Always size for the worst-case month. A UK off-grid system must work in January, not just July. The worst-case month determines your battery bank size and panel array. If you design for average annual generation, your system will run short of energy for three to four months every year.
Calculate your daily load in watt-hours
The first step is understanding how much energy you actually need each day. Underestimating is the most common cause of undersized systems.
How to calculate your daily load
For each appliance: multiply rated wattage × hours used per day = watt-hours (Wh). Then add 20–25% for inverter conversion losses and cable resistance.
High-draw items — electric showers (7–10kW), immersion heaters (2–3kW), and induction hobs (1–3kW per zone) — add significantly to both daily Wh and peak inverter demand. Consider whether alternatives are available: a heat pump water heater uses 3–4× less energy than an immersion heater for the same hot water output.
UK winter peak sun hours by region
Always size for the worst month. These are conservative December/January figures for south-facing panels at optimal tilt.
These are averages — individual cloudy days can produce 0.2–0.4 PSH. This is why battery autonomy (step 3) is essential: your system must store enough energy to cover 2–3 consecutive low-generation days.
Size the battery bank for autonomy
Your battery bank must cover the load for the number of consecutive cloudy days you want to withstand without a generator.
Battery bank sizing formula
Cold-weather capacity loss reduces effective battery size — a LiFePO4 bank in a 5°C shed delivers approximately 80–85% of rated capacity. Add 15–20% to the calculated bank size if the battery will be in an unheated space. See our cold weather battery guide for details.
Size the solar panel array for the worst-case month
The panel array must generate enough energy to fully recharge the battery bank within the available winter peak sun hours.
Select the MPPT controller and inverter
The MPPT controller must handle the full panel array current. The inverter must handle the peak simultaneous load with headroom for surge.