⚡ Nerd Stuff

Yes! BOLT Can Work With Your Existing Home Batteries: BOLT integrates seamlessly with your home battery system. There are two models:

• BOLT-16: 12 kW from the grid + 4 kW from BOLT internal battery (16 kW total)
• BOLT-20: 12 kW from the grid + 8 kW from BOLT internal battery (20 kW total)

IMPORTANT: Your home inverter determines how much power your batteries can supply. For example, if your inverter is 4 kW, your home battery can only supply 4 kW; any additional power needed comes from BOLT’s battery or the grid.

Central Heating: When running heating or space heating, BOLT does not use its internal battery. Heating draws only from your home battery (up to inverter limit) and the grid as needed.

Power Examples:

• Eco Shower (~10 kW): 6 kW home battery + 4 kW BOLT battery → 10 kW total, no grid required
• Standard Shower (~12 kW): 6 kW home battery + 4 kW BOLT battery + 2 kW grid → 12 kW total
• Fast Bath (~16 kW): 4 kW home battery + 4 kW BOLT battery + 8 kW grid → 16 kW total
• Two Showers (~20 kW): 6 kW home battery + 8 kW BOLT battery + 6 kW grid → 20 kW total

Battery Sizing Warning: Hot water draws high power. Even with BOLT, your home battery can deplete quickly. Recommended minimum battery size: 15 kWh usable capacity. kW, kWh & Flow Rate Explained for details.

Off-Peak Charging: Both your home batteries and BOLT can charge overnight at low-cost tariffs, providing cheap energy for showers and heating during the day.

Solar & Battery Integration: BOLT has its own inbuilt MPPT inverter if you don’t have solar panels. If you do have solar, BOLT can charge from your panels through your existing inverter at up to 2 kW. During high-demand periods, BOLT intelligently blends power from your home battery, its internal battery, and the grid for optimal efficiency.

• Solar-first charging when available
• Battery top-up from low-cost grid windows
• Seamless handover for backup scenarios

Off-Grid Heating

When you're off-grid, all heating power must come from your home battery system. This means your inverter size becomes the limit.

• 6 kW inverter → maximum 6 kW heating output
• Heating will warm the house more slowly but hold temperature efficiently once reached
• Battery capacity must be sized to the heat loss of your home for stable off-grid performance

Off-Grid Hot Water

BOLT can still deliver strong hot water performance off-grid by blending your home battery output with BOLT’s internal battery.

• With the 4 kW BOLT

• 4 kW from BOLT battery + 6 kW from home battery = 10 kW total
• Suitable for ~7 L/min (average comfortable shower)

• With the 8 kW BOLT

• 8 kW from BOLT battery + 6 kW from home battery = 14 kW total
• Suitable for ~10 L/min (strong shower or bath fill)

BOLT can typically run off-grid showers for around 45 minutes before its internal battery is depleted.

Increasing Off-Grid Capacity

• BOLT batteries can be doubled to extend shower runtime
• Your home batteries can also be expanded for longer heating performance

Solar Charging Options

Solar plays a key role in off-grid living, and BOLT has been designed to work flexibly with any solar setup. You can feed BOLT in multiple ways, depending on your installation:

1. Charge From Your Existing Solar Panels (Through Your Inverter)

• BOLT can charge from your existing solar system at up to 2 kW
• This uses the same path your home batteries use, so BOLT fits in naturally
• Ideal when you already have a full hybrid inverter setup

2. Charge Directly From Solar (BOLT’s Built-In MPPT)

• BOLT includes its own dedicated solar MPPT inverter
• Perfect for homes with no existing solar system, or for adding a second dedicated array
• Lets you add extra panels just for hot water, without touching your main inverter
• Means you can run hot water completely off-grid even with a very small home battery

3. Charge From Your Home Battery System

• BOLT can top itself up using surplus energy stored in your home batteries
• This ensures all solar you generate is used efficiently before export or curtailment

4. Expandable Solar Arrays

• If you have spare roof space, you can create a bolt-only solar array
• This keeps your main house battery topped up for heating while BOLT’s array handles hot water
• Ideal for high-demand off-grid homes that need separate energy channels

Together, these options allow you to run BOLT entirely off-grid, balancing your solar, house batteries, and BOLT’s internal battery to achieve reliable heating and hot water year-round.

Why heating doesn’t use BOLT’s internal battery:

Heating a home requires a large amount of stored energy compared to hot water. A typical 5-bed property can need around 30 kWh of heat per day in winter. A battery sized to handle this would be extremely large, expensive, and sit unused throughout summer.

For this reason, BOLT uses the grid or your home’s main battery system (or off-grid inverter/battery) for space heating. Home batteries are ideal for lower-power, steady heating loads and can recharge gradually from solar or low-cost grid windows.

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Why BOLT’s internal battery is essential for hot water:

Showers and baths require very high power for a short period — typically 10–20 kW. No standard home battery or inverter can supply that amount of power alone without tripping the system.

BOLT solves this by combining two power sources simultaneously:

• BOLT’s high-power internal battery
• Your grid connection or home battery/inverter

This dual-source design allows BOLT to safely deliver **up to 20 kW** for powerful showers and fast bath fills — without overloading the property’s supply.

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Simple explanation:

• Heating → high energy over many hours → best handled by home batteries.
• Showers → high power bursts → must use BOLT’s internal battery + grid/home battery together.

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Future flexibility:

BOLT may enable the internal shower battery to support heating in the future through a software update, if safe and technically feasible.

BOLT’s key innovation lies in using a high-power battery to rapidly heat flowing water in a compact, highly efficient design. Unlike traditional immersion tanks, which rely on a large volume of water and heat slowly, BOLT minimizes the water volume while maximizing the surface area in contact with the heater. This allows the energy from the battery to instantly heat the water, giving exceptional thermal responsiveness and reducing waste.

The thermal ramp time is incredibly fast, taking less than four seconds for the heating element to reach full output. At any given moment, less than 0.3 litres of water is in contact with the DC heater. Flow is carefully monitored and controlled, ensuring that each drop of water receives the perfect amount of energy without over-heating or wasting power. This means the user experiences hot water almost instantly at the tap.

BOLT also features a patented double-jacket system. Any heat escaping from the core is drawn back into the outlet, allowing the heat exchanger to operate at full 20 kW while remaining cool externally, without the need for insulation. Additionally, BOLT’s design leverages the central heating water as a solid-state heat exchanger. When the heating system is hot but water is not flowing, heat gradually penetrates the inner core. This pre-warms incoming water slightly, further improving overall efficiency and performance.

These innovations are protected by patents covering the low-volume, high-surface-area battery-to-water flash-heating system, precise flow monitoring and DC heater control, the double-jacket heat recycling, and the central heating pre-warming design. Together, they make BOLT unique, delivering instant, high-power hot water while maximizing efficiency and minimizing energy waste.

• Coming soon …

Understanding kW & Amps — and Why It Matters for Hot Water

Before diving into the details, it’s important to understand two key electrical terms: kW (kilowatts) and A (amps).

• Amps (A)

Amps are a measure of **current** — how much electricity is flowing at any moment. Your home has a fixed maximum, usually 60A, 80A, or 100A.

This limit determines how much power your home can supply to all appliances at once.

• Kilowatts (kW)

Kilowatts measure **power** — how much energy an appliance uses per second. Higher kW means stronger heating, faster boiling, or more intense electrical load.

• How Amps and kW Relate

In UK single-phase homes:
Power (kW) ≈ Amps × 230V ÷ 1000

Examples:

• 10A ≈ 2.3 kW
• 32A EV charger ≈ 7.3 kW
• BOLT at full power: 50A ≈ 11.5–12 kW

This relationship is critical because hot water requires very high kW, which in turn demands high amps. Most properties cannot deliver 20 kW from the grid alone — which is why BOLT uses smart load-sharing with its battery.

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Power in the Modern Home: Why Heating Water Dominates Energy Use — and How BOLT Manages It Intelligently

When most people think about heavy electrical loads, they picture ovens, tumble dryers, or an EV charger. But the largest energy demand in any household is hot water.

Why Water Heating Is So Power Hungry

Hot water requires heating large volumes continuously — not a small amount for a few minutes like a toaster or microwave. A shower or bath needs litres of water per minute raised from cold to 40–50°C while holding the flow steady.

Typical Household Loads:

• Kettle / Toaster — 2–3 kW
• Microwave — ~1.5 kW
• Dishwasher — 2.0–2.4 kW
• Washing Machine (heating cycle) — 2.0–2.5 kW
• Oven — 2.5–3.5 kW
• Tumble Dryer — 2.5–3.0 kW
• 7 kW EV Charger — ~30A
• Traditional electric water heater — 8–12 kW+

Water absorbs heat slowly, so the power required is enormous — this is why hot water dominates most energy bills.

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Household Power Limits — and Why They Matter

Most UK homes have 60A–100A supplies (100A for modern builds). But you can’t allocate that whole amount to one device — it must be shared across:

• Kitchen appliances
• EV chargers
• Fridges & freezers
• Washing machines
• Heat pumps or radiators
• Lighting & electronics
• Background standby loads

This is why electric boilers must be designed extremely carefully.

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How BOLT Fits Into the Home’s Power Limit

BOLT delivers full hot water performance without exceeding your property’s supply.

BOLT’s full-power mode:
• Draws 50A (~12 kW) from the grid • Still leaves headroom for cooking, EV charging, and household loads • Works safely even in homes with 80A main fuses

This is the crucial difference between something that works on paper vs. something that works in every home, every day.

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Eco Mode — For 60A Homes or Low-Impact Running

Many homes still have 60A main supplies. For these, BOLT offers Eco Mode:

• Grid draw limited to just 8Kw
• Internal battery supplies the remaining power
• Hot-water output equivalent to ~30A
• No electrical upgrades required

Perfect for taps, sinks, and normal hot water use. When a shower starts, the battery automatically fills the power gap.

Eco Mode makes BOLT:

• Compatible with nearly every household
• Safe for older wiring
• Extremely grid-efficient
• Capable of high-performance showers without upgrades

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Why Battery-Supported Operation Saves Money

Even in strong 80A+ homes, Eco Mode offers major savings:

• Heats water using cheap off-peak tariff energy
• Or free solar power stored in the battery
• Grid usage stays as low as 8Kw
• Shower boosts come from the battery, not the grid
• Avoids expensive peak tariff spikes

BOLT effectively becomes a smart, battery-powered hot water system that protects the grid while lowering your bills.

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Summary: Intelligent Power for Modern Heating

Hot water is the biggest electrical load in the home. Traditional electric boilers simply brute-force it — requiring huge grid draw.

BOLT solves this with:

• Smart load sharing
• Battery-supported heating
• Automatic grid protection
• Compatibility from 60A to 100A households
• Full 12 kW performance even when the grid alone cannot supply it

Whether running at full 50A or at 30A in Eco Mode, BOLT delivers reliable hot water that fits safely within your home’s electrical limits — while lowering peak demand and reducing energy costs.

Eco Mode

Eco Mode prioritises your battery for all hot water use. Low-power taps and mixers are fully supplied by the battery alone. If more power is required—such as during a shower—the boiler will intelligently top up the difference using:

• Your home batteries (depending on your inverter size)
• Your solar generation
• The grid

Eco Mode limits grid usage to 8 kW (≈30 A) out of BOLT’s 12 kW grid capacity. This makes Eco Mode ideal for homes with 60 A or lower mains supply, and helps reduce electricity costs.

Maximum Power in Eco Mode:

• 4 kWh internal battery → 12 kW total (≈7 L/min shower)
• 8 kWh internal battery → 16 kW total (≈9–10 L/min shower)

For best performance, we recommend using eco shower heads.

While in Eco Mode, the display/app shows a Bath Boost button. This temporarily switches to Normal Mode for 30 minutes—perfect for filling baths or occasional power showers—before automatically returning to Eco Mode.

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Normal Mode

Normal Mode inverts Eco Mode behaviour:

• The grid is used first to supply up to 12 kW (50 A).
• The internal battery only assists when demand exceeds grid capability.

Battery support occurs when:

• Two showers run at once
• A high-flow power shower is used
• A bath requires rapid heating

This mode guarantees maximum comfort and performance. Even if the internal battery becomes empty, the grid alone can supply one continuous shower.

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Auto Mode

Auto Mode intelligently switches between Eco and Normal modes depending on real-time demand.

• If flow demand is low (≈8 L/min or less), Auto Mode selects Eco Mode for maximum efficiency.
• If demand is high—such as for a shower or bath—Auto Mode switches to Normal Mode.

This gives you the best of both worlds: energy savings when demand is low and high performance when needed.

Priority Rule: If both Auto Mode and Eco Mode are enabled, Eco Mode always takes priority. The system will only defer to Normal Mode if:

• The internal battery becomes depleted

This ensures hot water is always available while maintaining efficiency.

Is Your Home Suitable for BOLT?

Most homes in the UK are compatible with BOLT, but there are a few things worth checking so the system can run safely and efficiently.

🔌 Electrical Requirements

BOLT needs a stable electrical supply because it heats water instantly. - In Eco Mode, your home must be able to provide at least 60 A. - In Normal Mode, we recommend an 80 A supply.

The boiler must be wired directly from the consumer unit using 10 mm² cable, or 16 mm² if the cable run is longer than 25 m.

💧 Water & Flow Rate

BOLT works with all common UK pipework — including microbore — and with both radiator systems and up to 100 m² of underfloor heating.

If your flow rate is unusually high, your installer may reduce it so BOLT can deliver consistent, stable hot water. You can read more in the Flow Rates section.

📦 Boiler & Battery Placement

- The boiler weighs 45 kg and measures 450 × 630 mm, with around 10 cm of clearance needed above and below.
- The battery should be placed within 3 m of the boiler.
- Batteries must not be installed in lofts, sleeping areas, or anywhere that could block escape routes in a fire.

🔥 Safety Basics

Your home needs working smoke or fire alarms, and a suitable location for the boiler and battery. Good insulation will also help keep running costs low — like any electric heating system.

💷 What About My Bills?

From our beta trials, we found that homes with household batteries and off-peak charging were often able to match the running cost of gas heating.

Some homes did see an increase during winter — mainly because electricity is generally more expensive than gas. But there's good news:

• Summer running costs dropped dramatically for most beta users.
• There’s no water tank to keep hot all day.
• BOLT heats water instantly and only when needed.
• A 12 kW shower no longer needs a huge 34 kW gas boiler to fire up, or a tank to be heated first.

In other words, BOLT is far more efficient in real daily use — especially when paired with solar or off-peak charging. Want to see real-world numbers? Take a look at our testimonials and case studies to learn how much our beta families spent across summer and winter.

Power, Energy, and Flow: Explained Simply

To understand your hot water system, there are three important concepts: kilowatts (kW), kilowatt-hours (kWh), and flow rate. Kilowatts tell you how fast energy is delivered — think of it like the size of a garden hose. The bigger the hose, the faster water flows. A higher kW means your water heats up more quickly.

Kilowatt-hours (kWh) measure the total energy used over time — like how much water ends up in the bucket at the end of the hose. kW is how fast you are “pouring,” and kWh is how much you actually delivered. Here’s a simple analogy: kW is like how fast a car is going, while kWh is how far you have driven (the speed times the time you were driving). A big engine (high kW) can get hot water very quickly, but the total energy (kWh) depends on how long you run it.

Flow rate tells you how much water is moving through your shower or tap, usually measured in litres per minute (L/min). Hot water comfort depends on all three together: power (kW), total energy used (kWh), and flow rate (L/min).

In the UK, incoming mains water is typically around 12 °C. This already accounts for slight pre-warming from central heating in winter, which means the hot water system only needs to raise the temperature by 31°C to reach the recommended shower temperature. The delivered temperature from BOLT in the examples below is around 42 °C, which is considered a safe upper limit for all showers. The max delivery is 55°C.

Typical Showers & Baths in the UK

Scenario	Flow Rate (L/min)	Temp Rise (°C)	Power Needed (kW)	Time to Fill / Run
Average shower	7 L/min	31°C	10–12 kW	Continuous (instant hot water)
Shower 16 kW	9 L/min	31°C	16 kW	Continuous
100 L bath	—	31°C	16 kW	≈ 13.5 min
100 L bath	—	31°C	20 kW	≈ 11 min
Two average showers	6–7 L/min each	31°C	20 kW	Continuous
High-flow shower	~14 L/min	31°C	20 kW	Continuous

BOLT matches kW, kWh, and flow rate carefully to deliver instant, comfortable hot water while using energy efficiently. This means your shower or bath is always hot without wasting electricity or overloading your household supply.

Sources for UK shower flow rates: bestelectricshower.co.uk, tapsuk.com, ccw.org.uk.

Using BOLT with Home Assistant

BOLT works well with Home Assistant, and many of our users already use it to build clever automations and dashboards. While we don’t yet offer an official “BOLT for Home Assistant” plugin, connecting is simple and fully supported.

🌐 Connecting to BOLT

On your BOLT display, you’ll see the device’s local IP address. Just enter that address into your web browser — this brings up the BOLT interface along with developer-friendly documentation.

Inside that page, you’ll find:

• A full list of WebSocket variables you can subscribe to
• Instructions for splitting the WebSocket stream into individual values inside Home Assistant
• Examples showing how to read temperatures, flow rates, battery data, modes, and more
• A guide to sending commands using our simple JSON protocol

🧠 Automations You Can Build

Once you’ve connected BOLT to Home Assistant, you can create powerful automations, such as:

• Turn on Eco Mode when your electricity tariff goes up
• Boost the boiler when occupancy sensors detect a morning shower routine
• Charge BOLT only when solar output exceeds a chosen threshold
• Track running costs, hot water usage, and system performance in custom dashboards

🛠 Sending Commands

The local IP page also includes examples for sending commands directly to BOLT using our JSON control interface. These commands allow you to switch modes, enable boosts, pull system data, and more — all from your Home Assistant workflows.

🎉 Have Fun — and Show Us What You Build

BOLT is designed to be open, flexible, and fun to integrate. We love seeing what people create — whether it’s smart tariffs, whole-house energy flows, or quirky automations. If you build something interesting, please share it with us!

Here at Bolt we're hardware enthusiasts. We've built the platform, but there's so much more to do to decarbonise the world. Our goal is to make the entire control platform open source — while the main internal safety board will stay closed source (for safety reasons), the screen interface and all data-processing units are ESP32-based and fully accessible to the wider tinker community!

We want this heating system to become an open-source ecosystem, directly compatible with all our future products (which we think you'll love). If you’d like to help us bring this open-source vision to life, get in touch!

Best Practices for Installing BOLT

Installing BOLT is straightforward, but a few simple choices help your system perform at its absolute best. If your home has good insulation or a battery system, you’ll get even better results — lower bills, smoother heating, and less reliance on the grid.

🔌 Electrical Setup

To power BOLT safely and reliably:

• Use 10 mm² cable for cable runs under 20 m, or 16 mm² if the run is longer.
• Install a double-pole isolator switch within 3 m of the boiler.
• BOLT must be protected by an MCB and RCD or RCBO, as with any high-power electric appliance.

🚿 Hot Water Optimisation

BOLT heats water instantly, so the right flow rate makes a big difference. We strongly recommend eco showerheads — they give a great shower while reducing the amount of power needed to maintain temperature.

🔋 Solar & Battery — two options

BOLT works well whether you already have solar or not — here’s how:

If you don’t have solar

BOLT includes an integrated solar charger that gives you a very affordable way to add solar to your home:

• Provides up to 2 kW of PV input and includes a 4 kWh internal battery option.
• Designed to be islanding-free and simple to install, making it a cheap, low-friction route to on-site solar generation.
• This is a good option if you want some solar benefit without the full cost and complexity of a separate PV + battery system.

If you already have solar panels

We recommend configuring your existing panels to charge BOLT first. Why?

• Once BOLT’s battery is charged, panels that are routed only to a charger with nowhere else to send energy may sit idle.
• If BOLT is configured to accept panel power, excess generation can be stored in BOLT for hot water or used later around the house.
• Having BOLT absorb panel output prevents wasted solar and lets your panels continue to power household appliances once the boiler is satisfied.

🏠 Insulation, Insulation, Insulation

It all starts here.

The single biggest factor in reducing heating costs — regardless of technology — is insulation. A well-insulated home holds onto heat for hours, meaning BOLT doesn’t have to work as hard to keep you comfortable.

• Loft insulation
• Wall insulation
• Properly sealed windows and doors
• Insulated pipework and hot water runs

BOLT is efficient, but retaining heat is what truly lowers bills.

🌡 Underfloor Heating Tips

Underfloor heating pairs exceptionally well with BOLT — especially on off-peak tariffs or with solar. Pre-warm the screed to about 23 °C during cheap periods (overnight or during sunny hours) and the floor will gently release heat through the day.

This approach:

• Keeps your home comfortable with minimal daytime electricity use
• Effectively turns the floor into a “thermal battery”
• Lets you take advantage of cheap or free solar energy

⚡ Eco Mode & Daily Use

For showers, using Eco Mode with an eco showerhead gives a consistent hot water experience while dramatically reducing grid load.

Following these installation best practices keeps your system safe, maximizes performance, and allows BOLT to deliver instant, reliable hot water with the lowest possible running costs.