I · How a water battery works
Blenheim-Gilboa is two reservoirs linked by a mountain. The upper pool sits on high ground; the lower pool sits along Schoharie Creek, roughly 1,000 feet below at last public report. At night, when demand is low and power is cheap, the plant runs its machines in reverse and pumps water uphill. During the day, when the city's demand climbs, it lets that water fall back through the same machines, now spinning as turbines, to make electricity.
The plant holds on the order of 1,160 megawatts of generating capacity across four reversible pump-turbines, per NYPA's project page, enough to matter on a tight afternoon. It loses some energy in the round trip, as every storage system does (you get back roughly three-quarters of what you put in, give or take). What it buys in return is timing: it moves cheap overnight power into the expensive peak, and it can ramp up fast, on the order of minutes, when the grid needs it.
II · Why it is not the same as the batteries on your block
Container batteries going into the five boroughs, the ones that draw fire-code hearings and community board meetings, are lithium chemical cells. They respond almost instantly and sit close to where the power is used, but each site holds a modest amount and discharges for a few hours at most. Blenheim-Gilboa is the opposite trade: far from the city, slow and costly to build (it took years and a mountain), but able to pour out power for many hours at a stretch.
Think of it as two tools for two jobs. The container batteries handle short, sharp needs and local reliability; the pumped storage handles the long evening ramp and holds a large reserve the grid operator can call on. NYISO (the New York Independent System Operator, which runs the state's bulk power market and keeps supply and demand matched) treats Blenheim-Gilboa as a dispatchable resource it can lean on, not a neighborhood asset.
III · The upgrade, and what it means for wind and sun
NYPA put the plant through a life extension and modernization program that wrapped up around 2010, at last public report, replacing aging turbine-generators and controls so the reservoirs can keep working for decades more. The upgrade did not move the mountain or the water; it renewed the machines between them.
Renewing those machines matters more now than when the work was first planned. New York is adding large amounts of upstate wind and solar, and new lines are being built to carry clean power, upstate wind and solar and Canadian hydropower, down toward the city. Those resources are strongest at the wrong times (wind often at night, sun near midday), and a water battery that can soak up a surplus and release it hours later is one honest way to keep that power from being wasted. Whether Blenheim-Gilboa alone is enough is a fair question, and the answer is no; it is one piece of a much larger build.
IV · Worth watching this month
1. NYPA's project page for Blenheim-Gilboa, where the capacity and modernization details are posted, is the first place any changed number should show up.
2. NYISO's regular power trends and reliability reports are where you can watch how much the state leans on long-duration storage as wind and solar grow.
3. FERC's hydropower licensing records (the federal license that lets the project operate) are worth checking for any relicensing or environmental review steps, which for a plant this age tend to be routine but public.
4. The major transmission lines bringing upstate and Canadian power toward the city have scheduled in-service milestones that have slipped before, so treat any new date as tentative until the operator confirms it.
5. NYSERDA's energy storage program updates are a reasonable gauge of how the state is valuing long-duration storage against the container batteries going into the boroughs.