Uptime · editable cold-weather scenario
Build the weather that actually happened
Drag either strip to reshape it. Top sets each day's mean temperature, bottom sets each day's solar resource. Load a preset and adjust, or recreate the cold snap you actually saw. Uptime is the fraction of the scenario the load was genuinely served: warm enough to discharge, and with charge left above the floor.
"Recorded" preset is modeled on Wausau, Wisconsin, Jan 19-26, 2026. Source: Wisconsin State Climatology Office, "Arctic Blast of January 22-25, 2026" (climatology.nelson.wisc.edu). Daily means derived from published lows and highs where available; days without a published Wausau figure are estimated to complete the shape.
Daily mean temperature (°F): drag to edit
Daily solar resource (peak sun hours): drag to edit
Applied evenly to all chemistries.
How this model works
Time resolution and energy balance
The simulator steps at 10-minute intervals through each day. At each step it computes available solar power, routes it to the heater or to charge, deducts load if the pack is online, and updates body temperature via a first-order thermal model.
Solar generation follows a half-sine curve pegged to the day's peak sun hours (PSH): P = P_peak × sin(π × fractional_daylight). Array derate is fixed at 0.85 to account for wiring losses, soiling, and inverter efficiency. Peak power is adjusted for a temperature coefficient of −0.35%/°C relative to STC (25°C).
Thermal model
Each enclosure is modeled as a single lumped thermal mass with a UA conductance and a heat capacity derived from published mass and surface area. Body temperature evolves as:
dT_body/dt = (Q_heater − UA × (T_body − T_amb)) / C_th
Ambient temperature follows a diurnal cosine with a 14°F peak-to-peak swing, minimum near 2 AM, maximum near 2 PM. The thermal time constant τ = C_th / UA determines how quickly each enclosure tracks ambient when the heater is off.
Discharge and charge floors
Two independent floors define the minimum usable SOC. The DoD floor is set by the slider (default 85% DoD = 15% floor). The cold retention floor is chemistry-specific: at temperature T, usable capacity = rated capacity × retention(T). Whichever floor is higher governs; they are not stacked.
Discharge is blocked entirely when body temperature is below the discharge cutoff. Charging is blocked when body temperature is below the charge floor. These cutoffs differ by chemistry and are listed in each card's explainer.
Uptime definition
A time step counts as "online" only when both conditions are met: body temperature is at or above the discharge cutoff, and SOC is above the active floor. Uptime is the fraction of total minutes that satisfy both conditions. A step that fails only the temperature condition is logged as "below cutoff" (red). A step that fails only the SOC condition is logged as "no usable charge" (amber).
Capacity retention curves
Sodium-ion: linearly interpolated between two published datasheet points: 85% at −40°F and 96% at 32°F. Intermediate values are estimated.
Lithium LFP (both heated variants): generic industry curve based on published LFP cell data. Not tied to a specific product.
Flooded lead-acid: derived from the Rolls Battery published temperature-capacity multiplier table by computing (1/multiplier) × 100. Source: support.rollsbattery.com.
Solver (minimum configuration table)
The solver exhaustively searches battery count (1-8 in parallel) and array sizes [200, 400, 600, 800, 1200, 1600, 2400, 3200, 4000 W] and returns the smallest configuration achieving ≥99.95% uptime with no depletion events. It runs a full simulation for each candidate; configurations searched to 8 batteries at 4000 W without success are reported as not achievable.
This is a directional sizing tool, not a warranty or field-validated guarantee. Use it to understand order-of-magnitude requirements and the sensitivity to array size versus battery count.
State of charge
Repeated deep discharge permanently erodes lithium capacity, and a pack held flat can suffer copper dissolution and lose the ability to recharge. Time below 10% SOC is a durability cost that uptime percentage alone does not capture.
Thermal clock at the coldest point
Minimum configuration for 100% uptime
Equal-energy basis of 3,648 Wh per unit, each product modeled with its own published mass and enclosure area. Heater wattages are published power densities scaled to this capacity. Sodium retention is linearly interpolated between two datasheet points (−40°F/85%, 32°F/96%); intermediate values are estimated. LFP retention is generic industry data. Flooded lead-acid retention is derived from Rolls Battery published multiplier table (support.rollsbattery.com); discharge cutoff is 0°F (internal resistance collapse under load), charge cutoff is 32°F per Battery University guidance. DoD and cold retention floors are independent; whichever is higher governs. 10-minute simulation steps, single-node thermal model, clear-sky solar within each day. Directional model, not warranty figures.
Uptime · editable cold-weather scenario
Build the weather that actually happened
Drag either strip to reshape it. Top sets each day's mean temperature, bottom sets each day's solar resource. Load a preset and adjust, or recreate the cold snap you actually saw. Uptime is the fraction of the scenario the load was genuinely served: warm enough to discharge, and with charge left above the floor.
"Recorded" preset is modeled on Wausau, Wisconsin, Jan 19-26, 2026. Source: Wisconsin State Climatology Office, "Arctic Blast of January 22-25, 2026" (climatology.nelson.wisc.edu). Daily means derived from published lows and highs where available; days without a published Wausau figure are estimated to complete the shape.
Daily mean temperature (°F): drag to edit
Daily solar resource (peak sun hours): drag to edit
Applied evenly to all chemistries.
How this model works
Time resolution and energy balance
The simulator steps at 10-minute intervals through each day. At each step it computes available solar power, routes it to the heater or to charge, deducts load if the pack is online, and updates body temperature via a first-order thermal model.
Solar generation follows a half-sine curve pegged to the day's peak sun hours (PSH): P = P_peak × sin(π × fractional_daylight). Array derate is fixed at 0.85 to account for wiring losses, soiling, and inverter efficiency. Peak power is adjusted for a temperature coefficient of −0.35%/°C relative to STC (25°C).
Thermal model
Each enclosure is modeled as a single lumped thermal mass with a UA conductance and a heat capacity derived from published mass and surface area. Body temperature evolves as:
dT_body/dt = (Q_heater − UA × (T_body − T_amb)) / C_th
Ambient temperature follows a diurnal cosine with a 14°F peak-to-peak swing, minimum near 2 AM, maximum near 2 PM. The thermal time constant τ = C_th / UA determines how quickly each enclosure tracks ambient when the heater is off.
Discharge and charge floors
Two independent floors define the minimum usable SOC. The DoD floor is set by the slider (default 85% DoD = 15% floor). The cold retention floor is chemistry-specific: at temperature T, usable capacity = rated capacity × retention(T). Whichever floor is higher governs; they are not stacked.
Discharge is blocked entirely when body temperature is below the discharge cutoff. Charging is blocked when body temperature is below the charge floor. These cutoffs differ by chemistry and are listed in each card's explainer.
Uptime definition
A time step counts as "online" only when both conditions are met: body temperature is at or above the discharge cutoff, and SOC is above the active floor. Uptime is the fraction of total minutes that satisfy both conditions. A step that fails only the temperature condition is logged as "below cutoff" (red). A step that fails only the SOC condition is logged as "no usable charge" (amber).
Capacity retention curves
Sodium-ion: linearly interpolated between two published datasheet points: 85% at −40°F and 96% at 32°F. Intermediate values are estimated.
Lithium LFP (both heated variants): generic industry curve based on published LFP cell data. Not tied to a specific product.
Flooded lead-acid: derived from the Rolls Battery published temperature-capacity multiplier table by computing (1/multiplier) × 100. Source: support.rollsbattery.com.
Solver (minimum configuration table)
The solver exhaustively searches battery count (1-8 in parallel) and array sizes [200, 400, 600, 800, 1200, 1600, 2400, 3200, 4000 W] and returns the smallest configuration achieving ≥99.95% uptime with no depletion events. It runs a full simulation for each candidate; configurations searched to 8 batteries at 4000 W without success are reported as not achievable.
This is a directional sizing tool, not a warranty or field-validated guarantee. Use it to understand order-of-magnitude requirements and the sensitivity to array size versus battery count.
State of charge
Repeated deep discharge permanently erodes lithium capacity, and a pack held flat can suffer copper dissolution and lose the ability to recharge. Time below 10% SOC is a durability cost that uptime percentage alone does not capture.
Thermal clock at the coldest point
Minimum configuration for 100% uptime
Equal-energy basis of 3,648 Wh per unit, each product modeled with its own published mass and enclosure area. Heater wattages are published power densities scaled to this capacity. Sodium retention is linearly interpolated between two datasheet points (−40°F/85%, 32°F/96%); intermediate values are estimated. LFP retention is generic industry data. Flooded lead-acid retention is derived from Rolls Battery published multiplier table (support.rollsbattery.com); discharge cutoff is 0°F (internal resistance collapse under load), charge cutoff is 32°F per Battery University guidance. DoD and cold retention floors are independent; whichever is higher governs. 10-minute simulation steps, single-node thermal model, clear-sky solar within each day. Directional model, not warranty figures.