If you have plug-in solar panels and want a daily kWh forecast without a subscription or an app that drowns you in ads you cannot close, here is exactly what to look for — and why setup should take two questions, not twenty.
Get your personalised solar PV forecast — free to download
Sun Hours gives you a 7-day kWh forecast calibrated to your actual system. Enter your postcode and best-ever daily output — that is it. Free on Android. If you want to remove ads, it is a one-time purchase. Never a subscription.
The one question every plug-in solar owner asks
You've got your panels up. The inverter app is showing kWh from yesterday. But the question you actually want answered is: how much will my panels generate today?
That is the job of a solar PV forecast app. Not historical logs. Not system configuration wizards. Not smart home integrations you don't use. Just: will today be a good solar day, when will it peak, and should I run the dishwasher at 11am or wait until tomorrow?
It sounds simple. Getting there without sitting through unskippable ads on every screen, or paying a monthly subscription just to see the next day, turns out to be harder than it should be.
What the reviews of other apps tell you
Spend ten minutes reading Play Store reviews of popular solar PV forecast apps and you'll see the same complaints repeat. Ads that can't be closed. Ads between every screen. Ads that redirect to app downloads when you tap the X. One reviewer put it bluntly: "I had to watch an ad to see each individual day, and even once you've watched an ad to see a day — watch another if you want to go back to it later."
Others mention overly complex setup. Panel wattage. Tilt angle in degrees. Azimuth. Efficiency rating. That is a setup form for a professional installer, not someone who just plugged 800W of solar into their balcony socket.
These are solvable problems. They are just not solved everywhere.
What a good solar PV forecast app should actually do
Give you a personalised kWh forecast. Not a generic "good conditions today" rating, but an actual number tied to your system and your location. If you have an 800W plug-in setup in Leeds, your forecast should look different from a 4kWp rooftop system in Plymouth — and it should look different on the same day.
Set up in under two minutes. Your postcode covers location and local weather patterns. Your best-ever daily kWh tells the app what your system can actually achieve in ideal conditions. Everything else — panel wattage, tilt angle, shading percentage — is detail that the app should infer or estimate, not interrogate you about.
Work with every system. Hoymiles, Deye, Marstek, Envertech, APS, Growatt, Solis — and every unbranded plug-in kit sold at Lidl, Iceland or City Plumbing. A forecast app that only works if your inverter has an API integration excludes most of the UK's growing plug-in solar market.
Show the hourly breakdown. Knowing today will generate 2.1 kWh is useful. Knowing that 1.4 kWh of that will happen between 10am and 1pm is what lets you decide when to run appliances.
Tell you when you're having a 100% day. Some days your system will hit its best-ever output. A notification that fires when that is forecast to happen — so you can run your EV charger, fill the hot water tank, run every appliance going — is the practical payoff of having a personalised calibration.
Be free, or honestly priced. Ads in a free app are a fair trade. What is not fair is charging a monthly subscription just to see tomorrow's forecast — data that comes from weather models that cost nothing to query. The right model is a one-time purchase to remove ads and unlock features. Pay once, own it. No renewal reminder, no annual fee, no "your subscription has expired" screen blocking the forecast you need right now.
How Sun Hours solves each of these
Sun Hours is built around a two-question onboarding: your postcode and your best-ever daily kWh. That is the entire setup. No inverter credentials, no panel specs, no azimuth slider.
The postcode maps to over 100 UK region sun-hour profiles — from Truro at 1,080 kWh/kWp/year down to Aberdeen at 880. The best-day kWh number is the calibration anchor. If your 800W system produced 2.6 kWh on your best day, the app knows that a perfect clear July day in your location produces 2.6 kWh for your system. Every forecast is built as a percentage of that number, applied to the day's forecast irradiance.
This means the forecast is automatically calibrated to your real panels — with their real shading, real mounting angle, real orientation — without you having to specify any of that. Your system told the app what it can do on its best day. The rest follows.
The hourly chart shows you generation potential across all seven forecast days, hour by hour. You can see that tomorrow peaks at 11am, drops off from 2pm, and that the best window for running heavy appliances is 10am–1pm.
The 100% notification fires when forecast conditions are expected to match your best day. In the UK that might happen 8–12 times a year depending on your location and orientation. When it does, you want to know.
Works with every system. Hoymiles owners: find your peak day in S-Miles Cloud (Reports → Daily, sort highest). Deye/SolarmanPV users: Generation report → Daily → find peak. Marstek: History → Day view. Envertech: EnverView → Statistics. No API connection needed — just the number.
No subscription, ever. The 7-day forecast, hourly breakdown, notifications, and home screen widget are all free. The app has ads. To remove them permanently, there is a one-time No Ads purchase. For unlimited Saved Locations and the Smart Appliance Timer on top of that, there is a one-time Pro upgrade. Both are lifetime unlocks — no recurring charge, no annual renewal, no subscription tier of any kind.
Finding your best-day kWh (takes 90 seconds)
The setup number that makes everything work is your best-ever single-day kWh. Here is where to find it by inverter brand:
Hoymiles (S-Miles Cloud): App → Reports → Daily generation → sort by highest value. Look for your summer peak, probably in May or June.
Deye / SolarmanPV: App → Generation report → Daily → scroll to find your highest single day.
Marstek: App → History → Day view → find highest.
Envertech (EnverView): App → Statistics → Daily generation → find peak.
APS (EMA app): App → Energy → Daily chart → find peak.
No app data yet, or brand-new system? Go to sunhours.app/solaruk, scroll to the NASA POWER calculator, enter your postcode and system wattage, and use the May figure divided by 31 as a starting estimate. You can always update it later once your system has been running for a few weeks.
Does location matter for a solar PV forecast?
Significantly. The UK's solar resource varies more than most people expect:
- Cornwall / Devon: 1,060–1,080 kWh/kWp/year — comparable to central France
- London / Bristol / Brighton: 1,010–1,030 kWh/kWp/year
- Birmingham / Midlands: 965–975 kWh/kWp/year
- Manchester / Leeds: 935–940 kWh/kWp/year
- Edinburgh / Glasgow: 895–900 kWh/kWp/year
- Aberdeen: ~880 kWh/kWp/year
A 800W south-facing system in Cornwall will generate around 3.0–3.2 kWh on its best day. The same system in Edinburgh generates 2.1–2.3 kWh. A forecast app that does not account for this — using a flat national average — will be systematically wrong for anyone north of Birmingham.
Sun Hours maps every UK postcode area to a measured solar yield figure. When you enter your postcode, you get a forecast for your specific location's sun hours, not the national average.
The appliance timing question
This is the daily practical use case that most forecast apps ignore. You have a solar-generating window of roughly 9am–4pm on a good UK day. Within that, output peaks for 3–4 hours around solar noon. Running your washing machine, dishwasher, or EV charger during that peak window means you consume your own free electricity rather than drawing from the grid.
Over a year, consistently timing your highest-draw appliances to solar peak hours can add £30–£80 in savings on top of your baseline bill reduction — without any change to your system. The forecast tells you when that window is. The habit of checking it takes about ten seconds.
Sun Hours shows you today's hourly curve on the home screen. You can see at a glance whether to run the dishwasher now or wait until the cloud cover predicted at 11am passes.
Further reading
What Are Plug-In Solar Panels? The UK Guide for 2026
Plug-In Solar · 5 min read
Sun Hours Is Now on iPhone — Free on the iOS App Store
App Update · 3 min read
UK Solar Breaks All-Time Annual Record: 269,000 Installations in 2025
Solar News · 6 min read
The Best Reddit Communities for Solar Panel Owners in the UK
Community · 6 min read
Do Solar Panels Add Value to Your Home? (And Why Plug-In Solar Changes the Maths)
Solar Guide · 6 min read
Get your personalised solar PV forecast — free to download
Sun Hours gives you a 7-day kWh forecast calibrated to your actual system. Enter your postcode and best-ever daily output — that is it. Free on Android. If you want to remove ads, it is a one-time purchase. Never a subscription.