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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.
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  • Written by solar engineers
  • UK winter figures throughout
  • LiFePO4 & lead-acid covered
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Philip · Apr 2026

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.

Step 1

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.

Example daily load calculation
LED lighting (10 × 10W, 5hrs): 500Wh
Fridge-freezer (150W, 24hrs, 35% duty cycle): 1260Wh
Kettle (2000W, 15 mins/day): 500Wh
Laptop (60W, 6hrs): 360Wh
Phone charging, misc (50W, 2hrs): 100Wh
Washing machine (1800W, 1.5hr cycle, 4×/week = 0.86/day): 550Wh
Subtotal: 3270Wh
+ 25% losses: 818Wh
Gross daily load: ~4090Wh

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.

Step 2

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.

Peak sun hours — worst-case month (Dec/Jan)
South England (Cornwall, Kent): 1.8–2.2 PSH
South Midlands, London: 1.6–2.0 PSH
Midlands, Wales: 1.4–1.8 PSH
North England (Lancs, Yorks): 1.2–1.6 PSH
North East, Cumbria: 1.1–1.4 PSH
Scotland (Central): 0.9–1.3 PSH
Scotland (Highlands): 0.7–1.1 PSH
Northern Ireland: 1.0–1.4 PSH

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.

Step 3

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

Formula
(Daily gross load × Days autonomy) ÷ Usable DoD = Battery bank capacity
DoD: 0.80 for LiFePO4 · 0.50 for lead-acid (flooded) · 0.55 for AGM/GEL
Example (4090Wh load, 2 days autonomy, LiFePO4)
4090 × 2 ÷ 0.80 = 10,225Wh minimum (≈10.2kWh)
At 48V: 10225 ÷ 48 = 213Ah — so 3 × Pylontech US5000 (222Ah) is the minimum viable configuration
LiFePO4
✓ 80% usable DoD
✓ 2000–6000 cycle life
✓ No maintenance
✓ Safe at partial SoC
Higher upfront cost
Lead-Acid (Flooded)
✓ Lower upfront cost
✓ Proven technology
50% usable DoD only
400–800 cycle life
Regular top-up required

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.

Step 4

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.

Panel array sizing formula
Required panel Wp = Daily gross load (Wh) ÷ Worst-case PSH
Example (4090Wh load, 1.4 PSH — North England, January)
4090 ÷ 1.4 = 2921W peak — use 3200–3500W panel array
e.g. 8 × 415W panels = 3320W · or 7 × 450W panels = 3150W
• String voltage: Check that the panel string open-circuit voltage (Voc) stays within the MPPT controller's maximum input voltage under all conditions. In cold weather (−10°C in UK), panel Voc can be 12–15% higher than the STC rating on the datasheet.
• Panel orientation: South-facing at 30–40° tilt maximises winter generation in the UK. East/west orientations reduce winter output by 20–30%. Fixed ground mounts at steeper tilt angles (50–60°) outperform roof mounts in winter due to better low-angle sun capture.
• Shading: In winter, the sun never rises high — horizon obstructions (trees, buildings, hills) that don't shade panels in summer will shade them in the morning and evening in January. Survey for winter shading before finalising panel placement.
Step 5

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.

MPPT controller sizing
Array Wp ÷ Battery voltage = MPPT current (A)
3320W ÷ 48V = 69A → select 80A or 100A MPPT
Verify: Max PV input voltage covers cold Voc. Victron SmartSolar 150/100 accepts up to 150V PV input, handles up to 100A charge current at 48V.
Inverter sizing
Peak simultaneous load + 25% headroom
2300W peak + 25% = 2875W → 3000VA inverter
Surge: Water pumps, compressors, and power tools draw 3–6× running current on startup. Check inverter surge VA rating covers the highest-surge load you'll connect.
For Victron off-grid systems, the standard combination is a SmartSolar MPPT controller (150/100 or 250/100 depending on string voltage) paired with a MultiPlus-II inverter-charger. The MultiPlus-II also accepts a generator AC input, providing a seamless backup charge source. See our MultiPlus-II commissioning guide and generator integration guide for configuration details.
Related off-grid guides
Victron off-grid guides
FAQ

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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  • Remote consultation from £195
  • UK winter generation figures used throughout
  • LiFePO4 and lead-acid systems covered

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