how much runtime does a portable solar power supply provide? | Insights by ShanPu

A common buyer question is "how much runtime does a portable solar power supply provide?" Use this engineering formula to answer it precisely: Runtime (hours) = (Battery capacity in Wh × Depth of Discharge (DoD) × Inverter efficiency) ÷ Load (W). Concrete breakout: battery Wh = nominal voltage × ampere‑hours (Wh = V × Ah). DoD is the fraction of that capacity the manufacturer or your operating policy allows (for Li‑ion LFP you can typically use 80–90% of rated Wh; for lead‑acid, 40–50% is prudent). Inverter efficiency converts DC battery energy to usable AC energy; typical modern portable inverters run 88–95% efficient under rated loads. Example: a 1,000 Wh battery with 90% DoD gives 900 Wh usable; with a 92% inverter efficiency the available AC energy ≈ 828 Wh, so a constant 60 W load will run ≈ 13.8 hours (828 ÷ 60). Always subtract inverter idle/standby draw and account for power factor on reactive loads.

Inverter efficiency directly reduces usable energy delivered to AC loads. Efficiency varies with topology and load percentage: many units are most efficient near 50–75% of rated power and fall off at light loads. Portable inverter efficiencies commonly range 88–95% at typical loads; modified sine designs are often at the lower end. Also include no‑load (idle) consumption—small portable inverters can draw 1–7 W when unloaded, which matters for long runtimes on small daily loads. For runtime forecasting, include both the percent efficiency and a fixed idle power term: Available AC Wh = (Usable battery Wh × inverter efficiency) − (idle W × hours). That converts to more accurate hour estimates, especially for low continuous loads like LED lighting or telecom standby.

Startup surges (motor compressors, pumps, some appliances) commonly draw 2–8× the steady running current for a fraction of a second to a few seconds. These surges are less about cumulative Wh consumption and more about whether the inverter can supply transient power without tripping or derating. The energy cost of a short surge (e.g., 600 W extra for 2 s) is small (~0.33 Wh), but the inverter must have the peak rating and thermal headroom to sustain it. If the inverter repeatedly operates near its surge limits, internal protection or thermal derating can cut continuous runtime. For procurement, specify both continuous and surge ratings (and the surge duration), and size the inverter so expected appliance inrush currents are within the device’s surge envelope to avoid premature shutdowns that disrupt runtime.

Recharge time depends on panel wattage, effective peak sun hours (PSH) at site, and end‑to‑end system efficiency (MPPT charge controller, wiring, battery charge acceptance). Use this formula: Recharge days or hours = Battery Wh to restore ÷ (Panel wattage × PSH × system efficiency). Typical PSH ranges widely by geography and season; a conservative mid‑latitude design number is 3–5 PSH/day. Account for energy conversion losses—charging, MPPT efficiency, and battery charge acceptance—using a system efficiency factor of ~0.75–0.9. Example: restoring 1,000 Wh with a 200 W PV array and 4 PSH at 85% system efficiency yields daily energy ≈ 200 × 4 × 0.85 = 680 Wh → ~1.5 days to fully restore. For mission‑critical designs, size PV and battery to guarantee multi‑day autonomy or include generator backup.

Battery capacity and available runtime are both temperature‑ and age‑dependent. Cold temperatures reduce instantaneous usable capacity and charge acceptance; for lithium chemistries this often shows as a measurable derating below 0°C and slower charge acceptance, while deep discharge at elevated temperatures accelerates calendar and cycle aging. Manufacturers publish derating curves—use those rather than ad hoc percentages. For aging, battery end‑of‑life is typically defined at 70–80% state‑of‑health (SoH); the battery will progressively deliver fewer Wh at end‑of‑life, reducing runtime by the same percentage. Operational recommendation: plan to oversize usable Wh in initial procurement to meet runtime requirements through expected capacity fade, and operate within recommended temperature and DoD limits to maximize retained runtime over the asset lifecycle.

Predictable runtime requires verifying measured, not nominal, metrics: true battery Wh (Ah × V — measured at your operating voltage), usable Wh (after applied DoD policy), inverter efficiency curves across loads, idle power draw, surge capability, and real PV output at site PSH. Relevant standards for safety and performance that procurement teams use to validate components include UL 1973/UL 9540 (energy storage systems and batteries), IEC 62109 (power converter safety for PV inverters), and UN 38.3 (transport testing for lithium cells) among others; these certify safety and basic performance, but runtime prediction is an engineering calculation combining manufacturer data sheets and field‑measured inputs. For contract requirements, request datasheet curves (efficiency vs load, charge acceptance, temperature derating) and perform a site‑specific energy balance—usable Wh out versus expected loads and daily PV Wh in—to produce reliable runtime projections.