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

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:

1. Solar produces; powers home + charges battery
2. Excess production exports to grid
3. At night, home draws from grid

Outage operation:

1. Grid disconnected (anti-islanding)
2. Battery + solar continue to power home (battery's "islanding mode")
3. System manages capacity vs. demand
4. Returns to normal operation when grid restored

Time-of-use optimization:

1. Battery charges from solar during day
2. Discharges to home during evening (high-rate hours)
3. Charges from grid during low-rate hours if depleted
4. 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

Lithium batteries have operating temperature ranges. Outdoor installations in extreme climates may need heated/cooled enclosures.

Electrical considerations

Code compliance

Backup capabilities

Customer perception management:

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:

1. Battery charges from solar during day (when rates are low or solar provides excess)
2. Discharges to home during evening peak rates
3. Daily savings = (peak rate - off-peak rate) × kWh shifted
4. 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:

• The battery is only part of the cost. The installed price includes the battery, the inverter or gateway that manages it, a backup load subpanel or a whole-home transfer device, conduit and conductors, permit and inspection, and utility interconnection paperwork. Quoting the battery alone sets up a bad conversation later
• Cost per usable kWh is the honest comparison, not cost per nameplate kWh. Usable capacity is less than nameplate, and it is what the customer actually gets during an outage
• A second battery is cheaper than the first, because the gateway, the subpanel, and most of the labor are already in place. That matters when a customer is deciding between one now and two now
• Panel work is the wildcard. An older service, a full panel, or a main breaker that cannot be derated for the interconnection can add a service upgrade to the scope, and that line is often the difference between an accepted and a rejected quote
• Incentives change the number substantially and change on their own schedule. Federal credit, state programs, and utility storage rebates all have their own eligibility and deadlines. Verify current terms before you put anything in writing, and never quote an incentive as a certainty
• Warranty term and throughput both matter. Batteries carry a year limit and a total energy throughput limit, whichever comes first, and heavy daily cycling for time-of-use arbitrage reaches the throughput limit faster than backup-only use

Frame the payback honestly. On time-of-use rates with daily cycling, there is a real arithmetic case. Without them, the customer is buying outage resilience, and that is a legitimate reason to buy that has nothing to do with payback.

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