Solar Battery Starter Sizing Calculator

Purpose and model boundary

Screens annual-energy, backup-with-recharge, or combined solar-plus-storage requirements against eight editable packages and exposes energy flow, power gates, first-year value, and discounted payback.

Do not treat this result as an electrical design, code-compliance determination, permit approval, utility commitment, equipment approval, safety certification, cost quote, or endorsement by a cited source. Service data, panel ratings, conductor properties, equipment loads, route lengths, manufacturer instructions, utility requirements, locally adopted codes, and approval thresholds remain the user's responsibility.

Inputs and units

• design route: a listed route or text value.
• selection objective: a listed route or text value.
• average daily load kwh: entered in kWh/day.
• annual offset percent: entered in fraction of design annual load.
• battery mediated fraction: entered in fraction.
• peak sun hours: entered in h/day.
• pv system loss percent: entered in fraction.
• critical daily load kwh: entered in kWh/day.
• backup autonomy days: entered in days.
• solar recharge days: entered in days.
• usable depth of discharge: entered in fraction.
• temperature factor: entered in fraction.
• end of life retention: entered in fraction.
• discharge efficiency: entered in fraction.
• round trip efficiency: entered in fraction.
• critical continuous kw: entered in kW.
• critical surge kw: entered in kW.
• load growth percent: entered in fraction.
• direct self consumption percent: entered in fraction of production.
• battery cycles per year: entered in cycles/year.
• retail rate per kwh: entered in user currency/kWh.
• export rate per kwh: entered in user currency/kWh.
• incentive amount: entered in user currency.
• discount rate: entered in fraction/year.
• analysis years: entered in years.

The package grid contains 8 fixed data rows plus one header row with these columns: Package label, Include, PV kW (kWdc), Battery kWh (kWh nominal), Inverter continuous kW (kW), Battery continuous kW (kW), Battery surge kW (kW), Installed cost (user currency), Annual O&M (user currency/year). Numeric columns retain the units shown here; text, route, and include columns are intentionally unitless.

Governing relationships

Annual design load is:

E_load = 365 E_day (1 + g)

For an annual-offset route, delivered offset and the PV production target are:

E_offset = p_offset E_load

E_PV,target = E_offset[(1-f_b)+f_b/eta_rt]

E_load, E_offset, and E_PV,target are in kWh/year. E_day is average daily load (kWh/day), g is the one-time load-growth fraction (dimensionless), p_offset is the target fraction, f_b is the battery-mediated fraction, and eta_rt is round-trip efficiency. Required offset PV is:

P_PV,offset = E_PV,target / [365 H_sun (1-l_sys)]

P_PV,offset is in kWdc, H_sun is peak-sun-hours per day, and l_sys is system-loss fraction.

Backup AC energy and nominal battery capacity are:

E_backup = E_critical d_auto

B_nom = E_backup/(DoD f_T f_EOL eta_dis)

E_backup and B_nom are in kWh. DoD, f_T, f_EOL, and eta_dis are usable-depth, temperature, end-of-life-retention, and discharge-efficiency fractions. Recharge PV is the backup energy, including the active round-trip loss, divided by recharge days and daily loss-adjusted solar yield. The route uses the offset requirement, backup/recharge requirement, or their maximum.

For package j, first-year production is:

E_prod,j = P_PV,j 365 H_sun(1-l_sys)

E_prod,j is in kWh/year. Direct use, battery input, battery-delivered energy, onsite use, and export are limited in sequence by production, annual load, battery capacity times annual cycles, and eta_rt. Net cost is installed cost minus incentive; first-year benefit values onsite energy at the retail rate and export at the export rate, less annual O&M. Discounted value is the level annual net benefit discounted over the entered whole-year horizon minus net cost. Only packages clearing every active energy, storage, continuous-power, and surge-power gate are ranked.

Calculation sequence

1. Validate every active selector, scalar bound, type, and every fixed-grid entry.
2. Apply the selected route and ignore values from inactive fields.
3. Normalize unit-dependent inputs into the workbook's calculation basis.
4. Evaluate every included row or candidate and aggregate the active schedule.
5. Calculate primary and supporting outputs, populate the authored chart series, and apply derived numeric checks.
6. Return the workbook status with the computed results; invalid input is never converted into a plausible engineering answer.

Outputs and interpretation

• required pv kw (kWdc): PV requirement from annual offset, backup recharge, or the larger of both.
• required battery kwh (kWh nominal): Critical AC energy grossed up by usable, temperature, retention, and discharge-efficiency factors.
• selected package: Passing package selected by the active objective.
• selected pv kw (kWdc): PV capacity in the selected package.
• selected battery kwh (kWh nominal): Nominal storage capacity in the selected package.
• year1 production kwh (kWh/year): Selected PV capacity times peak sun hours, days, and one minus system loss.
• year1 onsite use kwh (kWh/year): Direct self-consumption plus battery-delivered energy, capped at design annual load.
• net installed cost (user currency): Selected installed cost less entered upfront incentive, floored at zero.
• year1 net benefit (user currency/year): Onsite and export value less annual O&M.
• discounted value (user currency): Net installed cost plus the entered-horizon present value of a level year-one net benefit.

The chart is an explanatory view of the workbook's named chart ranges. It does not add a separate calculation or replace the numeric outputs.

Validation and status logic

The workbook evaluates Model_Status in the order shown. The first matching condition wins.

Assumptions and limitations

• All shipped default values are synthetic illustrative demonstrations; replace solar resources, package ratings, costs, tariffs, and incentives with project evidence.

• Peak sun hours and system loss are user-entered project assumptions; no location, weather file, or production guarantee is embedded.

• Round-trip efficiency is applied once to battery-mediated annual energy and solar recharge; discharge efficiency is separately used only to convert critical AC energy to nominal stored capacity.

• Annual energy flow uses a disclosed direct-self-consumption fraction and a battery throughput cap based on nominal capacity times equivalent full cycles; backup-only usable, temperature, and retention factors are not applied a second time to annual throughput.

• Discounted value treats the first-year net benefit as level for the entered analysis period; tax, replacement, degradation, escalation, financing, and residual value are outside this starter screen.

• This is a preliminary feasibility model, not a site survey, tariff analysis, production guarantee, interconnection study, permit package, structural review, electrical design, fire-code review, or emergency-power plan.

• Synthetic package sizes and costs are editable illustrations; use current vendor data, incentives, tariffs, shade and weather analysis, and utility export rules.

• Storage cannot make an unsafe or unsupported load suitable for backup; qualified professionals must verify transfer equipment, grounding, isolation, fault current, surge behavior, ventilation, clearances, and listed equipment compatibility.

• Do not purchase, install, energize, or modify solar or storage equipment from this workbook alone. Obtain required utility, permitting, inspection, and fire-safety approvals.

Restrictions and non-computing states

Each numeric control offers a working range chosen for usable entry, which is not always the limit this calculator enforces; a value outside that limit is rejected with a message rather than quietly accepted. The calculator additionally enforces allowed choices, active-field types, complete fixed-grid rows, and conditional or cross-field rules before computing.

Unsupported route values, incomplete rows, out-of-domain inputs, relational failures, nonfinite derived arithmetic, and workbook error tokens are non-computing states. Hidden inputs remain in the values sent to the calculator but must not affect results while those fields are inactive.

Errors and warnings

A rejected entry means the submitted state failed the published input rules. Workbook NOT VALID identifies an invalid active domain or unsupported derived arithmetic. Workbook CHECK retains a finite result while flagging a planning condition that needs review. OK means only that the delivered workbook logic closed within the entered assumptions; it is not code compliance, professional approval, or a safety guarantee.

This version is checked against 188 saved test cases, covering both calculated results and entries that should be refused, run against this exact workbook.

• NREL PVWatts Version 5 Manual - Primary public source for peak-sun-hours-style DC production and aggregate system-loss concepts. Official public U.S. national-laboratory publication; simplified energy arithmetic is independently implemented and no dataset, table, or prose is copied or reproduced.
• NREL Residential Battery Storage Cost and Performance - Primary public source for storage power, energy, efficiency, depth-of-discharge, and usable-capacity concepts. Official public U.S. national-laboratory publication; general engineering concepts are independently implemented and no product curve, dataset, table, or prose is copied or reproduced.
• NREL Behind-the-Meter Storage Assessment - Primary public source for battery throughput, round-trip efficiency, and residential use-case context. Official public U.S. national-laboratory publication; general formulas are independently implemented and no dataset, table, or prose is copied or reproduced.
• U.S. Department of Energy solar savings guidance - Official consumer guidance for site-specific cost, utility-rate, incentive, and payback considerations. Official public U.S. government guidance; financial arithmetic is independently implemented and no tariff, incentive table, or prose is copied or reproduced.
• U.S. Department of Energy solar and storage basics - Official source for solar-plus-storage energy-shifting and outage-resilience context. Official public U.S. government guidance; concepts are independently summarized and no figure, table, or prose is copied or reproduced.
• U.S. Department of Energy rooftop solar permitting and inspection - Official source for permitting, inspection, and local-authority review cautions. Official public U.S. government guidance; only general safety facts are used and no figure, table, or prose is copied or reproduced.
• UL residential energy storage safety testing - Official standards-organization source for listed-system, fire, electrical, and installation safety context. Official public standard-setter safety source; only general safety facts are used and no standard text, table, or figure is copied or reproduced.

Licensing conclusion: No licensed, proprietary, manufacturer, utility, product-price, code-table, forecast, or external dataset is embedded. The calculator independently implements general engineering arithmetic from cited public and official sources; all defaults, labels, conductor/load rows, equipment assumptions, diagrams, and test values are synthetic illustrations.