Energy Storage for Metalworking & CNC Shops: Engineering Sizing and Connection

Input Data: What We Need Before Sizing

We start with three things. First, a 1-second or faster load profile from your main incomer, recorded over at least one full production week. Without that, any kW figure is a guess. Second, your last 12 months of power bills—we need the demand charge line and the actual kWh to separate energy cost from peak cost. Third, a single-line diagram showing the transformer kVA, the cable lengths to the biggest welders, and where your CNC machines sit electrically.

If you have a power quality meter, we also want sag/swell logs. If not, we can install a temporary logger for two weeks. That data tells us how often your voltage dips below the 90% threshold that makes CNC servos trip. Do not skip this step; a BESS sized without it is a coin flip.

Power and Capacity Sizing: Peak Smoothing, Not Energy Arbitrage

Power sizing is straightforward. Take the 15-minute demand peaks from your load profile. Subtract the utility’s peak demand limit or your target (say 500 kW). The difference, plus a 20% margin, is the BESS power rating. For a typical metalworking shop with 150–800 kW load, we usually see peaks 30–60% above average, so a BESS between 100 kW and 500 kW covers most cases.

Capacity is trickier. You are not storing energy for hours; you are storing enough to ride out 5–15 minute peak events. Multiply the average peak power above the limit by the typical event duration, then add one event of margin. For example, a 200 kW peak over 10 minutes needs about 35 kWh, so we spec a 50 kWh battery. If you have induction furnaces that cycle every 20 minutes, we size for the worst-case overlap. At 0.4 kV, we integrate directly on the LV bus; above 800 kW we step up to 10 kV.

Connection Topology: Where the Battery Sits

Most metalworking shops are served by a 0.4 kV transformer between 250 kVA and 1250 kVA. We connect the BESS on the LV side, just after the main breaker, in parallel with your load. The inverter measures the transformer current and injects or absorbs power within 2 ms—that is fast enough to catch the first cycle of a welder’s inrush.

For sites with large induction furnaces, we sometimes add a passive harmonic filter, because the furnace’s rectifier throws 5th and 7th harmonics that the battery cannot fix. The BESS handles active power and voltage sag, but it is not a harmonic cure. That is a separate device. We always include a bypass contactor so the battery can be isolated for maintenance without shutting you down.

What Goes Wrong If Sized Badly

Undersize the power and the battery saturates on a welding peak. The voltage still sags, your CNC alarms, and you lose a part to scrap. Undersize the capacity and you run out of energy before the 15-minute peak ends—the battery drops to zero and the demand charge hits anyway. Oversize the power and you waste capital on an inverter that never operates near full rating, extending payback by two years.

Worse, if you ignore the physical location, a BESS on the LV bus cannot fix a sag that originates on the 10 kV line. We have seen sites where a welder 50 m away causes a dip that a battery at the incomer simply cannot react to in time, because the cable impedance is too high. That is why we always model the network impedance and, if needed, place a smaller battery near the sensitive load.

What a Properly Sized System Delivers

With the right BESS, your monthly peak demand drops to the target value. Expect a 20–40% reduction in the demand charge line of your bill, which in Ukraine can be 200–400 UAH/kW/month. For a 500 kW peak at 300 UAH/kW, that is 150,000 UAH per month saved. Payback is typically 2.5–4 years, depending on tariff.

Your CNC machines stop tripping on sags. A welder firing 10 m away will no longer reset a spindle drive. You also get a limited backup function: if the grid drops, the BESS can carry critical loads (lighting, CNC controllers, a server) for 30–60 minutes, depending on capacity. That is not a UPS replacement, but it keeps you from a full restart.