Solcast Solar Forecasting: Better Battery Decisions

Solcast provides solar generation forecasts using satellite weather data and your specific roof configuration. For UK solar owners with batteries, knowing tomorrow's generation before it happens is the key to smarter energy decisions.

What Is Solcast?

Solcast is an Australian company that provides solar irradiance and generation forecasting. Their system combines:

• Satellite cloud imagery (updated every 15–30 minutes)
• Weather forecast models
• Your specific roof configuration (location, tilt, azimuth, system size)

The result: a predicted generation curve for your system, updated multiple times daily, looking up to 7 days ahead.

Why Forecasting Matters

Without a forecast, your battery operates blind:

• Should it charge overnight from cheap grid electricity? (Depends on whether tomorrow is sunny — if it is, solar will charge it for free)
• Should it export now or hold for later? (Depends on whether more generation is coming)
• Should you run the washing machine now or wait? (Depends on when solar surplus will be available)

With a forecast, these questions have answers:

• Sunny tomorrow: Don't charge overnight, let solar fill the battery. Save the money.
• Cloudy tomorrow: Charge overnight at cheap rates because solar won't provide enough.
• Surplus expected at 11am–3pm: Schedule the washing machine for midday.

Setting Up Solcast

Step 1: Create a Free Account

Visit solcast.com and register for a free Home User account. This gives you:

• 2 rooftop sites (enough for most homes)
• 10 API calls per day (enough for regular forecast updates)
• Forecast data up to 7 days ahead

Step 2: Configure Your Rooftop

Enter your system details:

• Location: Postcode or coordinates
• Tilt: Roof pitch in degrees (30° is typical for UK)
• Azimuth: Compass direction the panels face (180° = south, 90° = east, 270° = west)
• System capacity: Total kWp installed
• Inverter capacity: Maximum AC output

If you have panels on two different roof faces (e.g., east-west split), create two separate rooftop sites — this gives a more accurate combined forecast.

Step 3: Get Your API Key

Solcast provides an API key for accessing forecast data programmatically. You'll need this for Home Assistant or Predbat integration.

Home Assistant Integration

The Solcast integration for Home Assistant (available via HACS) pulls forecast data into your HA dashboard:

What You See

• Today's forecast: Expected total generation in kWh
• Tomorrow's forecast: Expected generation for planning
• Hourly breakdown: When during the day generation is expected
• Remaining today: How much more generation to expect today

How to Use It

• Dashboard display: Show today's and tomorrow's forecast on your main energy dashboard
• Automations: Trigger actions based on expected generation (e.g., "if tomorrow's forecast is under 5 kWh, charge battery to 100% overnight")
• Predbat input: Predbat uses Solcast data automatically to make battery charging decisions

Accuracy in UK Conditions

Solcast's accuracy varies by forecast horizon and weather conditions:

UK weather is notoriously changeable, which limits long-range solar forecast accuracy. However, for next-day planning (which is what matters most for battery decisions), Solcast performs well enough to be useful.

Where it struggles:

• Rapidly changing weather (fronts passing through)
• Scattered cloud (hard to predict exact timing)
• Morning fog/mist (can burn off at varying times)

Where it excels:

• Clear sky days (easy to predict, high confidence)
• Fully overcast days (consistently low generation, easy to forecast)
• Seasonal patterns (longer-term trends are predictable)

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Practical Use Cases

Battery Charging Strategy

The most valuable use. Compare tomorrow's forecast with your expected consumption:

EV Charging

If tomorrow is sunny, delay EV charging until solar hours. If overcast, charge overnight on cheap rates. Solcast makes this decision data-driven rather than guesswork.

Hot Water

If heavy solar surplus is expected, rely on the PV diverter for hot water. If a poor solar day is forecast, schedule the boiler or immersion to heat water during cheap overnight rates instead.

Household Planning

Run energy-intensive tasks (washing, dishwasher, ironing) during expected peak generation. This maximises self-consumption and minimises grid import.

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Comparison with Other Forecast Services

Solcast is the most widely used and best-supported option for UK home solar owners, particularly through its Predbat integration.

Getting the Most from Solcast

1. Set accurate roof parameters — azimuth, tilt, and capacity matter
2. Use it through Predbat — automated decisions beat manual ones
3. Track forecast vs actual — use PVOutput to compare Solcast predictions with real generation
4. Adjust for local conditions — if your system consistently generates 10% less than Solcast predicts (due to local shading, trees, etc.), factor this in manually or through Predbat's damping settings

Solcast transforms solar energy management from reactive to proactive. Combined with a battery and smart tariff, it's the forecasting engine that makes automated optimisation possible.

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