Calculator Methodology & Limitations
Use our tools to explore off-grid power requirements before choosing equipment. Results are planning estimates. They are not a guarantee of runtime, an electrical design, or a substitute for equipment specifications and a qualified installer.
Start with energy, then check power
Power is measured in watts (W). Energy is measured in watt-hours (Wh). One kilowatt-hour (kWh) equals 1,000 Wh. For a constant load, energy use is watts multiplied by operating hours. For battery capacity entered in amp-hours, nominal energy is battery voltage multiplied by Ah. Use the battery manufacturer’s rated energy where available.
Battery runtime
Estimated runtime (hours) = usable energy delivered to the load (Wh) ÷ average load (W).
Usable energy can be lower than nameplate capacity because of the discharge limit, battery health, an extra reserve, inverter losses, and other system losses. Do not apply the same loss twice. If usable AC energy has already been measured, avoid derating it again.
Air-conditioner calculator assumptions
Simple mode estimates running watts as cooling capacity in BTU/h divided by an assumed EER. Its EER defaults are 10 for window units, 8 for portable units, 14 for mini-splits, 9 for RV rooftop units, and 12 for central AC. These are illustrative defaults; use measured electrical consumption or the equipment data sheet in Advanced mode when available.
Simple mode assumes LiFePO4, 90% depth of discharge, 92% inverter efficiency, 8% additional system loss, and a 72°F indoor target. Battery health and the additional reserve use the values displayed in the form. The model estimates duty cycle from outdoor temperature and AC type. These duty-cycle and startup-surge assumptions have not been validated for every unit or climate.
Advanced mode uses the running watts, duty cycle, other loads, battery chemistry, efficiency, health, and reserve entered in the form. Its main runtime result uses your entered duty cycle; the temperature comparison is a separate illustrative scenario. Average load is running watts × duty cycle + other simultaneous loads. The result describes elapsed operating time at that average load, rather than compressor-on hours.
For example: 5,000 Wh × 1.00 health × 0.90 depth of discharge × 0.90 additional reserve × 0.92 inverter efficiency × 0.92 remaining after system loss = 3,427.92 Wh. At 750 W × 60% duty + 120 W other loads = 570 W average load, estimated runtime is 6.01 hours.
What runtime does not establish
A long runtime does not prove that an inverter or battery can start or run the load. Check full running demand, startup surge and its duration, battery discharge-current limits, wiring, protection, and manufacturer requirements separately. Battery-only estimates do not include ongoing solar recharge unless a tool explicitly says otherwise.
Review status and corrections
On 23 October 2026, the AC calculator’s battery field selection, BTU-to-watts conversion, unit switching, and basic invalid-input handling were checked and corrected. This does not represent an independent engineering certification or validation of every calculator on the site. Other tools and older illustrative examples may use different assumptions; compare inputs before comparing results.
If results appear inconsistent, record the page URL, mode, units, input values, and displayed result so the calculation can be reproduced.