Off-grid system sizing Battery, panels & inverter — UK-specific
Undersized systems run out of energy in winter. Oversized systems waste budget. This guide gives you the UK-specific numbers — winter peak sun hours by region, autonomy calculations, battery chemistry comparisons, and inverter sizing for peak loads.
We size off-grid systems for UK conditions — daily load assessment, winter generation analysis, battery and panel specification, MPPT and inverter selection. Remote consultation from £75.
Book a consultation — £75 → How consultations workAlways 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.
Off-grid system sizing — common questions
In winter, the sun sits at a much lower angle — never rising above 15–18° above the horizon in northern England versus 60° in summer. Solar radiation hits panels at a steep angle, dramatically reducing effective energy capture. Combined with shorter days (7.5 hours in December versus 16.5 in June), December generation is typically just 8–12% of June generation. A system producing 30kWh on a sunny June day may produce only 2–3kWh on a dull December day.
A typical UK off-grid home with efficient appliances uses 3000–6000Wh per day. With 2 days of autonomy and LiFePO4 at 80% usable DoD, this requires 7.5–15kWh of battery capacity. In practice, most off-grid homes install 10–20kWh of LiFePO4 and supplement with a generator during extended low-solar periods. Lead-acid requires double the nameplate capacity for the same usable energy due to the 50% DoD limit.
Using worst-case January figures, a home consuming 5000Wh per day in northern England needs approximately 4000–5000W of panels to break even in the worst month. That's typically 9–12 panels of 400–450W. Southern England requires slightly fewer — around 3000–4000W. Most off-grid installations also include a generator for extended low-generation periods, allowing the solar array to be sized for average rather than worst-case generation.
Yes, but LiFePO4 is almost always the better long-term investment for a primary residence. LiFePO4 offers 80% usable DoD versus 50% for lead-acid, a much longer cycle life (2000–6000 cycles vs 400–800), no maintenance, and better cold-weather performance. For a cabin or occasional-use structure, flooded lead-acid with regular maintenance is viable. Over 10 years, the cost per kWh of delivered energy from LiFePO4 is typically lower despite the higher upfront cost.
Size for your maximum simultaneous load plus 25% headroom. For a domestic property where the highest draw is a kettle (2–3kW), a 3000–3500VA inverter is typically sufficient if you avoid running multiple large loads simultaneously. Electric showers (7–10kW) require a much larger inverter — either a single high-capacity unit or parallel MultiPlus-II units. Always check the inverter's surge rating — motors and compressors draw 3–6× their running current on startup.
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Tell us your location, daily load estimate, and available roof or ground space. We'll calculate battery, panel, MPPT, and inverter requirements using UK-specific winter generation data.
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