This post documents my full 2025/26 Victron ESS build designed purely for tariff arbitrage (Octopus 7p night rate → ~29p day rate).
After a year of planning and building my own ESS (no solar, battery only), I’m sharing real numbers, honest expectations, and a full bill of materials.
At a Glance - What to Expect
- Battery: Fogstar 16.1 kWh → ~12.44 kWh at the AC output of the inverter, corresponding to an overall AC‑to‑AC round‑trip efficiency of ~74% in my installation.
- Effective off‑peak delivered cost: ~9.40p/kWh (7p charge rate × measured round‑trip losses)
- Annual savings @ 29.14p/7p spread: ~£660/year
- Payback: ~6.5 years (system cost £4,318)
- If day rate drops to ~26p (April 2026+?): ~£548/year savings → ~7.9‑year payback
- Victron tip: Use Node-RED to turn on a low-limit SOC schedule to stop battery discharge when EV charging is detected via Octopus API (don’t force 100% SOC)
Home & Usage Context
- 4-bed end-terrace house (Hampshire)
- Gas heating/hot water; everything else electric
- Family of 5 (one usually away at university)
- EV household
- Daytime usage: ~10.17 kWh/day (excluding EV)
- Tariff: Octopus Intelligent Go: 7p off-peak, 29.14p peak
ROI, Efficiency & Savings - Real Numbers
Round-trip efficiency
- Max charge in: ~16.7 kWh (AC side measurement, includes inverter/BMS losses)
- Max deliver out: ~12.44 kWh to household
- Efficiency ≈ 74.49%
- Real cost per delivered kWh:
7p × (16.7 / 12.44) ≈ 9.40p
Arbitrage savings
- Peak unit rate: 29.14p/kWh
- Effective delivered cost off‑peak (incl. losses): ~9.40p/kWh
- Saving per delivered kWh: 29.14p − 9.40p = 19.74p
- Average daytime consumption based on 14 months of data (ex‑EV): ~10.17 kWh/day
- Unavoidable peak import (CT response + system losses): ~1.00 kWh/day
- Daily saving ≈ (10.17 − 1.00) × 19.74p ≈ £1.81/day
- Annualised ≈ £660/year
- Payback:
£4,318 / £660 ≈ 6.5 years
Full Bill of Materials (BOM)
Every item from the build is included exactly as purchased, with its exact price and a short explanation of purpose.
Battery Side £1,951.58
I opted for the battery version with longer warranty. IP65 outdoor version now exists as well.
| Item | Source | Price (inc VAT) | Purpose |
|---|---|---|---|
| 1x Fogstar Energy 16.1kWh 48V Battery | fogstar | £1799.99 | Main storage. Excellent value; Black Friday price. |
| 2m High-Flex Battery PVC Cable - 70mm² Black | voltacon | £22.94 | 70mm² required by Victron (Will sell Fogstar’s included 50 mm²) |
| 2m High-Flex Battery PVC Cable - 70mm² Red | voltacon | £22.94 | 70mm² required by Victron. |
| 1x Victron Fuse holder 6-way for MEGA-fuse - CIP050060000 | voltacon | £22.00 | Compact and tidy |
| Delivery | voltacon | £7.99 | |
| 1x 17mm all round band black (10m coil) | superlecdirect | £6.66 | Secure the battery against falling |
| 1x Hydraulic Crimping Tool for up to 70mm2 cable | temu | £14.21 | Excellent low cost 70mm² crimper |
| 1x Heavy-Duty 600A Rotary Battery Switch Isolator | temu | £4.50 | Recommended for safety and maintenance |
| 1x Electrician Pliers | temu | £3.26 | Cheap but good |
| 1x High-Leverage Ratcheting Cable Cutter | temu | £8.01 | For cutting 70mm² cable |
| 2x MEGA-fuse 200A/80V | butlertechnik | £12.70 | 1 required, 1 spare |
| Delivery | butlertechnik | £3.49 | |
| 1m red 20mm heat shrink | aliexpress | £1.67 | For nice professional finish |
| 1m black 20mm heat shrink | aliexpress | £1.65 | For nice professional finish |
| 1x Power Distribution Block M8 48V 250A | aliexpress | £4.94 | Negative bus bar |
| 200x Superlec 81STAP 8 x 1" Self Tapping Screws | superlecdirect | £3.54 | Overspent, but needed the right screws to fix battery switch |
| 11x Superlec ET70C8 70mm Cable Copper Crimp 8mm Hole | superlecdirect | £8.71 | Essential cable ends |
| 3x Superlec ET70C10 70mm Cable Copper Crimp 10mm Hole | superlecdirect | £2.38 | Essential cable ends |
Inverter Side £778.84
Victron 5000 model chosen because 5 kVA can fully recharge the battery within 4 hours. The 3 kVA would need ~8 hours - more than my off-peak window.
| Item | Source | Price (inc VAT) | Purpose |
|---|---|---|---|
| 1x Victron MultiPlus-II 48/5000/70-50 230V GX (4000W) | bimblesolar | £737.75 | G98 & G99 approved |
| Discount coupon 5% off Victron | bimblesolar | £-39.08 | Paid for the delivery  |
| Delivery | bimblesolar | £30.00 | |
| 1x GW70436P GEWISS 32A 4 pole rotary isolator | superlecdirect | £14.88 | Mandatory isolation point |
| 700x Mixed Ferrules Non-Insulated Set 0.5-6mm2 | aliexpress | £6.82 | Cable crimping |
| 70x EN10-18 Copper Non-Insulated Ferrule | aliexpress | £2.75 | Cable crimping |
| 2m Trinity (T3) wiring duct narrow slot L/GREY 40X60 | bpx | 5.82 | For neat cable routing |
| 4x 20mm Super Open Grommets | tlc-direct | £0.34 | For AC IN switch connections |
| 2m 10.0mm² Green Yellow 6491X Single Core Cable | tlc-direct | £3.05 | Earth wire (also used for enclosure doors) |
| 1x Blue PVC Electrical Insulation Tape 19mm x 33mtr | superlecdirect | £0.62 | Marking wires |
| 1x Brown PVC Electrical Insulation Tape 19mm x 33mtr | superlecdirect | £0.62 | Marking wires |
| 1x Green / Yellow Earth PVC Electrical Insulation Tape 19mm x 33mtr | superlecdirect | £0.62 | Marking wires |
| 1x 8 Way Earth Terminal Block - EB8 | superlecdirect | £2.58 | Main earth block |
| 1x Ferrule Crimper Plier HSC8 6-4A | ebay | £12.08 | Crimper for up to 10mm2 |
House Connections £223.64
This does not include RCBO required for house consumer unit (in my case EV unit), as it’s part of the electrician cost
| Item | Source | Price (inc VAT) | Purpose |
|---|---|---|---|
| 13m H07RN-F ExtremeFlex90 Flexible Cable 10mm² 3 Core EPR PCP AD8 | superlecdirect | £92.82 | Oversized intentionally for lower voltage drop and future expansion. |
| 1x 20m Victron Current Transformer 100A:50mA for MultiPlus-II | bimblesolar | £43.96 | Fast response. Superior to ET112 for this use case. |
| 1x Erbauer Diamond Core Drill Bit 28mm | screwfix | £21.99 | Drill holes in brick wall |
| 1x Mexco Diamond Core Drill Adaptor SDS Plus | toolstation | £7.98 | Drill holes in brick wall |
| 1x Essentials Rail End Sockets Polished Stainless Steel 32mm | screwfix | £4.49 | Hole cover |
| 1x 4-32mm Steel Step Cone Drill Bit | temu | £3.19 | Drill holes in the enclosure |
| 3x Titanium Coated High Speed Steel Step Drill Set | temu | £2.48 | Drill holes in the enclosure |
| 1x RJ45 Ethernet Pass through Network Tool Kit | temu | £12.64 | To make CAN-Bus BMS Type A Cable and right-sized ethernet cable |
| 2x Wiska 10111319 Sprint 32mm Tail Kit Gland | superlecdirect | £2.15 | Enclosure connections |
| 1x PVC Pipe Solvent Cement | superlecdirect | £3.84 | Enclosure connections |
| 1x Superlec CPG22/32 32mm Stuffing Gland IP67 To Fit 16-25mm Cable | superlecdirect | £2.70 | Enclosure connections |
| 3m Marshall Tufflex CR8BK 32mm PVC Conduit HG Black | superlecdirect | £7.80 | Enclosure connections (wall-to-enclosure conduit) |
| 2x Marshall Tufflex MAB4BK 32mm PVC Female Adaptor Black | superlecdirect | £3.41 | Enclosure connections |
| 1x 27.1mm OD B Type Spring Tube Bender 520mm Long for 32mm PVC Pipe | amazon | £14.19 | Bend conduit |
Ventilation / Fan System £166.02
Active ventilation is essential for my enclosed outdoor installation; it may not be required in well‑ventilated indoor spaces.
| Item | Source | Price (inc VAT) | Purpose |
|---|---|---|---|
| 1x Cudis Cpn CU05/2 IP65 5 Way Plastic Garage Unit with 63A RCD | superlecdirect | £29.94 | Breakers for fans and potential future AC loads. |
| 1x KNO011 1NO Thermostat, 0-60°C 17.5mm wide | charter-controls | £17.16 | Uses the least amount of electricity |
| 1x 24BS24BD1F GIC 24BS24BD1F Switch Mode Power Supply, 24Vdc, 0.5A | charter-controls | £28.90 | 24DC conversion for DC fan |
| 1x LK3238.024 24Vdc 148x148mm Filter Fan | charter-controls | £55.06 | DC fans have lower consumption and longer life |
| 1x LK3238.300 Exit filter with cotton filter mat | charter-controls | £12.76 | Keep insects and debris out of the enclosure. |
| Delivery | charter-controls | £9.60 | |
| 3x 20mm TriShot Gland + Locknut for Cable 6-14mm IP68 - Grey | tlc-direct | £1.26 | Consumer unit connections |
| 1m 6.0mm² Blue 6491X Single Core Cable | tlc-direct | £0.95 | Consumer unit wiring |
| 1m 6.0mm² Brown 6491X Single Core Cable | tlc-direct | £0.95 | Consumer unit wiring |
| 1x CHINT MCB 1P 2A Circuit Breaker AC 230V DIN | aliexpress | £2.01 | Fan MCB |
| 1m 6mm arctic blue 3183Y flex cable 3core | ebay | £5.10 | Consumer unit to Inverter |
| 1x DIN Rail | ebay | £2.35 | To mount thermostat outside the consumer unit |
Enclosure £651.13
Essential for outdoors installation. Steel one is not cheap, so consider housing the system in the garage (loft, hallways, bedrooms, under stairs, etc. is not allowed). Wooden or plastic enclosure might work if space allows.
| Item | Source | Price (inc VAT) | Purpose |
|---|---|---|---|
| 1x Schneider enclosure NSYS3D121230DP - 1200x1200x300mm | bxh | £460.88 | Large, safe clearances, weather-rated, drainage hole |
| 1x Schneider canopy NSYTJPLA123G | bxh | £52.78 | Cheaper option from a different model - I customized it to fit nicely |
| 1x Schneider heavy load kit NSYAEFAHLBSC | bxh | £27.79 | Very expensive for what it is but required for heavy load |
| 1x Schneider stainless steel brackets NSYAEFPFXSC | automationdatabase | 14.38 | Stainless steel grade for outdoors |
| 1x Schneider lock closure system NSYAEDLDINS3D | automationdatabase | £56.47 | Required for compliance |
| 1x Lock Single Half Euro Adjustable Cam 45mm | ebay | £12.00 | Required for compliance |
| 4x M8 x 95mm Threaded Anchor Through Bolt, Stainless Steel | gsproducts | £10.51 | Stainless steel grade for outdoors |
| 4x M8 A4 Stainless Steel Dome Nut | gsproducts | £1.15 | Stainless steel grade for outdoors |
| 25x M8 Stainless Steel A4 Grade Form C Washer (8mm) | gsproducts | £1.30 | Stainless steel grade for outdoors |
| Delivery | gsproducts | £5.99 | |
| 100x Drill Screw Cross Recess Wafer Head 4.2 x 13mm | tlc-direct | £2.28 | Easy self-drilling fasteners |
| 100x Drill Screw Pan Head 4.8 x 16mm | tlc-direct | £3.24 | Only used a couple of this size |
| 10x Superlec ET10C6 10mm Cable Copper Crimp 6mm Hole | superlecdirect | £0.84 | Earth cable crimps |
| 10x Superlec ET10C8 10mm Cable Copper Crimp 8mm Hole | superlecdirect | £1.56 | Earth cable crimps |
Stand £103.09
Enclosure stand is not strictly needed; this was my choice. As an alternative, the battery could be hanged inside the enclosure (£50 bracket). Schneider helpdesk confirmed the mounting plate can hold 380kg.
| Item | Source | Price (inc VAT) | Purpose |
|---|---|---|---|
| 3x 2 hole cantilever arm HDG | superlecdirect | £9.83 | Unistrut frame |
| 3m 41X21X2.5 slotted HDG | superlecdirect | £16.74 | Unistrut frame |
| 5x CF40 8 hole special fitting HDG | superlecdirect | £11.10 | Unistrut frame |
| Delivery | superlecdirect | £5.004 | Unistrut frame |
| 8x 10mm 1 Hole Joint Plate 40mm x 40mm | tlc-direct | £2.88 | Unistrut frame |
| 2x 41mm x 21mm Stop End (Black) | tlc-direct | £0.30 | Unistrut frame |
| 1x Draper Expert 6 Point 1/2 Inch Standard Socket | toolstation | £2.79 | I did not have this size in my toolbox |
| 1x Extra Strong Flexi Tub - 42L | wickes | £3.50 | For mixing concrete |
| 1x Roughneck Round Point Micro Shovel | screwfix | £10.99 | Old one broke |
| 4x No Nonsense C44 Concrete Grey 20kg | screwfix | £31.96 | Concrete base |
| 4x Vinyl flooring plank sample | B&Q | £8.00 | Spread the battery weight and achieve a flush base for battery |
Earth Rod £119.22
My electrician required an earth rod for safety in island mode, and it measured a nice, stable earth electrode resistance of 20 ohms.
| Item | Source | Price (inc VAT) | Purpose |
|---|---|---|---|
| 2x Pro-Fix Earth Rod 5/8" x 4’ | screwfix | £18.88 | Compliant rod must be either 2.4m deep or two rods installed 6 feet apart |
| 1x Pro-Fix External Earth Rod Coupler 5/8" | screwfix | £3.66 | Joins two 1.2m rods |
| 1x Greenbrook Inspection Earth Pit | screwfix | £24.24 | Best price, excellent kit |
| 2x Vimark Nylon Male Comp Glands 20mm Black | screwfix | £0.99 | Enclosure connection |
| 1x D-Line Speedway PVC Conduit Female Adaptor | toolstation | £0.29 | Enclosure connection |
| 1x Heavy Duty Conduit Bending Spring | toolstation | £3.99 | Makes bending conduit easy |
| 2m 16.0mm² Green Yellow 6491X Single Core Cable | tlc-direct | £4.46 | 16mm is the minimum unless compliant protection is provided |
| 1x Linian 1LERPD Earth Rod Pro SDS+ Driver | mastertrade | £23.42 | I couldn’t do 2.4m depth without this |
| 1x Linian 1LERPB Earth Rod Pro Bit | mastertrade | £17.54 | I couldn’t do 2.4m depth without this |
| 1x Unicrimp QEP58CC 5/8in G-Type Clamp Rod to Cable | mastertrade | £4.44 | I protected the finished connection with lots of Vaseline based on a YT recommendation. |
| Delivery | mastertrade | £9.00 | |
| 1x Build It Postfix | toolstation | £6.98 | Support the pit |
| 2m 20mm Heavy Duty Round PVC Conduit | toolstation | £1.32 | Would have to be 25mm to allow smaller cable size |
Electrician and Safety Signs £288.15
Expect far higher cost if not doing your own first-fix work
| Item | Source | Price (inc VAT) | Purpose |
|---|---|---|---|
| 1x Danger Isolate A.C. and D.C. switch before carrying out work / 100x75mm / 1mm PVC sign | safetysignsupplies | £1.49 | Mandatory for dual-supply systems |
| 1x Custom Battery Charging Sign. 100x150mm / 1mm PVC with Foam adhesive | safetysignsupplies | £5.47 | Recommended |
| 1x Custom Warning: Electricity / 100x150mm / 1mm PVC with Foam adhesive | safetysignsupplies | £5.47 | Mandatory |
| Delivery | safetysignsupplies | £4.74 | |
| Electrician | £270.98 | Connections, checks, G98 |
Mistakes (Honesty Section) £35.83
| Item | Source | Price (inc VAT) | Purpose |
|---|---|---|---|
| 25x Dewalt DFM141024S BLUETIP-BT8x100 Hex Hd ScrwBlt-ZncPlt-HP | mtmc | £10.04 | Not marine grade, I did not want to risk it |
| Delivery | mtmc | £4.00 | |
| 6x 8mm x 75mm Stainless Steel Coach Screw with Hex Head (M8) | gsproducts | £2.38 | I used through-bolts instead |
| 2x 25mm Super Open Grommets | tlc-direct | £0.24 | Not used |
| 2x 32mm Super Open Grommets | tlc-direct | £0.38 | Not used |
| 400x Superlec 811/2STAP 8 x 1" 1/2" Self Tapping Screws | superlecdirect | £5.16 | Not used, added to get free delivery |
| 1m XAI 6mm 3 Core Marine & Offshore Cable IEC60092-353 Black LSZHXAI | superlecdirect | £4.67 | Wrong cable selected, not flexible enough |
| 1m 3183TQ 3 Core 2.5mm² Flexible Mains & Control Cable | superlecdirect | £1.82 | Not used |
| 1m D-Line Speedway PVC Conduit Male Adaptor | toolstation | £0.29 | Not used |
| Schneider brackets NSYAEFPFSC | bxh | £6.85 | Wrong type, had to order stainless steel version for outdoors (NSYAEFPFXSC) |
Items I had
2m Fan supply cable; 15m Ethernet cable; Heat gun; more electrician tools; saws (incl. circular), saw table, safety wear, SDS drill, hand drill, bits, spanners, pencils, the usual stuff
Lessons Learned & Tips
- VRM vs smart meter mismatch (~1.5 kWh/day) is to be expected because CT metering updates faster and measures instantaneous power, whereas smart meters sample over longer intervals.
- Octopus reports ~1 kWh/day peak import during peak hours even when discharging. My grid setpoint is set to 20W.
- Fogstar’s 16.1 kWh results in ~12.44 kWh of inverter‑output‑measured usable energy, based on VRM measurements, which include inverter and BMS conversion losses.
- A Powerwall would be simpler, but the ROI is typically far worse.
- If you can install in a garage, do so. You avoid: enclosure costs, fans, ventilation noise, weather sealing, earth rod (depending on earthing arrangement)
- I built the system to be ready for solar, but looks like solar would provide a weak ROI if added. I have roof space for only 2 panels - 2×705 W (~3.12 kWh/day delivered) → ~£109/year benefit → 9.2-year payback at a hypothetical £1000 installed cost.
- Exporting would require switching to a tariff incompatible with my low off‑peak rate, which eliminates the arbitrage benefit in my case.
- Ventilation noise is noticeable when active. Not loud, but not silent. Planning to test anti-vibration gaskets.
- Node-RED tip: In my testing, forcing SOC=100% to block discharge during EV charging broke ESS module. Using a simple low‑limit SOC schedule triggered by Octopus API events proved reliable.
- Cable fishing under floors is horrible when going through 3 walls. I struggled even with an endoscope camera, but shining strong light through brick gaps from the other side helped.
- There is tariff risk: savings rely on a sustained large spread between off‑peak and peak rates. I believe that wind generation will likely keep off-peak cheap.
- VAT - 0% possible through MCS installer, but I thnk installer costs would have outweighed savings.
Wiring Diagram
Photos
I’m sure some of you will enjoy pointing out where you’d have done things differently, but I hope this breakdown helps others regardless.
