The simple recharge-speed formula
For a first-pass outage screen, start with average AC solar power during the useful solar window. Subtract the average backed-up household load. Then compare the remaining solar surplus with the battery system's maximum charge power.
Accepted charging power ≈ min(solar power − household load, battery-system charge limit)
The power that actually becomes stored battery energy is lower after conversion and charging losses. A simplified recharge-time estimate is:
Recharge time ≈ energy needed ÷ (accepted charging power × charging efficiency)
Example: 5 kW of average solar with a 1.5 kW backed-up load
This leaves 3.5 kW of average solar surplus before the battery charging limit. The table below starts each battery at 30% SOC, targets 90% SOC and uses a 90% solar-to-stored efficiency allowance.
| Battery | Usable capacity | Published charge limit | Energy needed 30→90% | Accepted solar | Simplified time |
|---|---|---|---|---|---|
| Tesla Powerwall 3 Specification profile → | 13.5 kWh | 5.0 kW | 8.1 kWh | 3.5 kW | 2.6 h |
| Enphase IQ Battery 5P Specification profile → | 5.0 kWh | Data gap | 3.0 kWh | Not calculated | Needs verified charge limit |
| FranklinWH aPower 2 Specification profile → | 15.0 kWh | 8.0 kW | 9.0 kWh | 3.5 kW | 2.9 h |
This is deliberately not a promise of real-world recharge time. Actual PV output varies continuously, and battery controls may reduce charging power with temperature, state of charge or system conditions.
Why a bigger solar array may stop making the battery charge faster
If the home has 9 kW of solar surplus but the battery system can accept only 5 kW, the additional solar cannot increase battery charging above that 5 kW limit at that moment. The extra power may serve other loads, be exported when allowed, be curtailed or be handled differently by the installed system architecture.
This is why battery recharge planning needs both energy capacity in kWh and charge power in kW. Capacity tells you how much energy is needed to move from one state of charge to another; charge power limits how quickly that energy can be added. Continuous discharge power and short-duration surge answer different questions again; see the Battery Charge Power vs Discharge Power guide before treating one kW rating as interchangeable with another.
Fast recharge does not guarantee multi-day resilience
A battery can recover quickly on one clear afternoon and still fail a longer outage if the first night is too long, the next day is cloudy, the critical load is high or the following solar window cannot replace the energy used overnight. Multi-day resilience depends on the sequence of charge and discharge periods, not only one recharge-time number.
The 3-Day Solar Battery Outage Calculator uses 15-minute steps across 72 hours and lets each day have a different solar multiplier. It also screens the minimum full battery-unit count that survives the selected sequence.
Current charge-power data in the Home Solar Atlas catalog
Tesla Powerwall 3
5.0 kW published maximum continuous charge power per full unit
Powerwall 3 itself is rated for 5 kW maximum continuous AC charge power. Tesla lists 8 kW for one Powerwall 3 with up to three Expansion units, so expansion-based systems should use the manual system charge-limit override instead of simple per-unit scaling.
Enphase IQ Battery 5P
Separate maximum continuous charge power not verified in the current source review
The current North American IQ Battery 5P datasheet publishes continuous and peak AC output plus round-trip efficiency, but this Home Solar Atlas review did not find a separate maximum continuous charge-power value. Enter a current manufacturer or installer system charge limit before relying on the charge-window result.
FranklinWH aPower 2
8.0 kW published maximum continuous charge power per full unit
FranklinWH publishes 8 kW continuous real charge power for aPower 2. Multi-battery system behavior still depends on aGate design, conductors, controls and the installed architecture.
Five inputs that change recharge time
- Starting state of charge. A battery at 20% has much more energy to recover than the same battery at 70%.
- Target state of charge. Reaching 80% requires less energy than reaching 100%, and some systems may taper charging near high SOC.
- Solar surplus after loads. During an outage, refrigerators, HVAC, pumps and other backed-up circuits can consume solar that would otherwise charge the battery.
- Battery-system charge limit. This can cap accepted power even when the array could provide more.
- Effective solar window and weather. Four strong equivalent solar hours are not the same as twelve clock hours between sunrise and sunset.
Outage recharge is not automatic for every solar home
A standard grid-tied PV system may shut down when the grid fails. Solar charging during an outage requires compatible battery, inverter, islanding and transfer/control architecture. Ask the installer to show exactly how the proposed system operates when the utility grid is unavailable.
Method and source policy
Home Solar Atlas uses direct manufacturer specifications for equipment data where available and records a review date. When a separate maximum continuous charging-power figure cannot be verified, the comparison reports a data gap instead of inferring a number from discharge power.
The recharge examples are screening calculations, not electrical designs. They do not model minute-by-minute PV output, battery thermal behavior, SOC-dependent charge taper, export restrictions, AC/DC clipping, or every manufacturer-specific multi-battery power-sharing rule.
Read the Home Solar Atlas source policy →