Tomorrow’s PV forecast
Enter your location and roof settings, then check the hourly curve. Compare the estimate with the actual array’s recent performance.
Battery planning
How much should you charge a home battery overnight?
Enough to cover the hours before useful solar arrives, plus your protected reserve and a forecast margin. A sunny-day target may be well below 100%.
Short answer
Charge for the forecast deficit, not from habit.
A fixed rule such as "charge fully at 02:00" treats every day as the same. Forecast-based charging asks how much energy the home is likely to need before the next good charging opportunity, then adds reserve and an uncertainty margin.
The result may be a full battery before a cloudy winter day, a partial charge before strong midday solar or no grid charge when stored energy is already sufficient.
Five inputs
A weather forecast alone is not a battery plan.
Load forecast
Expected household consumption, including EV, heating, cooling and unusual routines.
Prices
Actual retail import and export values, not only a wholesale chart.
Battery state
Usable capacity, current charge, power limits, efficiency and permitted operating range.
Protected reserve
Energy kept for outage risk, essential loads and household priorities.
Practical model
A target can be simple enough to explain.
target stored energy ~ reserve + energy needed before recharge + marginFirst choose the period you need to bridge: for example, the end of the overnight cheap window until useful PV surplus arrives. Add up the energy the battery must supply during that period, allowing for losses. Solar expected at noon cannot cover a shortfall at 07:00.
This is a heuristic, not a universal controller formula. A real system also converts between state of charge and usable energy, accounts for losses and checks power, warranty, tariff and export constraints.
Worked example · not a measured home
A 10 kWh battery: why a 60% morning target can be enough.
All figures below are assumptions chosen to make the calculation visible. They are not recommended settings for a particular battery.
| Input | Assumption | Meaning |
|---|---|---|
| Usable battery capacity | 10 kWh | The example treats 0–100% as this usable range; your app may define it differently. |
| Stored energy now | 3 kWh | The starting point before the overnight charging window. |
| Protected reserve | 2 kWh | Energy unavailable for normal household scheduling. |
| Battery draw before useful PV | 3.2 kWh | Expected draw from stored energy, already including discharge losses. |
| Uncertainty margin | 0.8 kWh | An illustrative allowance for later clouds or higher morning use. |
| Target at the end of charging | 2 + 3.2 + 0.8 = 6 kWh | 60% of the assumed usable capacity. |
| Charge efficiency | 95% | A separate example assumption, not a round-trip efficiency rating. |
| Grid energy for battery charging | (6 − 3) ÷ 0.95 ≈ 3.16 kWh | Additional house consumption during that window is separate. |
Now check the space for solar.
After the expected morning draw, the battery holds 6 − 3.2 = 2.8 kWh and has 7.2 kWh of room. If 6 kWh of PV surplus remains after simultaneous household use, storing it at the assumed 95% charge efficiency adds 5.7 kWh. The battery ends that window at about 8.5 kWh, if timing and charge-power limits allow it.
Starting at 100% would leave only 3.2 kWh of space after the same morning draw. That may mean more export or curtailment; it does not automatically mean a financial loss, because export prices and later demand matter too.
If the next forecast delays useful solar or adds 2 kWh to the expected battery draw, the same arithmetic raises the target from 6 to 8 kWh. If the required target exceeds capacity or the charging window is too short, plan for grid supply or reduce optional loads instead of assuming the battery can cover everything.
Check tomorrow’s PV forecast, then compare its strongest hours with your household’s actual morning use.
A five-minute check before changing a schedule.
- Read the forecast date and update time. Use the correct location, installed panel capacity, roof direction and tilt. A typical monthly yield is not tomorrow’s forecast.
- Choose when reliable recharging is next possible. Check hourly production and load, not just daily totals. Allow for an EV departure, heating demand or a cloudy morning.
- Protect reserve first. Convert the energy target to the state-of-charge scale used by your battery and retain the manufacturer’s operating limits.
- Check the tariff and available charging power. Use your retail import price, export value and device limits. Confirm that grid charging is permitted for your installation.
- Recheck in the morning. Compare actual production with the updated curve. Leave flexibility when the forecast is uncertain; do not spend protected reserve on a forecast that has not arrived.
In autumn and winter, useful solar may arrive later and contribute less. The 5 kWp winter-output comparison gives seasonal context; a dated forecast and your own recent measurements should set tomorrow’s plan.
Three different days
The same battery should not receive the same instruction.
| Forecast | Likely plan | Why |
|---|---|---|
| Sunny, export-limited day | Keep useful battery space for midday PV. | A full morning battery may force low-value export or solar curtailment. |
| Cloudy day, expensive evening | Consider charging in a permitted cheap window. | Stored low-cost energy may avoid later expensive imports while preserving reserve. |
| Uncertain weather or outage risk | Increase the margin or protected reserve. | The cost-optimal plan should not outrank resilience when forecast confidence falls. |
Replanning
A good plan expires quickly.
Forecasts are wrong in useful ways. Clouds arrive early, household demand changes and an EV plugs in unexpectedly. The controller should re-evaluate the remaining day when material inputs change rather than execute a 24-hour schedule blindly.
The fallback matters just as much. If price data or weather is stale, the home should preserve defined reserve, respect device limits and use a conservative local policy until fresh inputs return.
This is also why forecast accuracy cannot be judged only by a weather metric. The real test is whether updated decisions improve cost, solar use and reserve compliance.
Verification
Measure the decision, not only the forecast.
- Net energy costImport cost minus export value under the actual contract.
- Solar curtailment or spillGeneration that could not be used, stored or exported.
- Battery throughputExtra cycling created to achieve the result.
- Reserve complianceWhether the protected minimum was preserved when required.
- Forecast errorPV and load error by the intervals that changed decisions.
The main conclusion remains: the best target is the lowest sensible charge that covers the expected deficit and reserve while leaving room for likely solar.
FAQ
Forecast charging questions.
What is forecast-based battery charging?
It is a control method that updates battery charging from expected solar production, household load, electricity prices, current state of charge and reserve requirements.
Should a home battery charge to 100% every night?
Not always. A full battery can be useful before a cloudy or high-price day, but it can leave no room for expected midday solar. The target should follow the household objective and forecast.
What happens when the solar forecast is wrong?
A robust system keeps an uncertainty margin, replans as new data arrives and falls back to safe limits when forecasts or prices are stale or unavailable.
Does forecast charging always save money?
No. The result depends on tariff rules, import and export prices, battery efficiency and wear, solar size, household demand, weather and the quality of control.
Can every battery charge from the grid?
No. Capability, warranty, installation settings, metering, tariff and local rules vary. The exact system and contract must allow grid charging.
How much battery space should I leave for tomorrow’s solar?
Estimate solar left after simultaneous household use, convert that surplus to stored energy using charge efficiency, and compare it with the space available when solar arrives. Account for battery discharge before that window; a full-day solar total alone is not enough.
Sources
Technical references.
- SMA Sunny Home Manager operating manual
Documents forecast-based energy management using expected PV, consumption, tariffs, battery state and export constraints.
- Tesla Powerwall Savings Mode user guide
A manufacturer example of continuously forecasting solar and consumption while balancing time-based rates and backup reserve.
- Australian Government: home batteries
Official consumer guidance on battery losses, export limits, tariff configuration and conditional economics.
- NREL BLAST-Lite battery lifetime model
Shows why battery lifetime modelling includes temperature, state of charge, depth of discharge and power, not only a cycle count.
- Assessment of forecasts in PV-battery systems
Research on how load and PV forecasting methods affect residential and commercial battery control under different constraints.
Make it personal
Forecast your own roof, then get tomorrow by email.
Use your location and system size instead of a generic benchmark. The result includes expected output, uncertainty and the strongest solar hours.