Reference

Methodology & sources

Every constant this calculator uses, where it comes from, and the formulas applied at each stage. Nothing here is proprietary — it is the standard engineering you can verify yourself.

Stage 1 — Daily energy

For each device, daily energy is watts × hours × quantity. AC and DC loads are summed separately because AC loads pass through the inverter and pay an efficiency penalty; DC loads drawn straight from the bank do not. See the run-time guides for typical wattages of common appliances.

Stage 2 — Battery bank

Effective daily energy = (AC energy ÷ inverter efficiency) + DC energy. Multiplied by days of autonomy gives the usable energy the bank must store. Nominal capacity is then the usable figure divided by the chemistry's safe depth of discharge, its round-trip efficiency, and a temperature derating factor:

nominal Wh = usable Wh ÷ usableDoD ÷ roundTrip ÷ tempDerate
bank Ah   = nominal Wh ÷ system voltage

Chemistry constants

AC loads additionally pay an inverter efficiency of 92% (pure-sine typical); DC loads do not. Lithium is held to 80% usable as a conservative longevity-protecting default even though many packs allow more. Temperature sensitivity reflects how sharply usable capacity changes with ambient temperature — high for flooded lead-acid, low for lithium.

Sources

1. [1] LiFePO4 manufacturer datasheets (e.g. Battle Born, Victron) — 80–100% usable depth of discharge, 95–98% round-trip efficiency, 3,000–6,000 cycles.
2. [2] Trojan Battery flooded lead-acid datasheets and user guide — 50% recommended depth of discharge, ~80% round-trip, ~1,200 cycles, high temperature sensitivity.
3. [3] Victron / Renogy AGM and gel datasheets — 50% recommended depth of discharge, ~85% round-trip efficiency.
4. [4] Lithium NMC cell datasheets — ~85% usable, ~95% round-trip, 1,000–2,000 cycles.
5. [5] NREL PVWatts documentation — default overall system derate (~0.75) covering wiring, soiling, temperature and controller losses.
6. [6] Peak sun hours by city: solar irradiation from PVGIS (EU Joint Research Centre), horizontal-plane long-term annual average; city names and coordinates from GeoNames (CC-BY 4.0). Values are annual averages for planning; verify against a local solar resource map.
7. [7] Wiring and interconnection practice follows NFPA 70 (NEC) Articles 690, 705 & 706 — PV source circuits, interconnected power sources and energy storage systems.
8. [8] Days-of-autonomy and depth-of-discharge sizing follows the general practice in IEEE 1013, Recommended Practice for Sizing Lead-Acid Batteries for Stand-Alone PV Systems.
9. [9] Battery safety certification: UL 1973 (stationary/motive batteries) and UL 9540 (energy storage systems & equipment).
10. [10] Lithium cell safety and transport testing: IEC 62619 and UN 38.3 (UN Manual of Tests and Criteria, Section 38.3).
11. [11] Stationary lead-acid battery testing: IEC 60896 series (VRLA and flooded).
12. [12] Inverter safety and grid interconnection: UL 1741 and IEEE 1547-2018.
13. [13] Typical appliance energy use referenced in the run-time guides: ENERGY STAR Product Finder and U.S. EIA — Electricity Use in Homes; always check the nameplate rating of your own device.
14. [14] Installation standards for specific use cases: NFPA 855 (stationary energy storage systems) and NFPA 1192 (recreational vehicles).

Stage 3 — Series / parallel

Series count = system voltage ÷ battery voltage (rounded up). Parallel strings = required bank Ah ÷ single-battery Ah (rounded up). Run your own bank through the series/parallel calculator — above four parallel strings it suggests a higher system voltage or higher-Ah batteries, because many parallel strings complicate balancing and current sharing.

Stage 4 — Solar array

array W = daily Wh ÷ (peak sun hours × system loss factor)

The system loss factor defaults to 0.75, consistent with NREL PVWatts guidance [5]. It is the product of several real-world losses: wiring (~0.98), soiling (~0.95), temperature (~0.90) and charge-controller (~0.895), which multiply to about 0.75. Peak sun hours is location-dependent; enter the value for your site.

The days-to-recharge figure estimates how many full-sun days of surplus energy (array output minus that day's load) it would take to refill the bank's usable capacity after a deep drawdown. If the array is only sized to keep up with the daily load, there is no surplus to recharge a depleted bank faster, and you should size the array up if quick recovery matters.

Stage 5 — Charge controller & inverter

Controller current = array watts ÷ system voltage, with 25% headroom (consistent with NEC-style continuous-load practice). MPPT is recommended for larger arrays or 24V/48V systems; PWM is acceptable only for small, voltage-matched arrays. Inverter continuous rating is sized to 120% of simultaneous AC load; surge rating to the largest motor starting surge or twice continuous, whichever is greater.

Inverters also draw a small idle (no-load) current whenever they are powered on — roughly 1% of their rated continuous power. On a small daily load this idle draw can be a meaningful share of total consumption (left on 24 hours, a 300W inverter can use ~72 Wh/day). The calculator flags this when idle draw reaches about 10% of your daily energy, so you can switch the inverter off when idle or add the figure to your load.

Limitations

This is a planning tool, not an electrical design. Real installations must account for wire gauge and fusing, code compliance, panel orientation and shading, battery aging, and local conditions. Always verify with measurement and have a licensed installer confirm a system before purchase or wiring. See the common sizing mistakes guide and the FAQ for the questions this comes up in most.