How the Calculator Estimates Energy, Array, Battery, and Budget

1. Purpose & Planning Boundaries

OffGridSolarCalc documents a set of calculations intended to help readers explore off-grid energy needs and compare planning assumptions. The examples and outputs are not certified designs, and this page does not claim independent technical validation. Electrical-system design requires product-specific data, site conditions, and local code review.

2. Daily Energy Load Calculation ($E_{daily}$)

Daily energy is calculated from the selected appliance wattages, entered hours per day, and quantities. If no appliances are selected, the calculator uses entered daily kWh; if that is also empty, it estimates 12 kWh per 1,000 sq ft of home size. The resulting estimate has a 1 kWh/day minimum.

• Appliance mode: The appliance list uses the entered average hours directly; it does not apply an additional duty-cycle factor.
• Manual mode: A positive daily kWh value is used as entered. The home-size fallback is 12 kWh/day per 1,000 sq ft, subject to the 1 kWh/day minimum.
• Inverter standby: No inverter idle load is added. The calculator does not infer inverter continuous or surge ratings from daily energy.

3. Solar Resource & Peak Sun Hours (PSH)

A solar panel produces rated wattage only under Standard Test Conditions (STC: $1,000 \text{ W/m}^2$ irradiance, $25^\circ\text{C}$ cell temperature, AM 1.5 spectrum). Because solar intensity changes throughout the day, solar engineers model generation in Peak Sun Hours (PSH):

Array sizing uses the selected location's stored avgSolarHours value. In year-round mode, the calculator multiplies that value by 0.72; in 3-season mode, it uses the full value. The same resulting daily sun-hours estimate is used throughout the year, so this is not a monthly or worst-month model and does not guarantee winter performance. A separate local JSON file contains monthly irradiance and annual-output fields for display features; its source provenance is not recorded in this project, and it is not the input to the array-sizing formula.

4. Solar Array Estimate & Module Area

The calculator estimates a panel count using a 25% sizing factor and an assumed 400 W module. It does not model separate temperature, wiring, shading, inverter, snow, or MPPT losses; the 1.25 multiplier is not a measured system-efficiency factor.

The annual production comparison annualizes that same daily estimate as $E_{annual,model}=365 \times Panel\ count \times 0.4 \times PSH_{used}$. It is a simplified comparison, not a monthly energy balance, outage model, or reliability prediction.

Estimated Module Surface Area

The 18%, 20%, or 22% selector changes only this estimated module surface area. It does not change panel count, array wattage, budget rates, or modeled energy production. The estimate excludes gaps, access paths, roof setbacks, tilt, and usable-roof constraints; it is not a roof layout.

5. Battery Bank Capacity & Chemistry

The calculator's current baseline battery estimate uses the following relationship. It reports nominal energy before inverter, charge/discharge, temperature, aging, or product-specific corrections:

• Days_Autonomy: User-selected reserve days without solar generation; the appropriate reserve depends on loads, weather, backup sources, and risk tolerance.
• Depth of Discharge (DoD): Values such as 0.90 for some LiFePO4 products or 0.50 for some lead-acid products are examples, not universal limits. Use the exact battery model's manual, warranty, and cycle-life data.
• Rounding: The displayed nominal capacity is rounded up to the next 0.1 kWh. The deployed calculator does not apply inverter or battery efficiency factors; this is not a final component size.

6. Inverter Sizing (Continuous & Surge)

Inverter sizing must satisfy two distinct electrical criteria: continuous wattage and momentary motor starting surge.

Continuous Output Rating

Compare the expected simultaneous running load with the inverter's continuous rating, environmental derating, and manufacturer instructions. Any required margin depends on the applicable rules and system design; this page does not select an inverter.

Peak Motor Surge Capacity

Starting demand depends on the connected equipment. Compare manufacturer starting-current or locked-rotor data, power factor, and startup duration with the inverter's surge magnitude and duration ratings, while accounting for simultaneous loads and battery/DC support. A general multiplier is not enough to confirm compatibility.

7. Charge Controller Sizing (MPPT)

Maximum Power Point Tracking (MPPT) controllers step high solar array DC voltage down to the exact absorption or float voltage required by the battery bank. Controllers are sized by continuous output current and maximum open-circuit voltage:

Output Current Sizing

The displayed current is a rough estimate: array nameplate watts divided by a heuristic system-voltage value, multiplied by 1.25, then rounded up to the next 10 A. The voltage heuristic is 12 V below 1.8 kWh/day, 24 V from 1.8 through 4.5 kWh/day, and 48 V above 4.5 kWh/day. These daily-energy thresholds do not establish a compatible battery voltage or controller. Final selection must use actual charging voltage, corrected array voltage/current, equipment ratings, manufacturer instructions, and applicable requirements.

Maximum Array Voltage Ceiling ($V_{oc\_cold}$)

Use the exact module's Voc temperature coefficient and the site's design minimum cell temperature to correct string voltage. Compare the result with the controller's absolute maximum input rating and all manufacturer limits before selecting a string; this page does not calculate a safe array configuration.

8. System Voltage Selection (12V / 24V / 48V)

For a fixed power, the ideal current calculation is $I = P \div V$. The table below is arithmetic only; the calculator's voltage output is a daily-energy heuristic, not a voltage selection. These values do not determine conductor size, allowable current, or a safe installation.

9. Cable Sizing & Allowable Voltage Drop

This equation estimates voltage drop for a stated conductor and load; it does not select a conductor or establish a safe installation. Voltage-drop targets depend on the system and applicable requirements:

This simplified two-conductor DC estimate uses one-way route length and assumes equal outgoing and return conductors. Use resistance for the actual conductor material and operating temperature. Final conductor selection must also check ampacity after installation, ambient-temperature, and grouping corrections; insulation and terminal ratings; routing; manufacturer limits; and local rules. This site does not provide cable, fuse, or grounding designs.

10. Illustrative Worked Example

The following example shows how selected inputs flow through the equations on this page. It is illustrative and is not an independently checked test case or a recommendation for a particular installation:

Step 1: Total Daily Energy Demand

$E_{daily} = 3.86 \text{ kWh/day}$. No inverter standby consumption is added.

Step 2: Solar Array Size

Each assumed 400 W module produces $0.4 \times 3.6 = 1.44 \text{ kWh/day}$ in this simplified model. The panel count is $\lceil 1.25 \times 3.86 \div 1.44 \rceil = 4$, for a 1.6 kW nameplate array. The modeled annual production is $4 \times 1.44 \times 365 = 2,102.4 \text{ kWh/year}$; this annual comparison is not a daily or seasonal reliability result.

Step 3: Estimated Module Surface Area

$4 \times [400 \text{ W} \div (1,000 \text{ W/m}^2 \times 0.20)] \times 10.7639 = 86.1 \text{ sq ft}$ of estimated module surface area. This excludes layout gaps, roof setbacks, and access space.

Step 4: Battery Storage Capacity

$C_{baseline} = (3.86 \text{ kWh/day} \times 2 \text{ days}) \div 0.90 = 8.58 \text{ kWh}$, rounded up to $8.6 \text{ kWh}$. This reproduces the calculator's baseline arithmetic only; it excludes conversion losses and all product- and installation-specific corrections. It is not a battery-bank recommendation.

Step 5: Indicative Controller-Current Output

The calculator's rough voltage heuristic assigns 24 V at 3.86 kWh/day. Its current estimate is $(4 \times 400 \text{ W} \div 24 \text{ V}) \times 1.25 = 83.3 \text{ A}$, rounded up to the next 10 A increment (90 A). This does not check module Voc, cold correction, controller input limits, charging voltage, or manufacturer requirements and is not a controller selection.

Illustrative Budget and Projection Assumptions

The budget uses fixed model rates, not local quotes: array $0.50/W; battery $300/kWh for the LiFePO4 option or $160/kWh for lead-acid; inverter allowance $200/kW; miscellaneous allowance 10% of the hardware subtotal; and, in professional-install mode, labor allowance 25% of that same subtotal (zero labor in DIY mode). For this example's 1.6 kW array and 8.6 kWh LiFePO4 result, the model subtotal is $3,700 and the professional-mode total is $4,995. These figures are not product pricing or an installation quote.

Illustrative first-year cost recovery is the lesser of modeled annual production and entered annual use, multiplied by the selected location record's reference electricity tariff. The tariff is a bundled estimate, not a live utility rate or a value entered from your bill, and the calculator does not offer a custom tariff input. Simple payback divides the modeled system total by that first-year amount. The 25-year projection applies 0.5% annual production decline and 3% annual tariff escalation. It excludes maintenance, battery replacement, financing, incentives, backup generation, and changes in usage or tariffs. These assumptions can materially change the result.

11. Known Limitations & Unsupported Setups

These outputs are simplified estimates. They do not replace equipment specifications, a site survey, or electrical design:

• Solar and loads: The array estimate uses one stored daily sun-hours value and annualizes it. It does not calculate month-by-month energy balance, weather variability, shading, orientation, tilt, or site-specific losses.
• Electrical design: The tool does not select an inverter, verify controller compatibility, design strings, conductors, overcurrent protection, or grounding, or check equipment-specific voltage/current limits.
• Economics: Budget rates and projection factors are fixed illustrative assumptions. The output is not a quote, guarantee of savings, tax determination, or forecast of actual payback.
• Battery performance: The capacity estimate does not model temperature, aging, charge/discharge efficiency, power limits, or a specific product's BMS and warranty constraints.

12. Documentation Revision

Notice a formula discrepancy or have a calculation suggestion? View our Technical Sources or submit an errata report via our Corrections & Feedback Page.