Storage changes when electricity is used. It does not reduce the amount of heat a building needs, and it does not make electricity-to-heat conversion equivalent to a heat pump.
A battery that stores heat
Heat21 uses the term heat battery for a water-tank thermal store. Energy is held as heat in water rather than stored as electricity for later electrical use. The system transfers that heat to heating and hot-water demand through the building's designed heating arrangement.
In the Heat21 approach, a hydronic cold plate captures heat from SHA-256 computing hardware and carries it towards the store. Our compute heat-recovery guide explains that process and its connection to Bitcoin mining.
Usable storage depends on more than tank volume. The operating temperature range, the temperatures the building needs, insulation and heat losses all affect how much heat can be delivered later. A tank's maximum temperature alone cannot tell you how long it will keep a home warm.
For the broader storage options, see the Energy Saving Trust's thermal energy storage guide.
Buy power at one time. Use heat at another.
Heating demand often rises when people wake up or return home. Those times do not necessarily coincide with the cheapest electricity. A thermal store gives a system room to shift some electricity consumption to other periods while continuing to supply stored heat.
This is useful with a suitable time-of-use tariff, where electricity prices change during the day. The potential benefit depends on the actual prices available, how much operating time can move, and whether the store can cover demand until the next charging opportunity.
A cold-day test matters. A schedule that works on a mild day may not leave enough stored heat through a winter peak. A feasibility assessment should consider peak demand as well as annual energy use.
The aim is controlled flexibility, with heating requirements built into the operating plan. Cheap periods are not guaranteed, and an electricity tariff is not proof that a particular unit of electricity came from otherwise-curtailed wind power.
See tariff-responsive water heating in practice
The GWhFI live electric-heating dashboard, a related Heat21 project, shows measured electricity use from immersion heating alongside Octopus Agile prices. Compare the power trace with the tariff and the scheduled heating windows to explore when the heater actually draws electricity.
The displayed average price is weighted by measured consumption: electricity used in each priced interval contributes to the average in proportion to its kWh. A scheduled ON window describes the control plan; it is not proof that an element drew power throughout that window. Check the dashboard's update times when interpreting the readings.
What the demonstration measures. This is an electric water-heating demonstration. It does not measure the Node's Bitcoin proceeds, useful heat delivery or avoided wind curtailment. Electricity cost is also separate from the cost of installing and maintaining the system. Our heating-cost guide explains those distinctions.
Thermal storage and heat pumps do different jobs
A heat pump uses electricity to move heat from a source such as outside air into a building. Its coefficient of performance, or COP, compares heat output with electrical input. Heat pumps can deliver more heat energy than the electricity they consume because they also draw energy from that outside source.
Compute heating recovers heat generated by electricity-consuming hardware. Storing that heat does not create extra energy. Computing revenue can reduce its effective financial cost; that is a cost offset, not an increase in thermal COP.
Heat pumps can also work with thermal storage. For a fair comparison, assess the available systems against the same building demand, temperatures and tariffs, and include installation and running costs. Heat21 focuses on an alternative where a heat pump may be difficult to accommodate; this does not mean every such building will suit a Node.
Read the Energy Saving Trust's air-source heat pump explanation for more on that technology.
What should a site assessment cover?
- The demand profile: annual consumption, cold-day heating demand, hot-water peaks and summer use.
- The heating circuit: the existing system and the temperatures it needs to deliver comfort.
- The space: room and access for the Node, storage, associated equipment and maintenance.
- The electricity supply: available capacity alongside other building loads and suitable tariff options.
- The operating plan: usable storage, expected losses, control strategy and provision during downtime.
These are questions for system design, rather than a do-it-yourself installation checklist. Start with existing bills, the type of heating installed and a description of available space when contacting Heat21.
Then test the economics separately. The offset calculator explores compute revenue; our heating-cost guide explains the additional information needed to turn a unit-cost illustration into a building-specific comparison.