When homeowners ask about solar batteries, the conversation usually lands on one question: "Will it keep my whole house running?" The honest answer — and the one good installers give — is it depends on what you want to back up and for how long. Home battery backup sizing isn't complicated once you break it down, but skipping this conversation leads to undersized systems, disappointed customers, and callbacks.
What "Whole-Home Backup" Actually Means
Whole-home backup means every circuit in your panel stays powered during a grid outage — including your HVAC system, electric range, water heater, EV charger, and other large loads. It sounds ideal, but the math gets large fast.
A typical 2,500 sq ft home in Texas might have a 4-ton HVAC unit drawing 4–5 kW during the compressor cycle, a 240V electric range pulling 8–10 kW at full blast, and an electric water heater adding another 4.5 kW. Run those simultaneously and you're pulling 15–20 kW continuously — before counting the refrigerator, lights, and everything else. Backing up all of that requires serious inverter capacity and enough battery to sustain it overnight.
Inverter sizing matters as much as battery capacity here. A Sol-Ark 15K-2P or EG4 18kPV can handle whole-home loads in most residential installations. The Sol-Ark 30K-3P-208V steps up for larger homes or light commercial. The inverter's continuous output rating is the ceiling on what loads you can run simultaneously — battery kWh only determines how long you can run them.
The Case for Partial Backup: Smarter for Most Homeowners
For most grid-tied solar customers, partial backup is the right answer — and a much easier sell once you frame it correctly. Partial backup means you select a critical loads subpanel and back up only what matters: the refrigerator, some lighting, the Wi-Fi router, medical equipment, a few outlets, and perhaps the HVAC air handler (not the full compressor).
This approach dramatically reduces the battery capacity and inverter sizing required. A single FranklinWH aPower 2 (15 kWh usable, 10 kW continuous output) can comfortably run a partial load panel for 12–24 hours depending on usage. The EG4 WallMount 280Ah (14.3 kWh, LFP chemistry, rated for 8,000 cycles) handles similar scenarios at a different price point. For homeowners who want to start small and expand, the Sigenergy SigenStor is modular — starting at 6 kWh or 9 kWh per pack and stackable to 54 kWh — which makes it easy to add capacity as budgets allow without replacing anything.
Home Battery Backup Sizing: The Math That Actually Works
Here is a practical framework for sizing conversations with homeowners:
- List critical loads. Refrigerator (~150W average draw), LED lighting (~200–400W for a partial panel), a mini-split or window unit (800–1,500W), phone and laptop charging, Wi-Fi router (~15W). Add these for a realistic continuous draw estimate.
- Estimate backup duration. Does the homeowner need to bridge overnight (8–10 hours)? Through a 24-hour Texas ice storm? A multi-day hurricane outage?
- Calculate gross energy needed. Continuous draw (kW) × hours = kWh needed. Add ~15–20% for inverter and battery round-trip efficiency losses.
- Account for solar recharge. If the system includes panels, outage duration matters less than overnight capacity. A battery that gets partially recharged by solar each day can stretch indefinitely through extended outages.
Example: A homeowner backs up a refrigerator (0.15 kW), LED lighting (0.3 kW), a mini-split (1.2 kW), and phone/Wi-Fi (0.1 kW) for 12 hours overnight — roughly 1.75 kW × 12 hours = 21 kWh gross. With a 15% efficiency buffer, that is about 24 kWh usable. Two FranklinWH aPower 2 units (30 kWh combined) or a Sigenergy SigenStor stack at 27 kWh covers this with room to spare.
Don't Overlook Surge Capacity
Continuous load ratings tell only half the story. Motors — HVAC compressors, well pumps, refrigerator compressors — draw two to three times their rated current on startup for a fraction of a second. Always verify the inverter's surge rating and confirm the battery's peak discharge current can support it. The Sol-Ark 15K-2P and EG4 18kPV carry surge ratings well-suited for HVAC-inclusive backup, and LFP batteries like FranklinWH or EG4 WallMount handle high peak discharge rates better than older NMC chemistries.
When Time-of-Use Rates Change the Sizing Conversation
For grid-tied customers not primarily focused on backup, time-of-use (TOU) optimization is often the primary ROI driver. Under TOU rates common in California and increasingly in Texas deregulated markets, electricity can cost three to four times more during evening peak hours (4–9 PM) than during midday solar generation hours. A customer optimizing for TOU arbitrage needs enough battery to offset evening peak consumption — typically 8–15 kWh — rather than sizing for multi-day outages. A single FranklinWH aPower 2 or EG4 WallMount often hits the sweet spot here, with the option to add a second unit for backup depth later.
Get the Sizing Right First
Home battery backup sizing is not a one-size-fits-all spec sheet exercise. The best installations start with a real load analysis, an honest conversation about what the homeowner actually needs during an outage, and a system designed with room to grow. Oversell whole-home backup to a budget-constrained customer and you lose the deal. Undersell a partial backup to someone who genuinely needs whole-home coverage and you get a frustrated callback after the first major storm.
Get the sizing right, and the equipment — whether FranklinWH, EG4, or Sigenergy — does exactly what it promises. That is what builds referrals.