Understanding Data Center Power: kW, Amps, and What You're Actually Paying For

A colocation contract looks like it is selling you a cabinet, a cage, or a suite. It is not. What you are actually buying is power, delivered to a defined footprint, cooled to a defined temperature, connected to networks, and guarded by physical security. Everything else on the invoice — space, cross-connects, remote hands — is priced around the power commitment. That is why understanding kW, amps, and circuits is the difference between a well-negotiated contract and a monthly bill that quietly grows for years.

A few terms show up on every colocation quote. They mean specific things: • Volts (V) — the electrical pressure. In North American data centers, cabinet circuits are typically 120V or 208V. • Amps (A) — the current flowing through a circuit. Circuits are rated at a maximum amperage (for example, 20A or 30A). • Kilowatts (kW) — the actual power delivered. This is what heats up your servers and what the utility bills for. kW = Volts × Amps × (a derating factor for safety) ÷ 1000. • Circuit — one delivery path for power, terminating in a PDU or receptacle inside your cabinet. • Redundancy (A+B) — two independent circuits from separate upstream feeds, so a single failure does not drop the rack. • PDU (Power Distribution Unit) — the strip inside the cabinet that fans power out to your servers. If a facility quotes you "a 30-amp 208V A+B circuit," they are describing the delivery method. The number that matters for capacity planning and cost is the kW those circuits can safely support.

Electrical code requires that continuous loads on a circuit not exceed 80% of the breaker rating. This is not optional and it is not a facility preference — it is how the electrical system is designed to operate safely. So a 30-amp 208V circuit does not give you 30 × 208 = 6,240 watts of usable power. It gives you 30 × 208 × 0.80 = 4,992 watts, or roughly 5 kW. On A+B redundant circuits, you still only get the usable capacity of one side, because the other side has to be able to carry the full load if its partner fails. Buyers who forget the 80% rule budget for the nameplate number, provision servers to fill it, and then find out at deployment time that they are already at the safe limit with no headroom. Always convert amps to usable kW before you plan the rack.

There are three common billing models. Know which one you are signing: • Committed kW (subscribed). You pay for a fixed amount of power every month whether you use it or not. Simple, predictable, and typical for larger deployments. If you subscribe to 5 kW and pull 3 kW, you still pay for 5. • Metered (usage-based). You pay for the actual kWh you consume, usually with a minimum commitment. Common for smaller cabinets. This can be cheaper if your utilization is genuinely low, but it exposes you to variability. • Circuit-based flat fee. You pay a flat monthly rate per circuit (for example, X dollars per 20A 208V circuit). Simple, but you are effectively paying for the peak the circuit can deliver, not what you use. The important thing is not which model is best in the abstract. It is that you compare quotes on the same basis. A "$500 per kW" quote and a "$300 per circuit" quote cannot be compared until you translate both into dollars per usable kW per month.

Buyers who are unsure of their real load tend to over-subscribe power. They estimate high, add a comfort margin, and then pay for capacity they never use. Over a 3-year term, over-provisioning by a few kW per cabinet can add up to real money. A better approach is to measure. If you already have servers running, capture actual power draw from your PDUs or from the servers' out-of-band management interfaces over a week or two. Use the observed peak plus a modest growth allowance, not the nameplate wattage on the back of the server. Nameplate ratings are worst-case-plus-margin and are almost always well above real draw. Under-provisioning is a real risk too, but it is a different failure mode and it is easier to remediate mid-term than a fixed over-commit.

Two facilities can quote you the same kW but at very different prices, and the difference is often density. A facility rated for 5 kW per cabinet has to spread that same total power across more floor space, more cooling capacity, and more infrastructure per kW delivered than a facility rated for 15 kW per cabinet. As a general trend, higher-density-capable facilities can deliver kW more efficiently, but they may also charge a premium for that capability. Lower-density facilities may be cheaper per kW but force you to lease more cabinets to reach the same total capacity. The right choice depends on how much power your equipment actually needs per rack and how much you value contiguous footprint. AI and GPU-heavy workloads have pushed cabinet-level power requirements up sharply in the last few years. If you are deploying accelerated compute, the density conversation is not optional — it is the whole conversation.

A colocation quote should not just list a price. It should let you answer these questions cleanly: • What is the usable kW per cabinet, after applying the 80% rule? • Is the power committed or metered? What is the overage rate if we exceed the commit? • What is the redundancy model (A+B, 2N, N+1) and does the quoted kW assume single-side or full-load capacity? • What are the annual escalators on the power rate? • What happens at renewal — can we adjust the commitment down as well as up? • What is the process and lead time to add power mid-term if we grow? • Are cross-connects and remote hands billed separately? At what rate? A facility that gives you clear answers is a facility that has been through this conversation with sophisticated buyers before. A facility that dodges these questions is telling you something about how the relationship will feel three years in.

Right-sizing power and translating between billing models is exactly the kind of work vendor-neutral advisory exists for. We help clients measure real load, model 3-year cost across facilities on an apples-to-apples basis, and negotiate contract terms — commit levels, escalators, overage rates, and expansion mechanics — that do not quietly punish growth. If you are evaluating a new colocation contract, renewing an existing one, or just trying to figure out whether you are paying too much for the power you actually use, contact us. A single review conversation is usually enough to know whether there is a meaningful number on the table.