1. Understanding the island solution
An island solution ("off-grid") consists of four building blocks: generation (solar, wind, generator), storage (battery bank), conversion (charge controller, inverter) and distribution (fuses, cables, loads). Self-sufficiency means: consumption and storage must be matched to each other — and generation must cover even the worst day (winter, clouds).
2. Storage technologies: an honest comparison
| Technology | Cycles | Depth of discharge | Cost | Use |
|---|---|---|---|---|
| LiFePO₄ (LFP) | 3,000+ | 80–90 % | expensive | Today's standard — safe, no memory effect, built-in BMS required |
| Lead-gel / AGM | approx. 500 | only 50 % | cheap | Proven, heavy, maintenance-free — destroyed by deep discharge |
| Saltwater batteries | high | high | large & expensive | Very safe, but rarely available |
| DIY 18650 packs | variable | 80 % | moderate | Only with BMS and tested cells — fire hazard with DIY solutions! |
Lead-acid battery bank: cheap and proven — but only 50 % of the capacity usable (photo: Pseudoznanstvenik, CC BY-SA 3.0, via Wikimedia Commons)
3. Sizing: how to calculate correctly
- Measure consumption: what does the household need per day? Example: LED lighting 100 Wh + fridge 600 Wh + laptop/phones 200 Wh + water pump 100 Wh = 1,000 Wh/day
- Autonomy days: how many days without sun/wind? Realistic: 2–3 days
- Storage maths: 1,000 Wh × 3 days = 3,000 Wh ÷ depth of discharge → LFP (90 %): approx. 3.3 kWh; lead (50 %): 6 kWh
- Generation: PV panels deliver only 10–25 % of summer output in winter — size for winter sun or generator backup!
4. Charging and grid electronics
- MPPT charge controller between panel and battery (10–30 % more yield than PWM)
- Sine-wave inverter (not "modified") — motors, fridges and electronics need a clean sine wave; size = continuous load × 1.3 + peaks (motors!)
- RCD + fuses per circuit — an island grid is just as dangerous as the public grid
- Grounding of the system and lightning protection (arresters) at antennas/masts
5. Load strategy: the right order
- Priority 1: lighting (LED), communication (phone, radio), water pump
- Priority 2: fridge (efficient model! 12/24-V version directly on the battery), freezer
- Taboo in island operation: immersion heaters, hotplates, tumble dryers, fan heaters — for heat/cooking: gas, wood, solar thermal
- Switch off standby vampires — 10–30 W of continuous load is a whole battery day
6. Building the battery bank: practice
- Fuse directly at the positive terminal of the battery (especially lead: the short-circuit current is brutal)
- Cable cross-sections generous (voltage drop!) — long thin cables = heat + losses
- Temperature: lead loses capacity in the cold (up to 30 % at −10 °C), LFP must not be charged below 0 °C
- Ventilate lead batteries — charging produces oxyhydrogen gas (see warning above)
- Balancing: in series connection use balancers/BMS; never charge/discharge cells separately
7. Backup: the generator as a fallback
A petrol/diesel generator is the backbone during prolonged sunless periods: either via a charger (battery charging) or — for larger systems — directly at the inverter input (hybrid inverter). Important: separate feed-in — never run the generator and the island inverter in parallel on the same grid (island-grid coupling box or re-plug). Plan the runtime: 1–2 hours of generator charging is usually enough for a day.
8. Checklist: island-grid basic set
- ☐ Battery bank (LFP recommended) with BMS + fuse at the positive terminal
- ☐ Solar panel(s) + MPPT charge controller
- ☐ Sine-wave inverter (size: continuous load × 1.3)
- ☐ RCD + outgoing fuses
- ☐ Emergency charger: generator + battery charger
- ☐ Power metering (smart plug) for calibration
9. Did you know?
- LiFePO₄ batteries survive 3,000+ charge cycles — with daily charging that is over 8 years. Lead often manages only 1–2 years in cycle operation.
- Island grids are standard on boats, cabins and mountain huts — the technology is proven, and so are the mistakes: 90 % of all problems are wrongly dimensioned cables, missing fuses or overcharged/deep-discharged batteries.
- A smart plug (metering socket) for €10 shows you the real consumption — without measuring, sizing is guesswork.