The UK could be in for another winter of discontent, with tight electricity supplies and high demand squeezing the National Grid within watts of its max capacity, according to new analysis by a research company that monitors the wholesale electricity market.

According to Enappsys, electricity supplies last winter were the tightest they've been for a decade. Over the past year, 11 coal- and gas-powered plants have been fully or partially shut down, with only two new gas plants to replace them.

Enappsys says that, if the demand for electricity this winter matches last winter, there will be 85 hours where the grid has less than 2 gigawatts (GW) of spare capacity. Further, the research company estimates there will be 12 hours where demand will exceed capacity. When that happens, the National Grid will have to fall back on some of its last-resort measures.

The UK's power requirements fluctuate between about 25 gigawatts (GW) on summer nights to about 35-40GW through the winter months, peaking at 4pm to 8pm on the coldest days. Dealing with such variance is difficult, but the National Grid has lots of knobs that it can pull to meet demand, from lifting the sluices on small hydro systems, to firing up older coal and gas plants.

While most other countries have grid interconnections with neighbouring countries, the UK has just a single 2GW subsea cable between Folkestone and Sangatte in France, and a smaller 1GW link between Kent and the Netherlands. In both cases, electricity nearly always flows to the UK. A second 2GW link further down the coast, between Hampshire and Normandy, has been proposed, but it won't be deployed until at least 2020.

When those conventional methods of balancing the books can't keep up with demand, the National Grid has to use one of its last-resort measures. The most basic of these is firing up a bunch of smaller generators, which are mostly diesel and gas. These reserves (or Short Term Operating Reserves in National Grid parlance) must have a minimum capacity of 3MW, must be online within 240 minutes of the National Grid telling them that a shortfall is incoming, and must be able to produce a contracted amount power for at least two hours.

The National Grid pays a lot for this short-term electricity: last winter, at least one of those smaller power generating companies was paid £2,500 per megawatt-hour (MWh). The going rate for electricity produced by a large CCGT (combined cycle gas turbine) plant—the backbone of UK power generation—is about £60 per MWh.

Another option available to the National Grid is frequency response load balancing, where large power users (steel works, water pumping stations) automatically turn off when the electricity frequency falls below a certain threshold (if it falls below 49.8Hz, there's usually trouble brewing). These sites agree to stay offline for at least 30 minutes, during which time the National Grid can power up a ton of diesel generators, to prevent any potential blackouts—and then they can turn back on again.

Finally, the National Grid back in 2014 started paying some businesses to reduce their energy usage, or use on-site backup generators, when demand was highest. At the time, the National Grid said it was looking to shave off about 330MW in national energy demand that winter.

The UK is currently in a bit of an awkward spot when it comes to power generation. Both the previous and the current governments have made solid progress towards phasing out coal power, but new power sources aren't being brought online fast enough to make up for the shortfall.

A number of planned nuclear power plants are still in bureaucratic limbo: Hinkley Point C, which was going to be the first of a new batch of nuclear plants in the UK, has once again had its final decision delayed until at least September 2016. The two proposed European pressurised reactors (EPR) at Hinkley would produce about 3,200MW, or 7 percent of the UK's needs. Throw in Sizewell C—which would produce a further 3,200MW from two more EPRs—and suddenly the UK would have a lot of power generation. At this point, if EDF finally gives the greenlight, construction on Hinkley Point C probably can't begin before 2019. Sizewell C would be some time after that.

Both on- and off-shore wind power is doing well in the UK, but it's not reliable enough: while we have a theoretical wind power capacity of 13GW, the actual amount of power produced is highly variable. Back in November 2015—the last time we had a major power shortfall in the UK—our wind installations only produced around 400MW.