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

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13 Trustpilot reviews
Trustpilot Google

I have a GivEnergy system consisting of two batteries, two inverters and a controlling EMS (Energy Management System) which has not worked since Nov 2025. After six months I discovered Solar Tech Support, reached out to them and Ron phoned me back – how often do you get that service? Could not be more helpful – worked directly with me over the phone, outside what I would call normal working hours. Lucid explanations and we were able to discuss the issues and history using camera and email history. As this was a very rare setup, Ron was able to access an EMS expert in the field to confirm the solution. One sunny day in, I am now only paying for standing charge and a few pence for spikes in grid consumption while battery catches up with house demand.

Ian · May 2026 Trustpilot

When my GivEnergy system had an issue, I was completely left without support and had honestly lost all hope. Thankfully, I searched online and found Ron, which completely turned things around. After sending him a message, he responded incredibly fast and called me to assure me that he would get the problem fixed. I really admire his dedicated, supportive nature and his determination to find a solution. With this kind of outstanding attitude and customer service, he has absolutely secured a future customer in me.

Sree · May 2026 Trustpilot

Ron want out of is way to help, nothing was to much. He was very thorough in what he did Very knowledgeable I would highly recommend Ron and his company He did a fantastic job for me. if you have any problems, he'll do his best to help you out and resolve your problem. I wouldn't hesitate to recommend them

Dennis Brown · May 2026 Trustpilot

Ron took me through a diagnostic to confirm my GivEnergy Inverter had a fault. A common one as it turns out with the AC Inverter. As GivEnergy is defunct there is no immediate fix, aside from sourcing 2nd hand replacement. It may be that a fix becomes available over the summer which would make a lot of GivEnergy customers happy (Again)

Tony Deacon · May 2026 Trustpilot

Contacted Ron with a problem and he sorted it out quickly with no problems at all. Very knowledgeable on anything solar/ batteries. I would recommend him to anyone

Phill · May 2026 Trustpilot

I've spoken to Ron a couple of times with issues with my Givenergy installation. Such a friendly knowledge guy very highly recommended. Thank you very much for resolving my issues

michael fairhurst · May 2026 Trustpilot

Contacted Solar Tech Support in desperation. After explaining the issues I had with my system a diagnosis was made and a solution proposed. Fantastic service, even contacted a manufacturer to arrange replacement parts for me. Great communications, explained all they were doing and what I had to do, clearly and precisly. Followed up to confirm all was ok. Excellent service.

Mr Machin · May 2026 Trustpilot

After GivEnergy went into liquidation, just my luck, my battery started playing up (internal board crashed). Contacted my installer - not interested! Found Solar tech support on a Google search. Sooo glad I found this company! Ron is extremely helpful and has plenty of experience. He soon confirmed what the fault was, and helped me to get my system up and running again. Now moved my GivEnergy account to Solar tech support, and will definitely use again if I have more issues. Unusual to find such a helpful company in these times, no morons reading scripts, just direct contact with the engineer.

Keith Ballard · Apr 2026 Trustpilot

Contacted Solar Tech Support when trying to understand what my Givenergy inverter problem might be and what might be my options. Received good/honest advise which backed up my thoughts.

Hugh Speirs · Apr 2026 Trustpilot

Ron is a super star. Two months ago my GivEnergy battery failed a firmware upgrade leaving it a brick. My installer couldn't/wouldn't fix it. GivEnergy couldn't/wouldn't fix it. Then they went into administration and all hope was lost. A flurry of emails later and Ron had diagnosed the fault (failed USB flash drive, something I'd suspected) and talked me through resolving it. Two months of nothing resolved in about 3 hours. It's great to work with someone who pays attention to the details, knows that they're doing (not just following a script) and gets stuff sorted without a fuss or up-charging.

Christopher · Apr 2026 Trustpilot

I can add to the list of customers who had already 'given up' on GivEnergy due to their appalling customer service, and that was before they went into administration (their Trustpilot reviews don't lie!). So you can imagine my desperation when, having changed my ISP and my Inverter, predictably, proving to be the only device that didn't connect automatically to my new network, I found zero prospect of any customer support with GivEnergy having called in the administrators just five days earlier! The salvation came from Solar Tech Support. My IT advisor stumbled across their web site and some very helpful tips for beleaguered GivEnergy customers, as well as an offer to provide direct assistance. Nothing ventured, I decided to drop them an E-Mail, with very low expectations based on my experience of GivEnergy customer support. Within an hour Ron had responded with some pin point advice, and after a few exchanges of E-Mails he had nailed the problem, enabling the combined efforts of my IT advisor and solar installer to resolve it and reconnect my Inverter. Thank you Solar Tech Support, and Ron in particular, for coming to the aid of a deserted and despondent GivEnergy customer. Expert, razor sharp advice and first class customer service, even though I wasn't officially a customer.

customer · Apr 2026 Trustpilot

This company are a rare gem, I had a very unusual problem following a failed firmware upgrade on my GivEnergy kit. I then found out GivEnergy were in administration and had dismissed all their support staff! None of the usual fixes to try and restore my inverter comms would work, and I looked everywhere, forums, GivEnergy youtube support videos - even AI couldn't figure it out. My installer was talking about huge sums for system replacements, and being vague / evasive about if they'd even install replacement GivEnergy inverter. Enter Solar Tech Support, reassuring and knowledgeable from the very start, I've learnt loads about my solar system though the friendly chat while my engineer worked as he diagnosed the problem and figured out a fix procedure that I've not found anywhere else - amazing . If you need solar system repairs - especially if you like me have been left high and dry by GivEnergy, I cannot recommend this company enough. Give them a call.

Andy Thomas · Apr 2026 Trustpilot

I sent a message on their website regarding a problem I have on my Givenergy system. Although not supplied by Ronald, I thought it was worth an email. Within the hour on a Saturday, he phoned and we discussed the problem. He logged in remotely and gave excellent advice. I'm too far away for his on-site help but he did diagnose the problem and was happy also to chat through my thoughts about an upcoming solar/battery install I'm planning. Great bloke.... if only he was nearer!

Philip · Apr 2026 Trustpilot

I have a GivEnergy system consisting of two batteries, two inverters and a controlling EMS (Energy Management System) which has not worked since Nov 2025. After six months I discovered Solar Tech Support, reached out to them and Ron phoned me back – how often do you get that service? Could not be more helpful – worked directly with me over the phone, outside what I would call normal working hours. Lucid explanations and we were able to discuss the issues and history using camera and email history. As this was a very rare setup, Ron was able to access an EMS expert in the field to confirm the solution. One sunny day in, I am now only paying for standing charge and a few pence for spikes in grid consumption while battery catches up with house demand.

Ian · May 2026 Trustpilot

When my GivEnergy system had an issue, I was completely left without support and had honestly lost all hope. Thankfully, I searched online and found Ron, which completely turned things around. After sending him a message, he responded incredibly fast and called me to assure me that he would get the problem fixed. I really admire his dedicated, supportive nature and his determination to find a solution. With this kind of outstanding attitude and customer service, he has absolutely secured a future customer in me.

Sree · May 2026 Trustpilot

Ron want out of is way to help, nothing was to much. He was very thorough in what he did Very knowledgeable I would highly recommend Ron and his company He did a fantastic job for me. if you have any problems, he'll do his best to help you out and resolve your problem. I wouldn't hesitate to recommend them

Dennis Brown · May 2026 Trustpilot

Ron took me through a diagnostic to confirm my GivEnergy Inverter had a fault. A common one as it turns out with the AC Inverter. As GivEnergy is defunct there is no immediate fix, aside from sourcing 2nd hand replacement. It may be that a fix becomes available over the summer which would make a lot of GivEnergy customers happy (Again)

Tony Deacon · May 2026 Trustpilot

Contacted Ron with a problem and he sorted it out quickly with no problems at all. Very knowledgeable on anything solar/ batteries. I would recommend him to anyone

Phill · May 2026 Trustpilot

I've spoken to Ron a couple of times with issues with my Givenergy installation. Such a friendly knowledge guy very highly recommended. Thank you very much for resolving my issues

michael fairhurst · May 2026 Trustpilot

Contacted Solar Tech Support in desperation. After explaining the issues I had with my system a diagnosis was made and a solution proposed. Fantastic service, even contacted a manufacturer to arrange replacement parts for me. Great communications, explained all they were doing and what I had to do, clearly and precisly. Followed up to confirm all was ok. Excellent service.

Mr Machin · May 2026 Trustpilot

After GivEnergy went into liquidation, just my luck, my battery started playing up (internal board crashed). Contacted my installer - not interested! Found Solar tech support on a Google search. Sooo glad I found this company! Ron is extremely helpful and has plenty of experience. He soon confirmed what the fault was, and helped me to get my system up and running again. Now moved my GivEnergy account to Solar tech support, and will definitely use again if I have more issues. Unusual to find such a helpful company in these times, no morons reading scripts, just direct contact with the engineer.

Keith Ballard · Apr 2026 Trustpilot

Contacted Solar Tech Support when trying to understand what my Givenergy inverter problem might be and what might be my options. Received good/honest advise which backed up my thoughts.

Hugh Speirs · Apr 2026 Trustpilot

Ron is a super star. Two months ago my GivEnergy battery failed a firmware upgrade leaving it a brick. My installer couldn't/wouldn't fix it. GivEnergy couldn't/wouldn't fix it. Then they went into administration and all hope was lost. A flurry of emails later and Ron had diagnosed the fault (failed USB flash drive, something I'd suspected) and talked me through resolving it. Two months of nothing resolved in about 3 hours. It's great to work with someone who pays attention to the details, knows that they're doing (not just following a script) and gets stuff sorted without a fuss or up-charging.

Christopher · Apr 2026 Trustpilot

I can add to the list of customers who had already 'given up' on GivEnergy due to their appalling customer service, and that was before they went into administration (their Trustpilot reviews don't lie!). So you can imagine my desperation when, having changed my ISP and my Inverter, predictably, proving to be the only device that didn't connect automatically to my new network, I found zero prospect of any customer support with GivEnergy having called in the administrators just five days earlier! The salvation came from Solar Tech Support. My IT advisor stumbled across their web site and some very helpful tips for beleaguered GivEnergy customers, as well as an offer to provide direct assistance. Nothing ventured, I decided to drop them an E-Mail, with very low expectations based on my experience of GivEnergy customer support. Within an hour Ron had responded with some pin point advice, and after a few exchanges of E-Mails he had nailed the problem, enabling the combined efforts of my IT advisor and solar installer to resolve it and reconnect my Inverter. Thank you Solar Tech Support, and Ron in particular, for coming to the aid of a deserted and despondent GivEnergy customer. Expert, razor sharp advice and first class customer service, even though I wasn't officially a customer.

customer · Apr 2026 Trustpilot

This company are a rare gem, I had a very unusual problem following a failed firmware upgrade on my GivEnergy kit. I then found out GivEnergy were in administration and had dismissed all their support staff! None of the usual fixes to try and restore my inverter comms would work, and I looked everywhere, forums, GivEnergy youtube support videos - even AI couldn't figure it out. My installer was talking about huge sums for system replacements, and being vague / evasive about if they'd even install replacement GivEnergy inverter. Enter Solar Tech Support, reassuring and knowledgeable from the very start, I've learnt loads about my solar system though the friendly chat while my engineer worked as he diagnosed the problem and figured out a fix procedure that I've not found anywhere else - amazing . If you need solar system repairs - especially if you like me have been left high and dry by GivEnergy, I cannot recommend this company enough. Give them a call.

Andy Thomas · Apr 2026 Trustpilot

I sent a message on their website regarding a problem I have on my Givenergy system. Although not supplied by Ronald, I thought it was worth an email. Within the hour on a Saturday, he phoned and we discussed the problem. He logged in remotely and gave excellent advice. I'm too far away for his on-site help but he did diagnose the problem and was happy also to chat through my thoughts about an upcoming solar/battery install I'm planning. Great bloke.... if only he was nearer!

Philip · Apr 2026 Trustpilot
🌥️

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.
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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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.

  • Remote consultation from £75
  • UK winter generation figures used throughout
  • LiFePO4 and lead-acid systems covered

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