Battery Storage with Grid-Tied Solar
Why this matters
Grid-tied solar PV systems normally shut down when the utility power fails (UL 1741 anti-islanding requirement). Customers who want power during outages need an energy storage system (battery). The battery + control electronics integrate with the PV system to provide backup power, time-shift solar production for peak use, OR enable off-grid operation. Storage technology has matured significantly; Tesla Powerwall, LG RESU, Enphase IQ Battery, FranklinWH, and other systems compete in this growing market.
Why solar customers want batteries
| Customer driver | What the battery does |
|---|---|
| Outage backup | Battery + automatic transfer switch provide power during utility outages |
| Time-of-use rate savings | Battery stores solar production for evening use when rates are higher |
| Demand charge reduction (commercial) | Battery offsets peak demand to reduce demand charges |
| Off-grid living | Battery + larger solar = no utility connection |
| Grid services (some utilities) | Battery participates in utility programs for compensation |
| Energy independence philosophy | Some customers value reducing utility dependence |
The customer's primary driver determines the system design.
Battery technologies
Lithium-ion (dominant)
- Tesla Powerwall, LG RESU, Enphase IQ Battery, etc.
- High energy density (smaller per kWh)
- Longer service life (10+ years typical, 25 years possible)
- Higher upfront cost
- Less maintenance
Subcategories:
- NMC (Nickel Manganese Cobalt): high energy density, premium price, examples include Tesla Powerwall
- LFP (Lithium Iron Phosphate): safer chemistry, slightly lower energy density, growing adoption
- Solid-state lithium: emerging technology; not yet widely deployed residentially
Lead-acid (legacy)
- Cheaper upfront cost
- Lower energy density (larger for same capacity)
- Shorter service life (5-10 years)
- Require periodic maintenance (some types)
- Common in off-grid systems with budget constraints
Most current installations use lithium-ion. Lead-acid is occasionally used in specialized applications.
How the system integrates
The battery + grid + solar interconnection:
[Grid] ←→ [Smart Panel / Inverter] ←→ [Solar PV]
↕
[Battery]
↓
[Home Loads]
Normal operation:
- Solar produces; powers home + charges battery
- Excess production exports to grid
- At night, home draws from grid
Outage operation:
- Grid disconnected (anti-islanding)
- Battery + solar continue to power home (battery's "islanding mode")
- System manages capacity vs. demand
- Returns to normal operation when grid restored
Time-of-use optimization:
- Battery charges from solar during day
- Discharges to home during evening (high-rate hours)
- Charges from grid during low-rate hours if depleted
- Reduces utility electricity cost
Sizing the battery
The customer's goal determines battery capacity:
Outage backup for critical loads (refrigerator, lights, internet):
- 5-10 kWh battery
- Backup for 4-12 hours typical
Whole-home backup (some loads cycled):
- 13.5-30 kWh (one or more Powerwalls)
- Backup for 12-24 hours typical
- Load management to extend duration
Whole-home backup, extended duration:
- 30+ kWh
- Backup for multi-day outages
- Significant capital investment
Off-grid:
- 30-60+ kWh
- Designed to ride through cloudy days
- Larger battery + larger solar PV system
The customer's typical load curve + outage frequency + budget drive the sizing.
Common battery products
Tesla Powerwall
- 13.5 kWh capacity
- 5 kW continuous, 7 kW peak
- LFP chemistry (Powerwall 3)
- AC-coupled
- Premium price; well-known brand
- Tesla's monitoring app
LG RESU
- Various capacities (10-16 kWh)
- DC-coupled (integrates closer with the inverter)
- Higher round-trip efficiency
- LG monitoring
Enphase IQ Battery
- 3.36 kWh modular (stackable)
- Microinverter ecosystem integration
- Lower per-unit capacity (built in modules)
- Enphase monitoring
FranklinWH
- 13.6 kWh capacity
- Higher continuous output
- Backup transfer features built-in
- Emerging brand
Generac PWRcell
- 9-18 kWh modular
- DC-coupled with their inverter
- Familiar brand for generator customers
Each has advantages. Compare based on:
- Customer's existing solar inverter (compatibility)
- Capacity needs
- Brand preference
- Local installer support
- Warranty terms
Installation considerations
Location
| Location | Considerations |
|---|---|
| Garage | Standard; convenient access; temperature stable |
| Outdoor (wall-mount) | Sun + weather exposure; verify rated for outdoor use |
| Indoor utility room | Quiet; temperature-controlled |
| Outdoor in cold climate | Verify cold-weather operation; some require enclosures |
Lithium batteries have operating temperature ranges. Outdoor installations in extreme climates may need heated/cooled enclosures.
Electrical considerations
| Requirement | Detail |
|---|---|
| Dedicated circuit | Battery requires a dedicated AC circuit |
| Service panel coordination | The transfer switch / smart panel integrates with the main panel |
| Critical load panel (some installations) | Separate sub-panel for backup-only loads |
| Wiring | Larger gauge wires for the battery's current capacity |
Code compliance
| Code | Requirement |
|---|---|
| NEC 706 (Energy Storage Systems) | ESS-specific requirements |
| NEC 690 (Solar PV) | Integration with PV system |
| NEC 705 (Interconnected Power Sources) | Source coordination |
| Local fire code | Some areas require fire-rated installation |
Backup capabilities
Customer perception management:
| Backup capacity | Customer experience |
|---|---|
| Critical loads only (refrigerator, lights, internet) | Reduces "outage anxiety" but customer still feels constrained |
| Whole-home with load management | Customer experiences mostly normal life with occasional cycling |
| Whole-home with full capacity | Customer doesn't notice the outage |
Set expectations based on the actual capacity. A customer expecting "whole-home" capacity may be disappointed with a 13.5 kWh single-Powerwall installation.
Time-of-use savings
For customers on time-of-use rates:
- Battery charges from solar during day (when rates are low or solar provides excess)
- Discharges to home during evening peak rates
- Daily savings = (peak rate - off-peak rate) × kWh shifted
- Annual savings adds up over many days
For customers without time-of-use rates: the financial case is purely outage backup.
Cost framework
Battery storage is one of the larger residential electrical investments:
References
- NEC 706 (Energy Storage Systems)
- NEC 690 (Solar PV Systems)
- NEC 705 (Interconnected Power Sources)
- UL 9540 (Energy Storage System Safety)
- IEEE 1547 (Interconnection Standards)
- IRS Section 25D (federal tax credit)
- Manufacturer documentation (Tesla, LG, Enphase, FranklinWH, Generac)
- Manuall internal: Off-Grid and Battery Systems, Residential Solar Install SOP, Inverter Types and Sizing