The battery bank is what keeps a solar RTU alive through the nights and the cloudy stretches when the panel produces little or nothing, so sizing it is really about deciding how long the site can coast without sun. The calculation turns a daily load and a target number of days of autonomy into an amp-hour rating, then inflates that rating to cover the fact that you cannot fully drain the battery and that cold weather steals capacity. Get it right and the site rides through a week of overcast without blinking; get it wrong and the RTU dies partway through the first bad stretch. This page walks the full field workflow from daily load to installed amp-hours.
Battery Bank Sizing (Remote RTU) in one line: You size a remote RTU battery bank by taking the site's daily load in amp-hours, multiplying by the target days of autonomy to find the raw reserve needed, then dividing by the maximum allowable depth of discharge and again by a temperature derate factor to find the installed amp-hour rating you must actually buy. The days of autonomy is how long the site must run with no solar input, the depth-of-discharge limit keeps you from draining the battery to a level that shortens its life, and the temperature derate accounts for a battery losing usable capacity in the cold. The result is an installed capacity larger than the raw reserve, which is what protects the site through cloudy weather and winter.
Battery sizing starts from the same daily load that drives the whole solar design, but expressed in amp-hours per day at the battery's voltage rather than in watt-hours, since batteries are rated in amp-hours. You get it by taking the daily energy the site consumes and dividing by the battery bank voltage, which gives the amp-hours the load pulls out of the bank in a full day. This is the daily draw the battery has to supply whenever the panel is not keeping up, and it is the figure everything else multiplies against, so it is worth deriving it carefully from the real average load rather than from equipment nameplates.
The next decision is how many days of autonomy the site needs, meaning how many consecutive days it must run on battery alone with essentially no solar input before the panel recovers. This is a reliability choice driven by the local weather and the cost of a site visit: a location prone to long overcast or snowy spells, or one that is expensive and slow to reach, warrants more days than a sunny, accessible site. Unmanned remote wellheads and similar sites are commonly designed for several days of autonomy so a normal run of bad weather cannot take them down, with the exact figure set by how much risk of an outage the operation can tolerate.
Multiplying the daily amp-hour draw by the days of autonomy gives the raw reserve, the amount of energy the bank must be able to hand over during the worst run of sunless days. If a site pulls a certain number of amp-hours per day and you want it to survive several days with no charging, the product is the usable capacity the battery must actually deliver over that stretch. Crucially this is usable capacity, the energy you intend to draw out, not the size of battery you buy, because you cannot safely use all of a battery's rated capacity, which is what the next two steps correct for.
The raw reserve is the energy you plan to take out of the bank, but you must never plan to fully empty a battery, so the first derate is depth of discharge. Deep-cycle batteries last far longer when they are only partially discharged on each cycle, and draining them repeatedly to near empty shortens their life sharply, so you choose a maximum depth of discharge and size the bank so that even after supplying the full reserve it has not gone past that limit. In practice this means dividing the raw reserve by the allowable depth-of-discharge fraction, which inflates the required capacity: if you only allow yourself to use part of the battery, you need a bigger battery to get the reserve you need out of the part you are willing to use.
The second derate is temperature, because a battery's usable capacity falls as it gets cold, and remote sites are exactly where cold is a problem. A bank that delivers its full rating at a mild temperature can deliver noticeably less when it is deep into winter, which is a cruel coincidence, because winter is also when the panel is weakest and the days of autonomy matter most. To account for this you divide again by a temperature derate factor based on the coldest conditions the bank will see, further increasing the required installed capacity so that the bank still delivers the needed reserve at its worst operating temperature rather than only in the lab.
Applying both deratings in sequence takes you from the usable reserve to the installed amp-hour rating you must actually purchase, and the gap between the two is often substantial. A site whose raw reserve is a given number of amp-hours can easily need a considerably larger installed bank once you divide by the depth-of-discharge limit and then by the cold-weather derate. This is why battery banks always look oversized relative to a naive load-times-days calculation, and it is entirely deliberate: the extra capacity is what guarantees the promised days of autonomy are still there when the battery is cold and when you are refusing to drain it past its healthy limit, which is precisely the situation the site faces in the depth of winter.
The final installed capacity is then met with real batteries, which means choosing a bank of a size and configuration you can actually buy and wire, and this usually involves rounding up to standard battery sizes and arranging them in the series and parallel combination that reaches both the required voltage and the required amp-hours. As with the panel, you round up rather than down, because the whole calculation was built to guarantee a reserve and shaving capacity to hit a round number quietly erodes the days of autonomy you just designed in. It is also worth remembering that batteries age and lose capacity over their life, so a little extra headroom at installation buys margin against the bank being weaker in a few years than it is on day one.
The battery bank and the solar array are two halves of one system and are sized in concert, not separately. The array is sized to replace the daily load and recharge the bank during a normal worst-month day, while the bank is sized to carry the site through the run of days when the array cannot keep up, and the two have to fit together: a bank sized for many days of autonomy is wasted if the array can never recharge it, and a generous array is wasted if the bank is too small to hold a meaningful reserve. Good field design iterates between the two until the array reliably refills the bank between bad-weather stretches and the bank reliably covers those stretches.
Once installed, the bank's real behaviour is exactly what a cloud SCADA platform such as Merobix is there to watch, because the sizing was a prediction and only the field tells you whether it holds. Trending battery voltage and, where available, state of charge across nights and cloudy runs shows whether the bank is genuinely delivering the designed days of autonomy or dipping toward its depth-of-discharge floor sooner than it should, which is the early warning of an undersized or aging bank. Alarming on low battery voltage gives staff time to act, whether by dispatching before the site drops offline or by shedding load, and correlating the battery's decline against the season and the recent sun tells them whether they are seeing a normal winter dip or a bank that has lost capacity and needs replacing, which is the distinction that decides whether the site needs a visit or just patience.
Because repeatedly draining a deep-cycle battery to near empty shortens its life sharply, so you deliberately limit how deeply you discharge it on each cycle and size the bank so that even after supplying its full reserve it has not gone past that depth-of-discharge limit. This means the usable reserve you can draw is only a fraction of the rated capacity, so you divide the reserve by that fraction to find the larger rated capacity you actually need to buy. Sizing to the full rating would give you the reserve you need only by destroying the battery's lifespan in the process.
A battery delivers less of its rated capacity when it is cold, so a bank that meets its rating at a mild temperature can fall short deep in winter. This is a compounding problem, because winter is also when the solar panel is weakest and the days of autonomy matter most, so the cold hits when you can least afford it. You account for it by dividing the required capacity by a temperature derate factor based on the coldest conditions the bank will see, which increases the installed capacity so the bank still delivers its reserve at its worst operating temperature rather than only in mild conditions.
It depends on the local weather and how hard the site is to reach, since days of autonomy is how long the site must run with no solar input before the panel recovers. A location prone to long overcast or snowy spells, or one that is expensive and slow to visit, warrants more days than a sunny, easily accessed site. Unmanned remote wellheads and similar sites are commonly designed for several days of autonomy so that a normal run of bad weather cannot take them offline, with the exact number set by how much outage risk the operation is willing to accept against the cost of a larger bank.
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