Is a solar hot water diverter (Eddi) worth it if I have natural gas in Ireland?

In almost all cases, no. Natural gas in Ireland costs approximately 10c/kWh (or ~11.8c/kWh effective heat at 85% boiler efficiency). However, the Clean Export Guarantee (CEG) pays solar owners between 15.0c and 20.0c/kWh for exported electricity (e.g. 18.5c with Energia & Bord Gáis; 19.5c with Electric Ireland). Diverting 1 kWh of solar electricity to heat water saves ~11.8c of gas but sacrifices 18.5c–19.5c in guaranteed export cash—a net loss of over 7c per kilowatt-hour. The €750 diverter hardware will never break even.

When is an immersion diverter strongly recommended?

An immersion diverter is strongly recommended when your alternative water heating source is direct electric immersion (charging standard day rates of 34c to 42c/kWh) or expensive off-grid oil where the boiler is turned off during the summer. Displacing 36c electricity with solar yields an immediate 15c/kWh advantage over a 21c export rate, achieving hardware payback in 2 to 3 years.

Should solar charge my home battery before diverting to hot water?

Yes. Storing surplus solar electricity in a home stationary battery displaces expensive peak evening grid electricity (38c to 44c/kWh). In contrast, diverting solar to hot water only displaces cheaper fuel like gas (11.8c) or oil (14.4c). Therefore, battery charging must always take priority over hot water diversion.

How much energy does it take to heat a 150-litre cylinder of water?

Using the specific heat capacity of water (4.184 kJ/kg·°C or 0.001162 kWh/L·°C), heating a 150-litre cylinder from 12°C cold mains water to the 60°C anti-legionella threshold requires 150 × 48 × 0.001162 = 8.37 kWh of thermal energy. Adding daily standing jacket losses (0.6 to 1.5 kWh/day), total daily demand is approximately 9.0 to 9.9 kWh.

THERMODYNAMIC ENERGY & EXPORT ENGINE // IRELAND & UK·CEG ARBITRAGE

Hot Water Diverter vs. Battery vs. Export Engine

Evaluate whether diverting surplus rooftop solar into your hot water cylinder (e.g., Myenergi Eddi / Solar iBoost) saves money, or if exporting to the grid under the Clean Export Guarantee (CEG) yields higher cash returns.

Specific Heat: 4.184 kJ/kg·°CGas / Oil / Heat Pump / Immersion15c–20c CEG Export ArbitrageHome Battery Priority HierarchyEddi Hardware Payback Amortization

OPTION A: IMMERSION DIVERTER (EDDI)

HOT WATER STORAGE

Annual Displaced Heating Fuel:

€186/year

Based on 1,583 kWh diverted solar heat

Effective Heat Cost Displaced:€0.118 / kWh

Daily Cylinder Energy Demand:6.56 kWh/day

Hardware Capex (Eddi + CT Clamp):€750 installed

OPTION B: DIRECT GRID EXPORT (CEG)

FEED-IN CASH TARIFF

Annual Foregone Export Revenue:

€332/year

Cash payments received if surplus is exported instead of diverted

CEG Export Unit Payment:€0.210 / kWh

Capital Expenditure Required:€0 (Zero Capex)

TCA 1997 §216D Tax Exemption:Tax-Free up to €400/yr

STEP 1: WATER CYLINDER & CURRENT HEATING FUELTHERMAL CONSTANTS

What heat source currently heats your hot water?

Natural Gas Tariff:

c/kWh

6.0c10.0c (National Benchmark)20.0c

Hot Water Cylinder Volume:

Litres

120L (1–2 Bed)150L (Standard 3–4 Bed)210L (Large)300L (Commercial/HP)

Cylinder Jacket Insulation:

Standing losses continuously dissipate heat into the hot press / cupboard.

STEP 2: CEG EXPORT & HARDWARE SPECSARBITRAGE PARAMETERS

Clean Export Guarantee (CEG) Rate:

c/kWh

Rooftop Solar PV Array:

kWp

3 kWp (Compact)5 kWp (Standard)10 kWp (Full Roof)

Property Has Home Battery (ESS)

Battery prioritizes first to store energy for displacing 38c peak evening grid power before hot water diverter activates.

Diverter Installed Capex:

€

€550 (DIY / Supply)€750 (Standard Eddi)€1,000 (Complex Wiring)

Capital Recovery & Payback Horizon

Years to recover the €750 installed cost of an Eddi diverter vs. export revenue

PERMANENT ANNUAL LOSS

Payback Benchmark (7-Year Industry Target):Never Breaks Even

0 Years (Immediate)3.5 Years (Good)7.0 Years (Industry Max)15+ Years (Unviable)

📐Calculation Methodology, Thermodynamic Constants & Statutory Citations▼

1. Specific Heat Capacity of Water

Water requires exactly 4.184 kJ of energy to raise the temperature of 1 kilogram (1 Litre) by 1°C. Converted into electrical units:

c = 4.184 ÷ 3,600 = 0.001162 kWh / (Litre · °C)

For a standard 150-Litre unvented or vented cylinder heating from 12°C cold mains water to the 60°C anti-legionella baseline, the thermal energy required is:150 × (60 - 12) × 0.001162 = 8.366 kWh.

2. Standing Jacket Losses (EN 12897 Standard)

No hot water cylinder holds heat indefinitely. Factory spray foam insulated cylinders (polyurethane) lose approximately 0.6 to 0.8 kWh per 24 hours at 60°C. Older cylinders fitted with loose glass-wool jackets or uninsulated bare copper lose 1.5 to 2.2 kWh per day into the hot press.

3. Heating Fuel Replacement & Boiler Seasonal Efficiency

Natural Gas: High-efficiency condensing gas boilers achieve ~85% seasonal efficiency for hot water (Effective = Gas Tariff ÷ 0.85).
Kerosene Home Heating Oil: 1 Litre contains 10.0 kWh raw thermal energy; modern oil boilers deliver ~80% seasonal efficiency (Effective = (Cost per Litre ÷ 10.0) ÷ 0.80).
Direct Electric Immersion: 100% Joule resistive heating efficiency (Effective = Electricity Day Rate).
Heat Pump (DHW Mode): Operating at high temperature lift (12°C to 55°C) reduces COP to ~2.6 (Effective = Electricity Day Rate ÷ 2.6).

4. Irish Clean Export Guarantee (CEG) & Tax Rules

Under the Commission for Regulation of Utilities (CRU) Clean Export Guarantee framework and Section 216D of the Taxes Consolidation Act 1997, domestic microgeneration solar owners receive between 15.0c and 20.0c/kWh tax-free (up to an annual statutory exemption of €400 per domestic premises).

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