How to Use a PC Set Calculator for Off-Grid Battery Sizing

When sizing an off-grid solar or backup power system, guessing your battery capacity leads to either stranded power (dead banks by 8 PM) or wasted capital (oversized lithium gathering dust). The foundational tool to prevent this is the PC set calculator—a mathematical model that determines your exact daily Power Consumption (PC) Set, adjusted for inverter losses and appliance duty cycles. Before you buy a single battery or solar panel, you must calculate this value.

This guide breaks down the core PC set formula, walks through two real-world worked problems with strict unit tracking, and provides a hard decision tree to select your exact battery hardware.

The Core PC Set Formula and Symbol Definitions

The PC set represents the total daily Watt-hours (Wh) your battery bank must supply to the AC/DC loads, factoring in the energy lost as heat inside the inverter. Here is the master equation:

PC_set = [ Σ (P_i × t_i × DF_i) ] / η_inv

Symbol	Definition	Standard Unit	Typical Real-World Value
PCset	Power Consumption Set (Total daily energy required from the battery)	Watt-hours (Wh)	1,500 - 15,000 Wh
Pi	Nominal continuous power draw of appliance i	Watts (W)	10W (LED) to 1500W (Microwave)
ti	Total daily runtime of appliance i	Hours (h)	0.25h to 24h
DFi	Duty Factor (compressor cycling, thermostat limits, or intermittent use)	Decimal (0.0 to 1.0)	1.0 (continuous) or 0.35 (fridge compressor)
ηinv	Inverter conversion efficiency (DC to AC)	Decimal (0.0 to 1.0)	0.88 (low-end HF) to 0.95 (premium LF)
Σ	Summation across all n appliances in the load profile	N/A	N/A

Rearranged Forms for System Sizing

While the primary use of a pc set calculator is finding the total Wh, you will frequently need to isolate other variables when working backward from a fixed battery budget or hardware constraint.

• Solving for Maximum Continuous Load (P_max):
  Pmax = (PCset × ηinv - Σother) / (t × DF)
  Use when: You have a fixed battery bank and need to know the maximum wattage space heater or microwave you can run without draining the bank prematurely.
• Solving for Maximum Runtime (t_max):
  tmax = (PCset × ηinv - Σother) / (P × DF)
  Use when: Sizing a backup UPS for a specific router or CPAP machine to guarantee 8 hours of runtime during a blackout.
• Solving for Minimum Required Inverter Efficiency (η_min):
  ηmin = Σ (P × t × DF) / PCset
  Use when: Your thermal limits or battery capacity are maxed out, and you need to calculate how efficient your inverter must be to prevent a deficit.

Assumptions, Unit Traps, and Realistic Magnitudes

Before plugging numbers into your pc set calculator, you must understand the boundaries of the formula.

When this formula applies: This model is strictly for off-grid or hybrid systems where the battery is the primary energy source for the loads (AC-coupled or DC-coupled). It assumes a steady-state daily profile and does not account for multi-day autonomy (cloudy days). To add autonomy, multiply your final PC_set by your desired days of backup.

Realistic Answer Magnitudes: A tiny off-grid telecom cabin or camper van typically yields a PC_set between 1,000 and 2,500 Wh. A standard full-time residential homestead (including well pumps, refrigeration, and cooking) usually lands between 8,000 and 18,000 Wh. If your calculator outputs 65,000 Wh, you have likely forgotten to apply the Duty Factor to your HVAC or refrigeration loads.

Worked Problem 1: The Remote Telecom Cabin (12V System)

Scenario: You are powering a remote monitoring station. The loads are four 10W LED security lights, one 60W ruggedized laptop, and a 45W Starlink Roam dish. You are using a budget high-frequency inverter with 88% efficiency (η_inv = 0.88).

Step 1: Calculate raw load energy (Numerator)

• LED Lights: 4 units × 10W × 5h × 1.0 (DF) = 200 Wh
• Laptop: 1 unit × 60W × 8h × 0.6 (DF - charging cycles) = 288 Wh
• Starlink: 1 unit × 45W × 24h × 0.8 (DF - snow melt/standby cycles) = 864 Wh
• Summation (Σ): 200 + 288 + 864 = 1,352 Wh

Step 2: Apply Inverter Efficiency

• PC_set = 1,352 Wh / 0.88
• PC_set = 1,536.36 Wh

Result: Your battery must deliver 1,536 Wh per day. Assuming a 12V system and an 80% Depth of Discharge (DOD) for LiFePO4, you need a battery rated for at least (1536 / 12) / 0.80 = 160 Ah.

Worked Problem 2: The Full-Time Off-Grid Homestead (48V System)

Scenario: A primary residence running a full-size fridge, a microwave, a shallow well pump, and networking gear. We are using a premium low-frequency inverter (e.g., Victron MultiPlus) with 93% efficiency (η_inv = 0.93). For detailed inverter efficiency curves and loss calculations, refer to the Victron Energy Wiring Unlimited guide.

Step 1: Calculate raw load energy (Numerator)

• Full-Size Fridge: 120W × 24h × 0.35 (DF) = 1,008 Wh
• Microwave: 1500W × 0.5h × 1.0 (DF) = 750 Wh
• Well Pump (1/2 HP): 800W × 1.5h × 0.4 (DF - pressure tank cycling) = 480 Wh
• Router & Network Switch: 40W × 24h × 1.0 (DF) = 960 Wh
• Miscellaneous (Lighting/Phones): 300W × 6h × 0.8 (DF) = 1,440 Wh
• Summation (Σ): 1008 + 750 + 480 + 960 + 1440 = 4,638 Wh

Step 2: Apply Inverter Efficiency

• PC_set = 4,638 Wh / 0.93
• PC_set = 4,987.1 Wh

Result: The homestead requires roughly 5,000 Wh from the battery bank daily. For comprehensive load profiling methodologies, the AltE Store off-grid sizing primer corroborates this approach for residential DC-coupled systems.

Decision Tree: Sizing Your Battery Bank from the PC Set

Once your pc set calculator spits out the final PC_set value, use this decision matrix to select your system voltage and battery capacity. This assumes Lithium Iron Phosphate (LiFePO4) chemistry with an 80% usable Depth of Discharge (DOD).

Calculated PCset Range	Target System Voltage	Minimum Required Ah (at 80% DOD)	Hardware Configuration
< 2,000 Wh	12V	208 Ah	1x 12V 200Ah LiFePO4 Drop-in
2,000 - 4,500 Wh	24V	234 Ah	2x 12V 200Ah in Series (or 1x 24V 200Ah)
4,500 - 10,000 Wh	48V	260 Ah	2x 48V 100Ah Server Rack in Parallel
> 10,000 Wh	48V	520 Ah+	4x 48V 100Ah Server Rack in Parallel

Final Component Pick Based on PC Set Results

If your pc set calculator yields a value in the most common off-grid residential band (4,500 to 10,000 Wh, as seen in Worked Problem 2), do not waste time building custom 12V parallel banks or dealing with massive 2/0 AWG cable runs. The math dictates a 48V architecture.