Solar Panels: Technical Answers to the 8 Most Common Questions

Thinking about investing in solar panels? A load of technical questions come up pretty quickly. What size system should I go for? How many MPPT trackers do I actually need? And how much difference does roof orientation really make to your output?

We've pulled together the 8 most common technical questions we hear from homeowners across Denmark, the UK and Ireland — with data-backed answers that apply wherever you are.

1. What efficiency can I expect from monocrystalline panels?

Modern monocrystalline panels typically deliver 20–22.5% STC efficiency. In real-world UK and Irish conditions — with our fair share of overcast days — the annual average efficiency tends to come in 1–2 percentage points below the STC figure.

That's because northern European climates have a lot of diffuse light hours, especially in autumn and winter, where panels don't quite hit their optimum operating point. Monocrystalline panels are still the best choice for UK and Irish roofs though, as they handle low-light conditions significantly better than older polycrystalline types.

Real-world example: A 4 kWp system with 430 W panels (roughly 9–10 units) on a south-facing roof in southern England can deliver 3,500–3,900 kWh annually — a real-world annual efficiency of around 17–19%.

2. How many MPPT trackers does my inverter need for a 4–5 kWp system?

Most 4–5 kWp domestic systems work perfectly well with 2 MPPT trackers, provided the roof sections have consistent orientation and minimal shading.

If you've got panels on both east- and west-facing roof planes — or significant shading from chimneys, trees or neighbouring buildings — you'll want an inverter with 3 MPPT trackers. This gives you the flexibility to handle different solar exposure across each string independently.

Why it matters: MPPT (Maximum Power Point Tracking) optimises voltage and current for each panel string independently. With just 2 trackers on an east/west array, you could see an annual production loss of 3–7%, as the weaker string drags the other one down.

3. What's the optimal system size for a typical UK semi-detached home?

For homes with annual electricity consumption of 3,500–5,500 kWh, we typically recommend a 4–7 kWp system.

Many homeowners go for 4–5 kWp, as it gives a solid balance between upfront investment and self-sufficiency — typically covering 40–60% of annual consumption, depending on usage patterns and whether you have a heat pump or EV.

Rule of thumb: About 1 kWp per 700–800 kWh of annual consumption gives the best economics. Over-sizing means you'll export a lot in summer at a modest Smart Export Guarantee (SEG) rate, while still buying expensive grid power through the winter.

4. Is inverter undersizing a common problem?

Inverter undersizing relative to the DC array — meaning the array's kWp is significantly larger than the inverter's nominal AC output (a DC/AC ratio above 1.15) — is actually quite common and often intentional.

The goal is to optimise production during the many hours when solar irradiance is below peak — particularly outside the summer midday window. The trade-off is occasional "clipping", where the inverter caps output on those few perfectly clear summer days with maximum irradiance.

What's optimal? For UK and Irish conditions, a DC/AC ratio of 1.1–1.3 is generally recommended. This keeps annual clipping losses very small (typically under 1%) while improving production in the other 95% of daylight hours.

5. How does roof orientation affect MPPT tracker choice?

An east/west split makes bigger demands on your inverter's MPPT configuration, because the two roof faces receive solar radiation at entirely different times of day.

For mixed orientations, we recommend a minimum of 3 MPPT trackers so that morning and afternoon sun can be tracked independently. With only 2 trackers on an east/west split, one string inevitably drags the other down during the hours when only one roof face is in direct sun.

Real-world data: For a customer in southern Ireland with a 50/50 east/west split, switching from 2 to 3 MPPT trackers reduced annual mismatch losses from roughly 5% to under 1%.

6. How efficient is MPPT tracking in winter conditions?

MPPT efficiency typically runs at 96–98% under optimal conditions with clear skies and moderate temperatures.

During UK and Irish winter conditions — with diffuse light, low sun angles, and temperatures below 5°C — MPPT efficiency often drops to 93–96%. That's still perfectly acceptable, but it underscores the importance of choosing an inverter with good low-light performance.

Tip: Look for inverters with a "wide MPPT voltage range" — they perform significantly better through the darker months, when panel voltage can swing widely.

7. Should I go for 400 W or 450 W panels for a 5 kWp system?

450 W panels mean fewer units (about 11 for 5 kWp vs. 12–13 for 400 W), which cuts installation time and the number of roof mounts needed.

On the flip side, higher-wattage panels also need more attention to cable sizing and fuse ratings, as the current per string increases.

Recommendation: For standard UK semi-detached homes in 2026, most installers are choosing 430–450 W panels. They offer the best balance between roof space utilisation and installation cost. 400 W panels can still make sense if roof space is tight and you want to squeeze in as many panels as possible within a given area.

8. How much difference do cable losses make on a domestic system?

Cable losses are one of the most overlooked factors in solar system design — but they can quietly cost you tens of pounds a year in lost production.

With standard 4 mm² DC cables and a 15-metre run from roof to inverter, ohmic losses typically come in at 1.8–2.4%. That doesn't sound like much, but over 25–30 years it adds up to 1,200–1,700 kWh of lost generation — enough to run your washing machine for a couple of years.

Our recommendation: Always use 6 mm² DC cables for runs over 12 metres to keep losses below 1.5%. The extra cost for thicker cables is minimal compared to the overall system — typically less than £50 in materials.

Summary: Your Key Technical Decisions

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