How to Size an Automatic Transfer Switch for Solar, Battery Storage & EV Charging Systems

Most ATS sizing guides assume a simple two-source problem: utility power on one side, a diesel generator on the other. That logic works fine for a hospital or a data center running a standby genset. It breaks down as soon as the backup source is a solar inverter, a battery energy storage system, or an EV charging station — because these loads don’t behave like a generator, and in a multi-source system, “the load” isn’t always the biggest number you need to worry about.

This guide walks through how to size an ATS correctly when solar, storage, or EV charging is part of the picture, with a worked example and a selection table mapped to MOREDAY’s MDQ series. If you’re not yet familiar with the basic mechanics of transfer switches, what a transfer switch is and how it works covers the fundamentals this guide builds on, and our overview of solar transfer switch applications is a useful primer if you’re new to PV-integrated backup power specifically.

ATS Sizing Quick Answer

Size an automatic transfer switch by identifying the largest continuous current the switch will ever carry — not just the generator or grid rating — applying a continuous-duty margin on top of it, and matching that figure to the ATS’s rated current, voltage, pole configuration, and transition type. In multi-source systems (solar + storage + EV charging), the governing current is usually the combined load side, not either individual source, because the ATS has to carry whatever the load demands regardless of which source is supplying it. When in doubt, size to the larger of (a) the connected load’s continuous demand or (b) the largest single source’s output current, then apply the safety margin.

Why Sizing an ATS Is Different for Solar, ESS and EV Loads

A generator-based ATS sizing problem has one clean rule: match the switch to the generator’s full-load output current, because the generator is a fixed, known capacity and the switch will never see more than that from the backup side. Once you add solar, battery storage, or EV charging, three things change.

There can be more than two sources. A residential or commercial system might switch between utility, a solar inverter, and a battery inverter — sometimes all three feeding the same bus at different times of day. The ATS has to be rated for whichever combination produces the highest current, not just the nominal capacity of any one source.

The load side can run at or near full rated current for extended periods. A generator-backed office might run at 40-60% of its rated load most of the time. An EV charging station, by contrast, is designed to run at or close to its full rated current for the entire duration of a charging session — often several hours. That changes how much margin you need, because “continuous duty” in the electrical code sense specifically targets loads that run at full current for three hours or more.

Power can flow in more than one direction. A battery energy storage system charges from the grid or solar array and discharges back to the load — and in some architectures, back to the grid. The ATS itself is not typically a bidirectional power-flow device (it selects a source; it doesn’t manage power flow direction), but the current magnitudes on both sides of the switch need to account for both charge and discharge current at the battery’s maximum rating, not just the discharge-to-load direction.

Step 1: Determine the Governing Current

The first step is figuring out which current value actually determines the switch size. This is the step most generic sizing guides skip, because in a pure utility-generator system there’s only one candidate. In a solar/ESS/EV system, there are several, and you need the largest one.

Grid-only backup (traditional case): Governing current = the utility service entrance rating, or the generator’s full-load output current, whichever the ATS sits behind. Most industrial guidance is explicit that the switch should be sized to the utility main breaker rating, not the generator nameplate, when the ATS is installed at the service entrance — because the switch must be able to carry full utility-side load even when running on generator power is not the constraint.

Solar + battery storage, switching between grid and inverter output: Governing current is typically the connected load’s total demand, calculated the same way you’d size a main panel — sum of all circuits that can be active simultaneously, with motor loads counted at their locked-rotor or starting current if that exceeds their running current. Compare this against the solar inverter’s and battery inverter’s maximum output current; size to whichever is larger.

EV charging station backup: Governing current is the charger’s continuous output rating — for a Level 2 AC charger this is typically the full nameplate current with no significant motor-starting-type inrush, but it holds that current continuously for hours rather than momentarily. For DC fast-charging stations, the AC-side ATS (if present ahead of the charger’s own AC/DC conversion) needs to be sized to the charger’s AC input current, not the DC output current, and multiple charging bays on a shared ATS need their currents summed with an appropriate demand factor for how many bays are likely to be active at once. This demand factor is project- and code-specific — verify it against the applicable local code and the charging equipment manufacturer’s installation guide rather than assuming a fixed percentage.

Step 2: Apply the Continuous Duty Margin

Once you have the governing current, add a safety margin before selecting an ATS current rating. The standard approach used across generator, solar, and industrial ATS applications is the same continuous-duty logic used for circuit breakers and conductors: for any load expected to run continuously for three hours or more, the protective and switching device should be rated for at least 125% of that continuous current.

Rated ATS Current ≥ Governing Current × 1.25

For loads with significant starting or inrush characteristics (large motors, compressors, some pool or HVAC equipment on the load side), the margin should be evaluated against the starting current as well, not just the running current, since the ATS’s contacts and mechanism need to survive the transient without welding or excessive wear.

This 1.25 factor is a widely used engineering convention, not a universal legal requirement in every jurisdiction — always verify the applicable local electrical code and the ATS manufacturer’s specifications before finalizing the rating, particularly for continuous EV charging loads where some jurisdictions apply additional demand-factor rules on top of the base continuous-duty margin.

Step 3: Match Voltage, Poles and Transition Type

Current rating is only one of four parameters that need to match the application:

• Voltage and frequency: The ATS must be rated for the system’s nominal voltage (e.g., 230/400V, 50Hz in most IEC markets) with adequate margin above the highest voltage any connected source can present, including solar inverter output under light-load conditions.
• Number of poles: Single-phase residential systems typically use 2-pole ATS units; three-phase commercial and industrial systems typically require 4-pole units with switched neutral, particularly where the two sources do not share a common neutral-to-ground bond (common in grid + generator or grid + battery inverter configurations). Within MOREDAY’s own 63A tier, this is also where the MDQ2-63 and MDQ3-63 diverge: the MDQ3-63 is available in 110V/220V/400V and 2P/4P configurations, giving it more flexibility across voltage regions and pole counts, while the MDQ2-63 is a fixed 220V/400V configuration. If your project needs 110V compatibility or a choice between 2P and 4P in the same current tier, that points to the MDQ3-63 rather than the MDQ2-63.
• Transition type: Open transition (break-before-make) is standard and lower cost, appropriate for most solar, storage, and EV backup applications where a brief interruption of 100-500 milliseconds during transfer is acceptable. Closed transition (make-before-break) avoids any interruption but is only needed where the load cannot tolerate even a momentary break — this is uncommon for EV charging or typical BESS applications and is generally reserved for critical continuous processes.
• Classification (PC vs CB class): Under IEC 60947-6-1, transfer switching equipment is classified by whether it can also interrupt fault current on its own (CB class, based on circuit breaker construction) or requires upstream overcurrent protection to clear a fault (PC class). This distinction affects how the ATS coordinates with upstream breakers and fuses — it’s covered in depth in a dedicated guide, but for sizing purposes, confirm which class your selected ATS falls under so your upstream protection is coordinated correctly.
• Standard vs. fire-pump-service configuration: Both IEC 60947-6-1 and UL 1008 carry a dedicated annex covering transfer switching equipment used with fire pump control equipment and other life-safety circuits (Annex F under IEC 60947-6-1, Annex E under UL 1008). A fire-service ATS is built to force the transfer to the available source for a life-safety load under conditions where a standard (“basic”) ATS might otherwise delay or block the transfer — for example, it is generally not permitted to re-transfer back to a source that hasn’t been confirmed stable in the way a standard ATS would. If your supplier’s datasheet lists both a standard and a fire-service version of the same current tier, the fire-service version is the one intended for fire pumps, fire lifts, or emergency lighting circuits — confirm the exact certified differences and applicable local fire code requirements with the supplier’s documentation before specifying it for a life-safety circuit, since the specific features covered by the fire-service listing vary by manufacturer and edition of the standard.

Worked Example

Consider a commercial site with a 100A three-phase utility service, a solar inverter rated for 80A output, a battery inverter rated for 60A continuous discharge, and a Level 2 EV charging station drawing 32A continuously during charging sessions. The ATS switches between utility and the combined solar+battery inverter output; the EV charger is one of the loads on the protected bus.

Determine the load-side governing current: The connected load includes general building circuits plus the EV charger. Assume general building load calculates to 70A, and the EV charger adds 32A continuous. Combined load-side demand = 102A.

Compare against source-side capacity: The solar+battery combined inverter output capacity is up to 80A + 60A = 140A under ideal conditions, but the practical governing figure for ATS sizing is the load-side demand it must carry, since the switch must handle whatever the load draws regardless of which source is active — unless the source itself is the limiting factor, in which case size to the smaller of source capacity and load demand and address the shortfall through load management, not switch sizing. In this case, load-side demand (102A) is the governing figure.

Apply the continuous-duty margin: 102A × 1.25 = 127.5A.

Select the ATS rating: The nearest standard rating above 127.5A points toward a 3-pole 4-pole, three-phase ATS rated at or above 160A on MOREDAY’s MDQ8 series — the MDQ2-63 and MDQ5R-100 ranges top out below this figure and would be undersized for this specific combined load.

Verify voltage, poles, and transition type against the specific project’s electrical drawings and the local code before finalizing — this worked example illustrates the calculation method, not a substitute for a stamped electrical design.

Selection Table: Matching Load Current to MOREDAY’s MDQ Series

This table is a starting point for narrowing down a series — final model selection should confirm exact current, voltage, pole count, transition type, and whether a standard or fire-service configuration is required, against the project’s electrical design.

Common Sizing Mistakes

Sizing to the generator or inverter nameplate instead of the load. The ATS has to carry what the load draws, not what the source is theoretically capable of producing. In multi-source systems this is the single most common oversizing or undersizing error.

Ignoring EV charging as a continuous load. Treating an EV charger’s current draw like an ordinary intermittent appliance load, rather than a load that can run at full rated current for hours, leads to undersized switches that experience excessive thermal stress over the system’s service life.

Forgetting neutral switching requirements. In systems where the two sources don’t share a common neutral-ground bond — a frequent situation with generator or battery inverter backup — using a 3-pole switch instead of a 4-pole switch with switched neutral can create ground fault protection and neutral-to-ground bonding violations.

Assuming closed transition is always “better.” Closed transition adds cost and complexity for a benefit (zero-interruption switching) that most solar, storage, and EV charging applications don’t actually need. Specify it only where the load genuinely cannot tolerate a 100-500ms interruption.

Skipping the demand factor for multi-bay EV charging. Summing the full nameplate current of every charging bay on a shared ATS without applying a realistic simultaneous-use demand factor typically results in a switch and upstream distribution that’s far larger — and more expensive — than the installation will ever actually need. Confirm the applicable demand factor with local code and the charging equipment manufacturer.

Standards and Safety Considerations

Automatic transfer switches used in solar, storage, and EV charging applications should be evaluated against IEC 60947-6-1 (transfer switching equipment, including the PC/CB classification referenced above) in IEC-framework markets, or UL 1008 in North American installations. Where the ATS is part of a system that also includes rapid shutdown requirements — for example, a rooftop PV + BESS installation switching between grid, solar, and battery sources — its role in the isolation sequence needs to be coordinated with the rapid shutdown design; MOREDAY’s guide to rapid shutdown solar systems covers this coordination in detail for NEC 690.12-governed installations.

Any project-specific certification requirement (CE, CB scheme, local grid-connection approval for the switching equipment) should be verified against the applicable local code and the manufacturer’s current certification documentation, since certification scope can vary by current rating and configuration within the same product series.

MOREDAY Solution

MOREDAY’s MDQ series covers the current range this guide walks through in a single product family: the MDQ2-63 for fixed 220V/400V residential and small commercial solar-plus-storage backup, the MDQ3-63 for the same current tier where 110V compatibility or a choice of 2P/4P is needed, the MDQ5R-100 for mid-size systems, and the MDQ8 for commercial and industrial installations up to 1250A, including multi-bay EV charging and larger BESS deployments. All four are certified under IEC 60947-6-1, with CB-class construction providing coordinated fault interruption alongside the switching function. For sites where manual operation is acceptable and budget is the primary constraint, the NSIS AC Changeover Switch covers the same voltage range in a manual configuration.

If a project involves fire pump or other life-safety circuits, ask about fire-service-configured variants within these series specifically — don’t assume the standard version is suitable without confirming against the applicable fire code.

If you have specific load data for a solar, storage, or EV charging project, talk to MOREDAY’s engineering team to confirm the correct model and configuration for your installation.

Conclusion

Sizing an automatic transfer switch for solar, battery storage, or EV charging comes down to the same three steps every time: find the true governing current — usually the combined load demand rather than any single source’s rating — apply the standard continuous-duty margin, and match voltage, poles, and transition type to the specific installation. The calculation is straightforward once the governing current is identified correctly; most sizing errors happen at that first step, not in the math that follows.

External references: IEC 60947-6-1 — Low-voltage switchgear and controlgear: Transfer switching equipment (iec.ch); UL 1008 — Standard for Transfer Switch Equipment (ul.com); NFPA 70 (NEC) Article 210/215 continuous load provisions (nfpa.org)

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