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Sizing a Residential Battery System: A Worked Example

Sizing a Residential Battery System: A Worked Example

Welcome

Hello, and welcome. This is Super Structures General Contractors — a national general contractor headquartered in Powhatan, Virginia — here to help you and your clients build something that lasts. We're glad you're with us, and we look forward to connecting with you.

Sizing is where storage design stops being theory. Too small and the customer is in the dark at 3 a.m.; too big and they paid for capacity they'll never use. This lesson walks through a real-world method, step by step, with the math shown.

Educational content only — not electrical design, fire-protection engineering, or hands-on installation training, and not a substitute for a licensed electrician or a NABCEP-certified professional. Follow the NEC and fire and building codes as adopted by your Authority Having Jurisdiction (AHJ), the equipment manufacturer's installation instructions, and your utility's interconnection rules. Code editions change and jurisdictions adopt different editions — always confirm the one your AHJ enforces.

Step 1: Pick the goal

Sizing starts with the use case from the first lesson. Backup sizing asks, "How many hours of which loads?" Self-consumption or TOU sizing asks, "How much evening energy does the home use, and how much midday solar is left over?" Many homes need both; we'll size for backup and check the daily cycling against it.

Step 2: List the critical loads — power and energy

Build a load table. For each load, note running watts, starting (surge) watts for motors, and estimated hours of use per day. Use nameplate data, measured data, or — better — a monitoring device's history. Example critical loads for one home:

Daily critical energy ≈ 11.5 kWh. These figures are illustrative — always measure or use real data for a design.

Step 3: Check the power (kW) requirement

Add the running loads that could run at the same time: 150 + 500 + 200 + 100 + 1,000 + 300 = about 2.25 kW continuous. Now the surge: the worst case is the well pump starting while the other loads run — roughly 2.25 kW minus the pump's 1 kW running plus its ~4 kW start = about 5.25 kW momentarily. The inverter/battery must handle about 2.25 kW continuous and about 5.25 kW of surge for the few seconds a motor starts. A system rated around 5 kW continuous with a higher peak rating covers this with margin. A soft starter on the well pump can reduce the surge if the margin is thin.

Step 4: Size the energy (kWh) for the outage duration

The customer wants one full day of critical loads without relying on solar (a cloudy winter storm). Start with 11.5 kWh, then adjust:

So the target is roughly 14–15 kWh of usable capacity. If a single battery offers 13.5 kWh usable, the designer can either accept slightly shorter runtime, trim loads (skip the outlets), or add a second unit. With solar recharging on a sunny day, the same system can run for many days — but design for the bad day the customer cares about.

Step 5: Check daily cycling and solar

If the customer also wants TOU savings, compare the evening peak-window usage to the battery's usable capacity minus the backup reserve. Then confirm the PV array produces enough surplus to recharge: a 7 kW array might produce very roughly 20–35 kWh on a good day depending on location and season, but far less in winter. A battery the array can never fill is wasted capacity.

Going Deeper (Intermediate)

Seasons change everything. Winter brings heating loads and short days; summer brings air conditioning. Size for the season that matters most to the customer's outage risk. Also confirm voltage: 240 V loads like well pumps require a system that can deliver split-phase 240 V in backup mode — not all configurations do.

Then fit the design to code limits: the IRC caps individual unit and aggregate energy in different locations (covered in the next lesson), which can affect whether two or three batteries can go where the customer wants them.

Advanced / Pro-Level

Pros size from interval data instead of guesses, run an hour-by-hour simulation of battery state of charge through a representative outage and a representative TOU day, and account for temperature derating of capacity in cold locations. For homes with large motor loads, they review locked-rotor amps on the nameplate, consider soft starters, and confirm the inverter's surge duration, not just its peak kW. They document the assumptions in the proposal so the customer knows exactly what the system was sized to do.

Practice Challenge

A customer's critical loads total 8 kWh/day with a 3 kW running peak and a 6 kW surge from a well pump. They want two days of backup without sun. Roughly how much usable storage and what power rating? (Answer: 8 × 2 = 16 kWh; ÷ 0.9 for losses ≈ 17.8 kWh; plus about 15% margin ≈ 20.5 kWh usable. Power: about 3 kW continuous with at least 6 kW of surge for motor starting — or add a soft starter to the pump.)

In Practice

Give every customer a one-page summary: which circuits are backed up, the estimated hours of runtime without sun, and what happens if they run the dryer or the central air during an outage. That page prevents most callbacks.

Common Mistakes to Avoid

Takeaway: Size storage in two passes — power (continuous and surge kW) and energy (usable kWh for the outage you're designing for, plus losses and margin) — and write down the assumptions.

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