willing to discharge it without wrecking it, and how much capacity the cold quietly steals. Each factor inflates the raw amp-hour figure, and skipping any one of them produces a bank that looks adequate on paper and goes flat in the field. This is the arithmetic that converts a power budget into an actual battery size.
Solar Battery Bank Sizing in one line: To size a solar battery bank, start from the site's daily load in amp-hours, multiply by the days of autonomy you need to ride through bad weather, then divide by the maximum depth of discharge you allow so the usable capacity meets the demand. Finally apply a cold-temperature derate, because a battery delivers less than its rated capacity when it is cold. The result is the rated amp-hour capacity the bank must have, always checked against the worst season rather than the annual average.
The starting point for battery sizing is the daily energy the site consumes, expressed in amp-hours at the system voltage rather than in watts. If the power budget is already in watt-hours per day, dividing by the nominal system voltage converts it to amp-hours per day, which is the unit a battery is rated in. This step matters because a battery's capacity is stated in amp-hours, so the whole sizing calculation lives in amp-hours from here on. A site that burns a certain number of watt-hours a day on a twelve volt system needs twice the amp-hours of the same site on a twenty-four volt system, which is one more reason higher system voltages ease remote power design.
The daily amp-hour figure has to be an honest one, built from a real load profile rather than a single nameplate number. It must include the continuous base draw of the RTU, transmitters, and modem running every second, plus the energy from every intermittent load weighted by how often it actually fires, plus any seasonal load such as heat trace that only appears in winter. The winter case is the one that governs battery sizing, because winter is when the load is often highest and the solar input is lowest at the same time, so the daily amp-hour number used for sizing should be the worst-case winter day, not the mild-weather day the site was measured on.
It is worth separating this daily load number cleanly before applying any of the multipliers that follow, because errors here propagate through every later step. If the daily amp-hour figure is understated by twenty percent, every downstream number, the autonomy reserve, the depth-of-discharge headroom, and the temperature derate, is applied to a load that is already too small, and the final bank is undersized by more than twenty percent once the factors compound. Getting the daily amp-hour draw right, and pinning it to the worst season, is the foundation the rest of the calculation stands on.
The first multiplier is days of autonomy, the number of days the bank must carry the site with little or no solar charging. Remote sites face runs of overcast winter weather where the array delivers a fraction of its rated harvest for days, and the battery has to bridge that whole stretch without going flat. Multiplying the daily amp-hour load by the chosen autonomy days gives the raw energy the bank must store to survive the worst credible dark run. Longer autonomy costs more battery but buys resilience, and the right number depends on the site's climate and how catastrophic an outage would be; a critical alarm site justifies more autonomy than a low-consequence one.
The second multiplier corrects for depth of discharge. You cannot use all of a battery's rated capacity, because deeply discharging a lead-acid battery repeatedly shortens its life dramatically, and even at the moment of use a battery that is fully drained sags in voltage and risks tripping the low-voltage disconnect. So you decide a maximum depth of discharge, a fraction of capacity you are willing to use, and the bank must be big enough that the autonomy energy fits within that fraction. In arithmetic terms, you divide the autonomy energy by the allowed depth of discharge to get the rated capacity, which inflates the bank: allowing only half the capacity to be used, for example, doubles the rated amp-hours you must install.
The third correction is temperature. A battery's usable capacity falls as it gets cold, so a bank that delivers its full rated amp-hours at a mild reference temperature delivers meaningfully less on a freezing winter night, which is precisely when it is being asked to carry the longest autonomy run. To account for this, the rated capacity is increased by a temperature derate factor drawn from the battery manufacturer's cold-capacity data, so that the derated cold capacity still meets the autonomy-plus-depth-of-discharge requirement. Stacking these three factors on the daily load, autonomy to cover dark runs, depth of discharge to protect battery life, and temperature derate to survive the cold, turns an honest daily amp-hour figure into the rated amp-hour bank the site actually needs.
The calculated bank size is a design starting point, and the only way to know whether the arithmetic matched reality is to watch the battery behave across a real winter. A monitoring platform that trends battery voltage and state of charge turns the sizing assumptions into something you can check: the depth to which the bank actually cycles during a bad-weather run, the number of consecutive low-harvest days the site really sees, and how close the battery comes to its low-voltage disconnect on the worst night. If the bank routinely dips deeper than the design depth of discharge, it was undersized or the daily load was understated, and the trend says so long before a failure.
This is where a cloud SCADA system such as Merobix earns its place in the power design rather than just the process monitoring. Trending state of charge across many remote sites reveals the real autonomy each site experiences and whether the temperature derate was adequate, because the cold-night voltage sag shows up directly in the data. A site whose battery grazes the disconnect threshold every cold snap is running with no margin, and the trend gives the operator the evidence to add capacity or expand the array before an outage rather than after one. The design math gets you close; the trended data tells you whether you were close enough.
Monitoring also protects the bank against the slow creep of load that undoes a good original sizing. Remote sites accumulate instruments over the years, and a battery bank sized correctly for the original load can quietly become undersized as a second transmitter, a hungrier radio, or an unplanned heater is added. When the load current and state of charge are trended, that creep appears as a battery that no longer recovers as fully as it once did, giving early warning that the bank has been outgrown. Treating the battery sizing as a living number, verified and revisited against field data, is what keeps a remote RTU alive through the winters after the one it was commissioned in.
Four numbers stack up: the site's daily load in amp-hours, the days of autonomy the bank must ride through with little charging, the maximum depth of discharge you allow so battery life is protected, and a cold-temperature derate because batteries deliver less capacity when cold. You multiply the daily load by autonomy days, divide by the allowed depth of discharge, and inflate by the temperature derate. Each factor must be based on the worst winter case, not the annual average.
Repeatedly discharging a lead-acid battery deeply shortens its life sharply, and a fully drained battery sags in voltage and risks tripping the low-voltage disconnect. So you pick a maximum depth of discharge, a fraction of capacity you are willing to use, and size the bank so the required energy fits within that fraction. Allowing only half the capacity to be used effectively doubles the rated amp-hours you must install, which is why depth of discharge is a major driver of bank size.
A battery's usable capacity drops as temperature falls, so a bank that delivers its full rating at a mild reference temperature delivers less on a freezing night. That matters because cold nights coincide with the longest low-solar runs, exactly when the bank is being asked to carry the most. Sizing applies a temperature derate from the manufacturer's cold-capacity data, increasing the rated amp-hours so the reduced cold capacity still meets the autonomy requirement.
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