Battery Bank Sizing Reference Guide | PES Supply

By Jennifer Kirstens · 2026-08-31 · 16 min read

Static reference tables for battery bank sizing — daily kWh consumption → backup hours → required usable kWh, with matched SKU recommendations from the PES Supply direct-distributor lineup. LFP defaults to 80 % DoD, NMC to 90 %, AGM to 50 %. For the interactive calculator that includes contractor pricing and inverter compatibility, use the PES contractor portal.

A battery bank sizing pass takes four inputs:

1. Daily kWh consumption — the average energy the site uses per day. Pull from a utility bill (monthly kWh / 30). Typical US residence: 25–35 kWh/day.
2. Backup hours needed — how many hours of that daily load the battery must cover during an outage. 8 hours (overnight), 24 hours (one full day), 72 hours (multi-day event).
3. Depth of discharge — the fraction of nameplate capacity you'll regularly cycle. LFP: 80 %. NMC: 90 %. AGM: 50 %.
4. Voltage architecture — 48 V low-voltage or 200–500 V high-voltage. Drives which brands are compatible.

The kWh sizing pass determines how much battery you need. Voltage architecture determines which battery brand you can use.

The most common sizing error is guessing at daily kWh consumption instead of measuring it. Three ways to get an accurate number:

An essential-loads panel captures a subset of the home's circuits — usually refrigerator, sump pump, garage door, some outlets, some lighting, HVAC or mini-split. It's what most residential batteries actually back up during an outage, because sizing a battery to whole-home load is expensive and rarely necessary.

Most residential LFP batteries parallel in 2–8 unit configurations. Rules of thumb:

• Same-brand only. Cross-brand paralleling breaks the BMS closed-loop protocol.
• Same firmware version. Update all units to the same BMS firmware before commissioning.
• Same state of charge at commissioning. Charge each unit to 50 % SoC on a bench before connecting in parallel.
• Matched cable length between battery and combiner. Unequal cable resistance causes uneven current sharing.
• Individual DC breaker per battery. NEC 706 requires per-unit disconnect for service.
• Per-unit balancing time. On first commissioning, allow 24–48 hours of low-current balancing before full-power discharge.

The static tables in this reference guide are enough for a rough sizing pass. Use the interactive calculator when:

• You need matched SKU pricing with contractor tier applied
• You want inverter-battery compatibility validated against the specific hybrid inverter you're spec'ing
• You need to compare 3–5 configurations side-by-side with total installed cost
• You want to model TOU arbitrage or peak shaving against a specific utility rate
• You need a quote-ready line-item breakdown for the customer

The interactive Battery Sizing Calculator lives at axis.pesdistribution.com/portal/battery-sizing-calculator on the PES contractor portal — free with contractor registration.

Battery kWh and inverter kW are two independent sizing decisions. Common combinations:

Below 0 °C, LFP batteries derate charge acceptance. Below -10 °C, most LFP won't accept charge at all without a self-heating BMS. Cold-climate sizing implications:

• Use a self-heating battery. EG4 PowerPro WallMount AllWeather, Fortress Avalon with cold-weather option, and select SimpliPhi models include self-heating BMS that keeps cells above 0 °C.
• Size up 20 %. Winter kWh consumption in cold-climate homes runs 30–50 % higher than summer averages (electric heat pump, heat strips, longer runtimes). Size to winter peak, not annual average.
• Locate indoors or in a heated garage. Even AllWeather batteries operate more efficiently in mild ambient. Attached garages with basic heating maintain battery performance year-round.
• Increase headroom for solar shortfall. Cloudy winter days with short daylight hours mean the battery discharges deeper and recharges less. Add 30–50 % capacity headroom over the mathematical sizing target.

Above 45 °C ambient, LFP batteries derate power output (charge and discharge). Above 50 °C, most batteries alarm and de-rate to 50 % power. Hot-climate implications:

• Ventilated cabinet or indoor mount. Battery cabinet must maintain internal temperature below 45 °C. Passive vents may not be enough in AZ summers; some C&I installs use active HVAC.
• Avoid direct sun. Wall-mount on north or east-facing walls, or provide shade structure.
• Size larger inverter. Higher summer AC load means the battery discharges harder, and the inverter has to keep up. Size inverter to peak load, not average.

A proper sizing pass yields a documentation package the customer, AHJ, and installer can all work from:

1. Load audit — 12 months of utility data with 90th-percentile daily kWh identified.
2. Backup scope — essential-loads panel list or whole-home scope defined.
3. Battery bank spec — usable kWh, nameplate kWh, chemistry, DoD, brand, model, quantity.
4. Inverter spec — continuous kW, peak kW, brand, model, closed-loop battery profile.
5. Voltage architecture — 48 V or HV, matched inverter-battery combination.
6. UL 9540 listing letter — from the manufacturer, addressed to the AHJ.
7. Site plan — cabinet locations, NFPA 855 clearance, service entrance route.
8. Installed cost estimate — hardware, labor, permit, interconnection.
9. ITC and financing math — post-incentive cost.
10. Warranty summary — years, cycles, end-of-warranty capacity, registration process.

The interactive calculator on the PES contractor portal produces items 3, 4, 5, and 8 automatically for any given input load profile — axis.pesdistribution.com/portal/battery-sizing-calculator.

Once the battery is installed and commissioned, verify the sizing was correct with a real-world test:

1. Full charge test. Charge the battery to 100 % from utility or PV. Log the total energy in — should match nameplate × charge efficiency (~98 %).
2. Full discharge to essential loads. Simulate a grid outage by opening the utility disconnect. Run essential loads on battery only. Log the total energy out and the runtime — should match usable kWh × discharge efficiency.
3. Peak load test. Turn on the largest inrush load in the essential-loads scope (AC compressor, well pump start). Verify the inverter delivers the peak surge without tripping.
4. Round-trip efficiency. Compute total energy out ÷ total energy in. Should be 90–96 % depending on the brand. Below 88 % suggests a wiring or configuration issue.
5. Comms verification. Confirm the inverter reads state of charge from the battery BMS (closed-loop) rather than estimating from terminal voltage. Should update in real time.

The contractor portal calculator at axis.pesdistribution.com/portal/battery-sizing-calculator collects:

• Daily kWh consumption (average) — with an option to upload a Green Button data file for utility-level accuracy
• Backup hours target — 8 / 24 / 72 or custom
• Chemistry preference — LFP / NMC / AGM
• Voltage architecture — 48 V / HV / auto-select
• Existing inverter (if retrofit) — dropdown of Sol-Ark, EG4, Fortress, SolarEdge, Enphase, Victron, Schneider, Outback
• Backup scope — essential-loads / whole-home / off-grid
• Climate zone — cold / moderate / hot for temperature-derate adjustments
• Existing PV kW — for self-consumption offset

The calculator returns:

• Required nameplate kWh
• 3–5 matched SKU configurations with contractor-tier pricing
• Compatibility verified against the specified inverter
• Installed cost estimate
• Post-ITC and financing options
• Line-item quote export (PDF)

Register at axis.pesdistribution.com/portal-registration to access.

For the installer field-reference use case, the fastest path from a homeowner's utility bill to a matched product is:

Note this is whole-home 24h backup sizing — if the customer only needs essential-loads backup (which is the more common scope), the required nameplate drops to roughly 30–40 % of the whole-home number, moving the recommended SKU one or two rows up the table.

DoD (depth of discharge)

The fraction of nameplate capacity you can cycle without violating warranty. 80 % for standard LFP, 90 % for BYD Premium and Enphase IQ, 50 % for AGM.

Round-trip efficiency

Energy out at AC output ÷ energy in at AC input. 90–96 % for typical LFP-plus-hybrid-inverter systems.

C-rate

Discharge rate expressed as a fraction of capacity. 0.5C means a battery discharges at half its capacity per hour (a 20 kWh battery at 0.5C discharges 10 kW for 2 hours).

Nameplate kWh

The full stored energy in the battery. Sizing target is nameplate × DoD.

Usable kWh

What you can actually cycle. Nameplate × DoD.

Closed-loop

Battery BMS communicates full state data to the inverter over a proprietary or published protocol. The inverter obeys the battery's operating envelope directly.

Grid-forming

Inverter can start loads on battery power without utility present. Required for whole-home backup.

PTO (Permission to Operate)

Utility approval to interconnect an ESS or PV system to the grid.

UL 9540

North American safety listing for stationary energy storage systems. Required by most AHJs for permitted installation.

About the author: Jennifer Kirstens leads distributor content at PES Supply. Last updated August 2026.