Article — Solar Battery Sizing
For an average U.S. home drawing 29 kWh per day, three days of autonomy on a LiFePO₄ bank at 80% depth of discharge and 92% round-trip efficiency works out to about 118 kWh of nameplate battery capacity, paired with roughly an 8.7 kW solar array at 4 peak sun hours. Off-grid sizing always reduces to the same chain: daily load, how long the bank must coast without sun, how deeply the chemistry can be cycled, and how much of the energy survives the round trip through inverter and wiring losses.
This article walks through the four variables the calculator uses, the trade-offs between chemistries, why voltage choice changes wiring cost more than capacity, and the seasonal adjustments that decide whether the lights stay on in January.
Solar battery sizing fundamentals
A solar battery bank is sized for two jobs at once: store enough usable energy to run loads through cloudy days, and accept enough charge during sunny periods to refill before the next cloudy stretch. The two jobs interact — a bigger bank needs a bigger array to refill in reasonable time, and an undersized array leaves a fully sized bank chronically half-empty.
The U.S. National Renewable Energy Laboratory's methodology for off-grid PV-plus-storage sizing builds the calculation around four inputs: daily energy demand, autonomy in days, depth of discharge tolerated by the chemistry, and the round-trip efficiency of the whole DC-AC-DC path. The calculator above uses the same chain and adds a seasonal multiplier for high-latitude winters.
Tesla Powerwall 3 ships in 13.5 kWh blocks; LG Chem RESU comes in 10, 13, and 16 kWh modules; SunPower SunVault uses 13 kWh stacks. Bank capacity is built up by stacking these modules in parallel rather than ordering one giant battery — which is why the calculator returns a module count alongside total kWh.
Choosing battery chemistry for solar storage
LiFePO₄ (lithium iron phosphate) has become the dominant chemistry for residential storage since about 2020. It tolerates 80–90% depth of discharge with no meaningful loss in cycle life, hits 92–95% round-trip efficiency, lasts 6,000–10,000 cycles, and is chemically stable under thermal abuse. The downside is energy density — an LFP pack takes about 25% more physical space than NMC for the same kWh.
NMC lithium-ion (nickel-manganese-cobalt) trades cycle life for density. Same 80% DoD but only 3,000–5,000 cycles, and the cobalt content makes thermal runaway a real consideration for indoor installs. NMC suits weight-constrained applications — EVs, marine, aviation — more than stationary home storage.
Lead-acid remains the cheapest upfront option, particularly flooded golf-cart batteries common in DIY off-grid setups. The penalty is two-fold: maximum 50% DoD before cycle life collapses, and 75–85% round-trip efficiency versus 90%+ for lithium. A 100 kWh lead-acid bank delivers only ~38 kWh usable after accounting for both penalties, while a 100 kWh LiFePO₄ bank delivers ~74 kWh.
A flooded lead-acid bank at $200/kWh and 1,000 cycles costs $0.20 per kWh cycled. A LiFePO₄ bank at $500/kWh and 6,000 cycles costs $0.08 per kWh cycled — less than half. Lead-acid pencils out only for systems cycled infrequently (backup-only) or where capital is severely constrained.
Depth of discharge and round-trip efficiency
Depth of discharge is the fraction of nameplate capacity you actually pull out before recharging. A 100 kWh bank cycled to 20% remaining has been at 80% DoD. The deeper the cycle, the more useful kWh per dollar of capital — but only up to the chemistry's threshold. Past that threshold, cycle life drops fast.
Round-trip efficiency captures the energy lost going from solar DC into the battery and back out through the inverter to AC loads. Modern hybrid inverters reach 94–97% peak efficiency on the AC side, but combined with charge/discharge losses inside the cell and standby parasitic draw, system round-trip lands around 90–95% for lithium and 75–85% for lead-acid. The U.S. Department of Energy's Office of Electricity tracks round-trip efficiency in its annual energy storage market report — it has crept up about 3 percentage points across the industry in the past five years.
- LiFePO₄ DoD — 80% is the standard design point, 90% acceptable for stationary use
- NMC Li-ion DoD — 80% standard, 90% reduces cycle life by ~30%
- Lead-acid DoD — never exceed 50% for daily cycling
- Lithium round-trip η — 90–95% for whole system
- Lead-acid round-trip η — 75–85%
- Inverter standby draw — 30–80 W continuous, 1–2 kWh/day lost to idle
Picking a solar battery sizing voltage
System voltage determines wire gauge, fuse ratings, and inverter compatibility — not capacity. A 20 kWh bank holds 20 kWh whether it is wired at 12 V, 24 V, or 48 V. But the current required to deliver a given wattage scales inversely with voltage. A 5,000 W load at 12 V draws 417 A; the same load at 48 V draws only 104 A. The wire cross-section scales with current, so the 12 V system needs four times the copper.
For residential whole-house off-grid, 48 V is the standard. The vast majority of hybrid inverters (Sol-Ark, Schneider XW, Outback Radian) are 48 V designs. Twelve-volt makes sense for RVs, small cabins under 5 kWh of storage, and marine systems where 12 V appliances are common. Twenty-four-volt is a niche — sometimes seen in mid-size systems or older European installs.
The U.S. National Electrical Code (NEC 690.7) requires PV-system DC disconnects, conductor sizing, and grounding rated for the maximum system voltage. Stepping from 12 V to 48 V at the design stage often cuts the copper-and-conduit budget by 40–60% for the same battery and load capacity.
Seasonal solar battery sizing adjustments
Peak sun hours collapse in winter at high latitudes. London averages 2.8 PSH annually but drops to 0.8–1.2 PSH in December and January. Berlin sees the same pattern. Even mid-U.S. cities like Chicago lose roughly half their solar yield from June to December. A system sized for summer PSH will starve through the winter unless the bank is oversized to coast longer or the array is oversized to harvest more on the few clear winter days.
The seasonal correction factor in the calculator (1.0–1.5) multiplies the bank size to account for this. Use 1.0 for temperate sites where winter is mild. Use 1.2 for cloudy temperate climates (UK, Pacific Northwest, northern Germany). Use 1.4 for northern continental winters (Scandinavia, upper Midwest). Beyond 1.4, off-grid solar alone rarely makes economic sense — a propane or wood backup generator is usually cheaper than another 50% of battery.
Sizing a bank to carry a high-latitude home through every winter cloudy stretch can double the battery capital cost. A common compromise is to size for 95th-percentile demand and add a small generator (3–6 kW) for the worst 1–2% of hours per year. The U.S. National Renewable Energy Lab calls this "hybrid sizing" and finds it usually beats pure off-grid on a 25-year levelised cost basis.
Common solar battery sizing mistakes
A 1,500 W microwave does not draw 1,500 W for the whole time it is on — it cycles. A "5 kW" heat pump pulls less than 1 kW most of the time and only peaks at 5 kW during defrost cycles. Sizing from nameplate-watts-times-hours overstates load by 30–100%. Use a measured kWh figure from a utility bill or plug-through meter.
An off-grid inverter pulls 30–80 W in standby mode — about 1 kWh per day — even with no AC loads connected. Refrigerator compressors, modems, alarm panels, and chargers add another 100–200 W of "always-on" load. Parasitic draw can easily reach 4–6 kWh/day before you turn anything on.
The single most expensive lead-acid mistake. A flooded battery rated for 1,000 cycles at 50% DoD will deliver fewer than 300 cycles at 80% DoD. The bank that should have lasted ten years dies in three. If 50% DoD does not provide enough usable kWh, switch to LiFePO₄ rather than over-cycling lead-acid.