Backup blackout kit 24V
24V system, designed for a -15C (5F) cold snap - saved 2026-08-16
Compatibility checks
12 passing checks - expand to see the math
passBattery matches the 24V systemSystem voltage coherence
LiTime 24V 100Ah LiFePO4 (25.6V nominal) is a 24V-class battery.
battery nominal 25.6V is in the 24V class == system 24V
passInverter matches the 24V systemSystem voltage coherence
Growatt SPF 3000TL LVM-24P DC input is 24V-class.
inverter DC input 24V is in the 24V class == system 24V
passString Voc stays at 96.8V at -15C - safe down to -226.7C (-376.1F)Cold-temperature string voltage
2 x Renogy 320W Monocrystalline Solar Panel (RSP320D) in series stays below the Growatt SPF 3000TL LVM-24P max PV input voltage at your design low temperature. The string would not reach the 145V limit until the temperature drops below -226.7C (-376.1F).
Why this matters: Panel voltage rises as temperature falls. A string that measures fine in summer can exceed the controller's absolute maximum input voltage on the first cold clear morning, which can permanently destroy the controller - and at these DC power levels, failures can start fires. This is the mistake experienced builders catch most often in DIY parts lists.
Voc_string = Voc x N_series x (1 + tempCoeff/100 x (T_design - 25))
= 43.82V x 2 x (1 + (-0.26%/100) x (-15 - 25))
= 43.82V x 2 x 1.1040 = 96.8V
Controller max PV input: 145V
Crossover: string Voc reaches 145V at T = 25 + (145/87.6 - 1) x 100/-0.26 = -226.7CpassString Vmp 75.7V has healthy margin over the 24V bankMPPT voltage margin
Clears the start floor cold and the hot-weather floor with sag included.
Vmp_string = 37.85V x 2 = 75.7V Start floor = V_batt + 5V = 24 + 5 = 29V Hot Vmp (~11% sag) = 75.7V x 0.89 = 67.4V vs hot floor 32V
passController can pass the array's full output (25A of 80A rated)Array size vs controller charge output
No clipping at rated conditions.
Array = 320W x 2 panels = 640W Potential charge current = 640W / 25.6V = 25A Controller rated charge current: 80A
passArray 640W is within the controller's 2000W rating at 24VMax array wattage for battery voltage
Within the published maximum array wattage.
Array = 320W x 2 panels = 640W Controller max array wattage at 24V: 2000W
passGrowatt SPF 3000TL LVM-24P supports LiFePO4 chargingBattery chemistry support
Controller has a lithium profile or user-adjustable charge voltages.
battery chemistry = lifepo4; controller lifepo4_profile = true
passGrowatt SPF 3000TL LVM-24P charger supports LiFePO4Battery chemistry support
Inverter/charger has a lithium charge profile.
battery chemistry = lifepo4; inverter charger lifepo4_profile = true
passBank BMS limit 200A comfortably covers the inverter's 117.2A drawInverter draw vs battery BMS limit
Healthy headroom at full inverter load.
Inverter draw = 3000W / 25.6V = 117.2A Bank BMS limit = 100A x 2 batteries = 200A Headroom band starts at 80% of limit = 160A
passBank BMS peak 560A covers the inverter's 234.4A surgeInverter surge vs battery peak limit
Surge loads within the bank's published peak rating.
Surge draw = 6000W / 25.6V = 234.4A Bank BMS peak = 280A x 2 batteries = 560A
pass2 in parallel is within the manufacturer limit of 4Battery parallel count
Supported parallel configuration.
2 in parallel vs manufacturer max 4
passCharge current 80A is within the bank's 200A capacityCharge current vs bank capacity
Charge rate within limits.
Controller rated charge = 80A Bank max charge = manufacturer max charge 100A x 2 = 200A
2 checks not run (missing published specs)
not checkedNot checkedArray current vs controller input limit
Growatt SPF 3000TL LVM-24P does not publish a PV input current limit in our data. MPPT controllers generally current-limit safely, but verify against the manual.
not checkedNot checkedClosed-loop battery communications
Comms protocols for LiTime 24V 100Ah LiFePO4 are not in our data. The system will still work open-loop with manually set charge voltages.
Parts list
Wire and fuse guidance
| Segment | Continuous | Run (one way) | Copper wire | V-drop | Fuse/breaker |
|---|---|---|---|---|---|
| Array to all-in-one PV input | 8.9A | 25 ft | 14 AWG | 1.49% | 15APV-rated (gPV) fuse or DC breaker |
| Battery bank to all-in-one | 117.2A | 5 ft | 1/0 AWG | 0.45% | 150AClass T |
Array to all-in-one PV input: show the math
Continuous current = 8.9A Design current = 8.9A x 1.25 (NEC continuous) = 11.2A Wire: 14 AWG copper (75C ampacity 20A); voltage drop 1.49% over 50ft round trip at 75.7V (target <= 3%) Fuse/breaker: 15A - two criteria, both required: carry the load: fuse >= design 11.2A -> next standard size = 15A protect the wire: fuse 15A <= 14 AWG ampacity 20A - OK construction: PV-rated (gPV) fuse or DC breaker
Battery bank to all-in-one: show the math
Continuous current = 117.2A Design current = 117.2A x 1.25 (NEC continuous) = 146.5A Wire: 1/0 AWG copper (75C ampacity 150A); voltage drop 0.45% over 10ft round trip at 25.6V (target <= 3%) Fuse/breaker: 150A - two criteria, both required: carry the load: fuse >= design 146.5A -> next standard size = 150A protect the wire: fuse 150A <= 1/0 AWG ampacity 150A - OK bank BMS continuous 200A >= fuse 150A: the bank can hold the fuse's rated load construction: Class T (interrupt rating matters on battery mains)
Mount the fuse close to the battery positive terminal - it protects the wire run, so the unprotected stub should be as short as possible.
Sized for 75C copper ampacity with the NEC 1.25 continuous factor and a 3% voltage-drop target. Round up when between sizes and verify against local code.
Balance of system - shopping list
| Item | Qty | For |
|---|---|---|
| 14 AWG copper wire | 1 | ~60 ft total (25 ft one way, round trip + slack) - Array to all-in-one PV input |
| 15A PV-rated (gPV) fuse or DC breaker | 1 | Array to all-in-one PV input |
| 1/0 AWG copper wire | 1 | ~15 ft total (5 ft one way, round trip + slack) - Battery bank to all-in-one |
| 150A Class T | 1 | Battery bank to all-in-one - include the matching fuse holder/block |
Derived from the wire and fuse guidance above. Wire lengths include round trip plus slack - verify against your actual layout, and verify ratings against local code. Buy fuses and wire from reputable electrical suppliers; marketplace listings for these items are a common source of counterfeit or mislabeled parts.
About this build
This build is a grid-down insurance policy: enough power to keep a fridge cold, lights on, phones charged, and a router running through an outage, without the cost of a whole-home system. The all-in-one unit keeps it simple - inverter, charger, and MPPT in one box that charges from the grid in normal times and flips to battery when the grid drops.
LiFePO4 matters more here than in any daily-use build because backup gear spends most of its life waiting: lead-acid self-discharges and sulfates on the shelf, while lithium holds charge and stays ready. Two batteries in parallel double the bank's continuous current capability and its runtime. The panel pair is the piece that turns a big UPS into an actual off-grid system - in an extended outage, the array recharges the bank every day instead of the kit dying when the batteries do.
Common questions
How long will this run a fridge?
It depends on the fridge's real consumption and what else you power, so put your actual loads into the builder rather than trusting a rule of thumb. The honest framing: a bank this size covers essentials for a day-plus, and the solar input is what makes multi-day outages survivable.
Should the batteries sit at full charge all the time?
LiFePO4 prefers not to sit at 100% for months. A common pattern for standby banks is storing somewhat below full and topping up when storms are forecast. Check what your battery vendor recommends and exercise the system a few times a year.
Can it backfeed my house panel?
Not as built - backfeeding requires a transfer switch or interlock installed at the panel, and doing it without one endangers line workers. This kit powers essentials via its own outlets and cords; panel integration is an electrician conversation.