The OSO Saga S Charge is a direct electric unvented hot water cylinder with a difference: it connects to your energy tariff, reads tomorrow's grid prices, and heats automatically during the cheapest half-hour windows — without you ever adjusting a timer. Here's the full explainer for homeowners, installers, councils, and all-electric developers.
AIZO Quality Heating · February 2025 · 10 min read
Hot water cylinders have barely changed in decades. They heat water, store it, and slowly lose some of that heat to the room around them. The OSO Saga S Charge does all of that — but it also connects to the internet, reads tomorrow's electricity prices, and decides autonomously when to heat so that you always have hot water at the lowest possible cost.
The result is a certified Energy Smart Appliance (ESA) — a category the UK government will eventually mandate for all new electric cylinders. OSO got there three years ahead of that deadline. Projected average savings are around £200 per year, achieved entirely automatically, by heating during the cheapest grid windows and avoiding expensive peak periods.
Who is OSO Hotwater — and Why Does Heritage Matter?
The story of OSO Hotwater begins with Reidar Frölich Braathen, who founded Oslo Sveisebedrift — OSO — by the Akerselva river in Oslo in 1932. A blacksmith and air force lieutenant skilled in electric and autogenous welding, Braathen built Norway's first fully welded hot water cylinder. By 1940 he had 100 employees. Three generations of the Braathen family have run the business ever since, each pushing the technology further — and OSO has never operated at a loss.
Today, OSO operates from Hokksund, Norway, and has been the country's most robotised manufacturing company since 2015 — a deliberate choice to keep production in Norway rather than move to lower-cost markets. That discipline shows in the product: OSO water heaters are found in two out of three Norwegian homes, and more than 4 million units have proven their durability over five decades.
In 2025, OSO became part of LG Electronics Eco Solutions, bringing Norwegian engineering heritage into LG's global sustainability platform. The Hokksund factory and product quality remain unchanged.
What sets OSO apart from most cylinder brands is a long-standing commitment to stainless steel construction at a time when competitors relied on glass-lined and ceramic tanks. OSO was among the first in the world to make stainless-steel hot water cylinders. Lighter, more corrosion-resistant, more energy-efficient, and radically longer-lived, their proprietary technologies — ULTRAWELD laser-welded Duplex steel, NANOPUR high-grade insulation, and TITANIUM IMMERSION elements — are the result of decades of refinement. The Saga S Charge is the UK market expression of their newest platform: SAGA 2.0 Supercharged.
How the OSO S Charge Actually Works
At its core, the S Charge is a direct electric unvented cylinder: cold mains water in, hot water out at mains pressure, no pump required. What makes it different is the OSO InCharge smart controller built into every unit. Here's how the intelligence works in practice:
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Day-Ahead Price Retrieval
The InCharge controller connects to your Wi-Fi and, once paired to a compatible time-of-use tariff, retrieves the next 24 hours of half-hourly electricity prices. These are published by your tariff provider (such as Octopus Energy) based on wholesale grid pricing. The cylinder knows — tonight — exactly what electricity will cost every 30 minutes tomorrow.
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Machine Learning Demand Prediction
Three internal temperature sensors track water use throughout the day. The AI in the InCharge controller builds a model of your household's hot water consumption — when showers happen, how much is drawn, how the tank recovers. It uses this model to predict tomorrow's demand with increasing accuracy over time.
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Optimal Heating Schedule
The controller combines price data with demand prediction and calculates the cheapest heating windows that will still guarantee sufficient hot water when you need it. It selects the lowest-cost 30-minute slots and schedules the immersion heater accordingly. Water is stored at 75°C — a high setpoint that maximises effective volume through temperature stratification.
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Continuous Adaptation
Every day the system refines its model. If your usage changes — new household member, seasonal shift — it adapts. If grid prices shift overnight due to a wind surplus, the schedule shifts too. The cylinder is always optimising, always ensuring hot water at the lowest possible cost.
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App Control & Override
The OSO InCharge app (iOS & Android) gives full visibility and control. See live tank temperature and available hot water volume. Boost immediately if needed. Set holiday/sleep mode. Review historical usage and cost data. If broadband goes down, the unit falls back to its last-known schedule — you never lose hot water, just smart optimisation until connectivity is restored.
OSO S Charge 150 (S2S 151L) — with S-DESIGN rear connections, integral G3 kit, and OSO InCharge smart controller built in
"Because water is stored at 75°C and blended down to 60°C at the mixing valve, a 150-litre S Charge cylinder delivers an effective usable capacity of 261 litres at a comfortable 40°C draw temperature — dramatically more than the nominal volume suggests."
OSO Technical Specification — S Charge 150 (S2S 151L)This is worth dwelling on. The high storage temperature means you effectively get significantly more usable hot water from a given tank size than with a conventional cylinder set to 60°C. For smaller flats or space-constrained installations, this means you can often install a smaller physical unit without compromising on hot water availability.
Octopus Agile, Time-of-Use Tariffs, and the ESA Standard
The S Charge's smart scheduling is designed to work with dynamic time-of-use electricity tariffs — tariffs where the price of electricity changes every 30 minutes based on wholesale grid pricing. In the UK right now, the main example is Octopus Agile, although other half-hourly pricing tariffs from Octopus Energy (such as Flux) are also compatible in principle, as they follow the same ESA communication protocol.
Octopus Agile is a time-of-use tariff from Octopus Energy that tracks the wholesale electricity market. Unlike fixed-rate or Economy 7 tariffs, Agile pricing changes every 30 minutes and is published 24 hours ahead — directly reflecting grid demand and renewable generation. When wind or solar output is high and demand is low, prices drop sharply. When demand spikes (typically 4–7pm on winter weekdays), prices rise.
Why Octopus, and What About Other Tariffs?
The S Charge communicates with your tariff provider's API to retrieve half-hourly pricing data. Octopus Energy provides a well-documented, open API for this purpose — which is why Octopus Agile is the primary compatible tariff cited in UK product documentation. If you are on a standard fixed-rate tariff, Economy 7, or a non-dynamic tariff, the cylinder works perfectly as a conventional premium unvented cylinder; the smart scheduling simply isn't active. You can switch to a compatible dynamic tariff at any time and the smart functionality activates automatically — no hardware change required.
"Octopus Agile prices have gone negative on hundreds of occasions, meaning the grid pays you to use electricity. During these periods the S Charge can heat aggressively, acting as a thermal battery charged at zero — or negative — cost."
The ESA Mandate — Why Specifying Now Matters
The UK government has signalled that new electric hot water cylinders will be required to meet the Energy Smart Appliance (ESA) standard — capable of responding to grid signals and flexible tariffs. The OSO S Charge is already certified to this standard, three years ahead of the expected mandate. For councils and developers specifying systems today, this means future-proofing is built in: the cylinder remains compliant as legislation evolves, and will continue to deliver savings as the dynamic tariff market expands beyond Octopus to other suppliers.
▶ OSO Hotwater — official product video · OSO Hotwater UK YouTube channelThe Technology Stack — What Makes an OSO Different
The S-DESIGN system is worth particular attention for installers and developers. All pipework, expansion vessels, and electrical cabling are concealed behind the unit with a fitting template included in the box for precise wall positioning. The entire G3 kit is factory-fitted — expansion vessels, pressure relief valve, strainer, tundish, and drain cock — arriving at the site as a complete, pre-plumbed, pre-wired unit. For a development of 50 flats, the cumulative reduction in on-site labour time is substantial.
| Specification | S Charge 150 (S2S 151L) | Notes |
|---|---|---|
| Nominal Volume | 151 L | Internal model: S2S 151 |
| Effective Capacity at 40°C | 261 L | Due to 75°C storage temperature |
| Dimensions (H × Ø) | 1110 × 595 mm | Including integral vessels |
| Standing Heat Loss (24 hrs) | 1.15 kWh/day | NANOPUR insulation — class-leading |
| Thermostat Setpoint | 75°C | Legionella-safe, high stratification |
| Mixing Valve Preset | 60°C | Factory-set for scalding protection |
| Immersion Power | 3 kW | Titanium grade, 240V single-phase |
| Max Operating Pressure | 6 bar | Full G3 kit factory-fitted |
| Expansion Vessels | 2 × 8L integral | Factory-fitted, concealed under lid |
| Cylinder Material | ULTRAWELD Duplex stainless | Laser-welded construction |
| Cylinder Warranty | 25 years | Longest in class for UK market |
OSO Saga S Charge — Three Sizes, One Smart Platform
All three cylinders share the same ULTRAWELD stainless steel construction, TITANIUM IMMERSION heater, OSO InCharge smart ESA controller, S-DESIGN pre-plumbed layout, and 25-year cylinder warranty. For maximum smart thermal battery benefit, OSO recommends sizing generously — the larger the tank, the more low-cost energy you can bank in any given cheap pricing window.
S Charge 150
S Charge 200
S Charge 250
Why the S Charge Is a Compelling Specification for Social Housing, New-Builds & All-Electric Flats
The S Charge's S-DESIGN conceals all pipework behind the unit — a clean, space-efficient solution for apartment and social housing installations
For local authorities, registered social landlords, housing associations, and private developers building or retrofitting all-electric properties, the OSO S Charge addresses several specification challenges at once. Here's why it belongs in a serious procurement conversation:
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Built for All-Electric, Gas-Free Developments
As the new-build sector moves away from gas connections under the Future Homes Standard and Part L Building Regulations, electric hot water becomes the default. The S Charge is built for exactly this: a direct electric cylinder requiring no gas boiler, no heat pump, and no external heat source. It works straight from the mains supply. For flat developments where gas infrastructure is impractical or prohibited, this is a complete, self-contained solution.
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Reduced Running Costs for Tenants — A Social Value Argument
OSO projects average savings of around £200 per year per household by shifting heating to off-peak pricing windows. For councils managing fuel poverty risk in their stock, a smart cylinder that materially reduces hot water bills without requiring tenants to actively manage their tariff is a meaningful intervention. The cylinder manages it automatically — no behaviour change required from the resident.
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Future-Proofed ESA Compliance — Ahead of Regulation
The S Charge is already certified to the UK's ESA standard, three years before the government mandate is expected. Councils and developers specifying now will not face retrofit costs when the legislation takes effect. This is particularly relevant for long-term asset management plans and Decent Homes Standards reviews.
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Up to 70% Installation Labour Saving — Lower Build Cost per Unit
The S-DESIGN pre-plumbed system — all pipework, expansion vessels, and wiring factory-fitted and concealed — can dramatically reduce installation time per unit. Across a 50-unit development, the cumulative saving in plumber labour hours is significant. The fitting template ensures precise positioning, and the unit arrives G3-ready with no separate component sourcing required.
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Carbon Reduction & Part L Compliance
A smart cylinder that shifts load to renewable-rich, low-carbon grid periods directly contributes to measured carbon intensity reductions — relevant both to Local Authority Net Zero commitments and to SAP/EPC calculations in new-build assessments. The S Charge's NANOPUR insulation, low standing heat loss, and ESA certification all support a credible carbon case in planning and procurement documentation.
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Remote Monitoring for Asset Management
The OSO InCharge app provides real-time temperature data and usage history per unit. For property managers overseeing multiple properties, this creates a digital performance audit trail. Legionella risk management is simplified: the 75°C storage setpoint keeps the system well above the 60°C Legionella threshold, and this can be verified remotely without a physical inspection.
We offer custom quotes and project pricing for councils, housing associations, and large-scale developers specifying the S Charge across multiple units. Request a custom project quote →
Frequently Asked Questions
Will it work if I'm not on Octopus?
The S Charge is designed for dynamic time-of-use tariffs that publish half-hourly pricing 24 hours ahead. Octopus Agile and compatible Octopus products are the main options in the UK right now. On a standard fixed-rate tariff or Economy 7, the cylinder functions as a conventional premium direct cylinder without smart scheduling. You can switch to a compatible tariff at any time — smart control activates automatically, no hardware change needed.
What happens if the broadband goes down?
The cylinder falls back to its last-known schedule or a default safe schedule. You will not lose hot water. Smart tariff optimisation pauses until connectivity is restored, and the app notifies you of any connectivity issues.
Is it suitable for hard water areas?
Yes. The TITANIUM IMMERSION heater is specifically engineered for hard water conditions. Standard Incoloy or stainless elements scale badly in hard water over time. The titanium-grade element in the S Charge is significantly more resistant to scale and corrosion — one reason OSO can offer a 2-year component warranty even in hard water areas.
Can it integrate with solar PV?
Yes. The OSO InCharge app supports solar PV integration via InCharge+ mode. The cylinder can be configured to prioritise heating during solar generation surplus, acting as a thermal battery for your PV output. This further reduces running costs beyond tariff optimisation savings.
What does it come with for G3 compliance?
The S Charge arrives with a factory-fitted G3 kit — the complete package for Building Regulations Part G3 compliance. This includes two integral expansion vessels, pressure relief valve, strainer, expansion relief, tundish, and drain cock. Your G3-qualified installer completes the commissioning record at handover.
What is the warranty?
The stainless steel cylinder body carries a 25-year warranty — one of the longest available in the UK market. Electrical components and the immersion heater carry a 2-year warranty. The warranty reflects OSO's confidence in their ULTRAWELD Duplex stainless construction.
Ready to Specify the OSO S Charge?
All three sizes in stock. Custom project quotes available for councils, housing associations, and developers.