The Sun Hours solar forecast app needs two things: your postcode and your system's best-ever daily kWh. Here's the fastest way to find that number for every common UK plug-in solar setup — Hoymiles, Deye, Envertech, Marstek, APS — plus a NASA-powered fallback if you have no data yet.
The setup question I get most often
People download Sun Hours, open it, and get to the second question: what's your best-day kWh?
Then they get stuck.
Not because it's complicated. Just because nobody told them where to look. So here's the complete answer for every common UK plug-in solar setup, plus a fallback if you have no data yet.
(If you're wondering why we ask for this instead of panel wattage, there's a reason — I'll explain at the end.)
The two questions
Sun Hours needs two things to give you a personalised daily forecast:
- Your location — postcode, city, or just let it use your phone's location
- Your best-ever single day in kWh — the highest daily output your system has ever recorded
That's it. No inverter login. No serial numbers. No technical specs.
Here's how to find that second number.
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Method 1: Your inverter app (fastest, most accurate)
Every plug-in solar system sold in the UK comes with an inverter — the small box that converts your panels' DC power to the AC that goes into your wall socket. And every modern microinverter has a companion app.
Open the app and look for a History, Daily, or Statistics tab. You want the highest single-day kWh figure you've ever recorded. Don't average it — you want the peak.
Here's where to find it in the most common UK apps:
Hoymiles (S-Miles Cloud / HMS app): Dashboard → tap the energy card → Daily → scroll back to find your highest day.
Deye (SolarmanPV app): Home → plant → Generation report → Daily → swipe back through months.
Envertech (EnverView): Statistics → Daily generation → sort or scroll to find peak.
Marstek: Dashboard → History → Day view → highest recorded.
APS (EMA app): My System → Energy → Daily chart → find peak day.
Solis / Growatt: Both have similar History or Reports sections in their respective apps.
If your app shows multiple months of history, look for a clear spring day — April and May produce the highest single-day figures for most UK locations because the days are long and the panels are cool.
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Method 2: Smart plug or energy monitor
Some plug-in solar kits (and some installers) include a smart plug or clamp meter that sits between the solar output and your mains. These log daily output independently of the inverter.
Shelly EM / Shelly Plus: Shelly app → device → Energy → History → Daily.
Emporia Vue / Sense: Both apps have daily energy history with per-device or per-circuit breakdowns.
Smart meter in-home display (IHD): If your IHD shows import/export separately, you may be able to infer generation from periods when your import dropped to zero (but this is indirect and less accurate).
A clamp meter or smart plug is also useful for verifying your inverter app figures — they should be within a few percent of each other.
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Method 3: No data yet — use the NASA calculator
Just installed? System's brand new? No history at all?
Go to sunhours.app/solaruk and use the NASA POWER postcode calculator. Enter your postcode and your system wattage (400W, 600W, or 800W — it's printed on the inverter). The calculator pulls decades of NASA satellite irradiance data for your exact GPS coordinates and returns a monthly kWh breakdown.
To get your estimated best-day figure: take the May column (usually the peak month for most UK locations), divide by 31. That's your estimated best-day kWh under good conditions.
Rough guides for an 800W south-facing system in the UK:
- Cornwall (TR) → ~3.0–3.2 kWh best day
- London (SW, SE, E) → ~2.7–2.9 kWh best day
- Bristol (BS) → ~2.6–2.8 kWh best day
- Birmingham (B) → ~2.5–2.7 kWh best day
- Manchester (M) → ~2.3–2.5 kWh best day
- Leeds (LS) → ~2.3–2.4 kWh best day
- Edinburgh (EH) → ~2.1–2.3 kWh best day
- Glasgow (G) → ~2.0–2.2 kWh best day
For 400W systems, halve these. For 600W, multiply by 0.75.
Once you've got your first real sunny spring day, replace the estimate with your actual recorded figure — it'll make the forecast more accurate.
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Why best-day kWh, not panel wattage?
Quick explanation, because it's a reasonable question.
Your panel wattage (400W, 800W) is a laboratory rating — what the panel produces under ideal test conditions: 1,000 W/m² irradiance, 25°C, no shading, perfect angle. Real-world output is always lower than this.
The actual difference depends on your specific setup:
- Which direction your panel faces (south = best, east/west = less)
- Whether there's any shading from walls, railings, or neighbouring buildings
- Your cable length and inverter efficiency losses
- Your local microclimate vs the theoretical model
Your real best-ever kWh figure already accounts for all of these. It's a measurement of what your system actually does in your garden or on your balcony. That makes it a much better calibration point than a theoretical spec.
A south-facing 800W system on a Cornwall balcony and a north-facing 800W system on a Manchester flat are both "800W systems" — but they have very different best-day figures. Sun Hours can only give you an accurate forecast if it knows which one you are.
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Once you have both numbers
Open Sun Hours, enter your postcode and your best-day kWh, and you're done.
What you get:
- A 7-day kWh forecast updated every morning with real weather data
- An hourly generation chart showing when your system will peak each day
- A daily quality rating so you know instantly whether it's worth timing your washing machine to your generation window
- All of it personalised to your specific system output in your specific location
No inverter login. No account. No subscription.
Free on Android. Download it here →
Enter your postcode and best-day kWh — that's it
Sun Hours uses those two numbers to build a personalised 7-day kWh forecast for your plug-in solar setup — updated daily with real weather data, no inverter login required. Free on Android.
Get it on Google Play →