Battery sizing (off-grid and hybrid) explained simply

Reading time: 8–9 min · Updated

The idea is to get three things right: capacity (kWh), system voltage (12/24/48 V or High Voltage), and charge/discharge power (kW) for your off-grid or hybrid grid-tied installation (with or without back-up).

One-minute summary

• Quick formula: Nominal capacity (kWh) ≈ Energy to cover / (η × DoD). η (overall efficiency) use 0.85 as a rule of thumb.
• Voltage (off-grid): up to ~1000 W → 12 V; 1500–2500 W → 24 V; ≥3000 W → better 48 V or HV (if your equipment allows). In hybrid systems: 48 V or HV.
• DoD (depth of discharge): lithium LiFePO₄ 80–100% typical; lead-acid ~50% usable.
• Charge/discharge: battery + inverter must charge with what your PV produces and discharge your peaks without drawing from the grid.
• Back-up: you can power only “critical” loads (fridge, lights, router) to extend hours, or the whole house if the back-up output provides enough power.

Index

• Key concepts: kWh/day, DoD, efficiency, C-rate
• Choose the voltage: 12/24/48 V or HV
• Method for off-grid (step by step)
• Method for grid-tied hybrid (with and without back-up)
• Charge/discharge power: practical rules
• Series and parallel made simple
• Practical examples
• Table / Checklist
• Typical errors (and how to avoid them)
• FAQs
• Related products + CTA

Key concepts: kWh/day, DoD, efficiency, C-rate

kWh/day: what you use per day (power in kW × hours). This is your starting point.

DoD (“Depth of Discharge”): % of the battery you can use without shortening its life. Lithium 80–100 % typical; lead ~50 % usable.

Round-trip efficiency (η): what you recover versus what you charge (battery + losses). Use 0.85 to be on the safe side.

C-rate: “rate” of charge/discharge relative to capacity. Tells you how many kW it can handle without damage.

Choose the voltage: 12/24/48 V or HV

• 12 V: up to ~1000 W (caravan, boat, cabin). Short cables and moderate currents.
• 24 V: around 1500–2500 W (small/medium home).
• ≥3000 W: better 48 V or HV to reduce currents and losses.
• Hybrid: normally 48 V (low voltage, LV) or HV. (Detailed differences in another guide).

Method for off-grid (step by step)

Quick rule / formula

Usable capacity (kWh) = Daily consumption (kWh/day) × Days of autonomy.
Nominal capacity (kWh) = Usable capacity / (η × DoD).
Example: 6 kWh/day × 2 days = 12 kWh usable.
With η=0.85 and DoD=0.90 ⇒ 12/(0.85×0.90)= 15.7 kWh.

Parameters to set

• Days of autonomy: 1–3 is normal; 3–5 if cloudy weather or critical use.
• Temperature: if operating at <10 °C add +10–20 % margin (batteries provide less).
• C-rate: ensure the battery can handle inverter peaks and the charge from the regulator/MPPT.

Method for grid-tied hybrid (with and without back-up)

First: when do you consume?

• Early morning and evening/night: it is better to have more battery to shift energy from the day to those hours.
• Daytime consumption: prioritize enough panels and a “just right” battery for the period without sun.

Easy calculation

1. Define the kWh without sun you want to cover (evening/night + early morning).
2. Apply: Nominal capacity = kWh without sun / (η × DoD). In hybrid systems: actual battery η ~0.90–0.95; use 0.85 if you want an overall margin.
3. Check discharge power (kW) of battery + inverter for your nighttime peaks.
4. Check charging power (kW): ensure the battery can absorb what your PV provides without “choking” the system.

And what about back-up?

The back-up supplies critical loads (fridge, lights, router…) when the power goes out. If you limit consumption, autonomy is extended. If the inverter's back-up output provides enough power, you can use the whole house, but be moderate to extend hours.

Charge/discharge power: practical rules

• Discharging: if your peaks are 3 kW, require continuous discharge ≥3 kW and a higher peak.
• Charging: if your PV provides 4–5 kW, ensure the battery/inverter can accept it or that the system limits/diverts it.
• Typical LiFePO₄ C-rate: 0.5C–1C (depends on brand). Always use the lowest value of the set.

Series and parallel made simple

• In series, the voltage is added; the Ah remain the same. The energy (Wh) increases due to the voltage.
• In parallel, the Ah are added; the voltage does not change. The energy (Wh) also increases.

Example: 2× 12 V 100 Ah batteries →
Series: 24 V, 100 Ah ⇒ 2400 Wh ·
Parallel: 12 V, 200 Ah ⇒ 2400 Wh.

Same total energy, different voltage/current. Always use connections permitted by the manufacturer.

Practical examples

Off-grid — Caravaning/Marine 12 V

Consumption: 0.8 kWh/day (lights, small fridge, mobiles). Autonomy: 2 days. LiFePO₄ DoD 90%, η=0.85.
Usable = 0.8×2 = 1.6 kWh → Nominal = 1.6/(0.85×0.90)= 2.1 kWh.
Recommendation: ~2.5 kWh at 12 V (≈200 Ah LiFePO₄) and suitable MPPT/charger.

Off-grid — Home 24 V (1.8–2.2 kW)

Consumption: 3.5 kWh/day. Autonomy: 2 days. DoD 90%, η=0.85.
Usable = 7.0 kWh → Nominal = 7.0/(0.85×0.90)= 9.15 kWh.
Recommendation: ~9–10 kWh at 24 V, 2 kW inverter, monitor C-rate and charge current.

Off-grid — Home 48 V (>3 kW)

Consumption: 6 kWh/day. Autonomy: 2 days. DoD 90%, η=0.85.
Usable = 12 kWh → Nominal = 12/(0.85×0.90)= 15.7 kWh.
Recommendation: ~16 kWh at 48 V with 5–6 kW inverter, margin for temperature and growth.

Hybrid on grid — No back-up (self-consumption)

Consumption without sun: 4 kWh (20:00–08:00). DoD 90%, η=0.90.
Nominal = 4/(0.90×0.90)= 4.94 kWh.
Recommendation: battery 5–7 kWh (48 V or HV), discharge ≥2–3 kW for night-time peaks.

Hybrid on grid — With back-up (critical loads)

Objective: 10 h at 0.6 kW (fridge+lights+router) = 6 kWh usable. DoD 90%, η=0.90.
Nominal = 6/(0.90×0.90)= 7.4 kWh.
Recommendation: battery 7–10 kWh, back-up output ≥1 kW continuous. Use wisely to extend hours.

Table / Checklist

Typical errors (and how to avoid them)

• Going "by eye" → Apply the formula and leave a 10–20% margin.
• Only looking at kWh → Check kW for charge/discharge and peaks.
• Incorrect DoD chosen → Lithium 80–100% / lead-acid ~50%.
• Too much power at low voltage → Switch to 48 V or HV.
• No margin for cold → add +10–20% if <10 °C.
• “Creative” series/parallel → Only approved configurations and with protections.

FAQs

How many kWh do I need? Off-grid: kWh/day × days / (η×DoD). Hybrid: kWh “without sun” / (η×DoD).

12/24/48 V or high voltage? 12 V up to ~1000 W; 24 V ~1500–2500 W; ≥3000 W → 48 V or HV. For hybrids, 48 V or HV.

What DoD should I use? Lithium 80–100% typical; lead-acid ~50% usable.

More battery or more panel? Daytime consumption: panel. Nighttime consumption: battery (do not forget discharge power).

Can I expand later? Yes, but many brands require identical modules and specific timeframes. Plan for growth.

Related products + CTA

• Collection: 48 V Lithium Batteries (LV)
• Collection: High Voltage Lithium Batteries (HV)
• Collection: Hybrid Inverters
• Pylontech Force L1 (LV)
• Huawei LUNA2000 (HV)
• GoodWe Lynx Home U 5.4 kWh (LV)
• Victron MultiPlus-II 48 V

Need help? Tell us your location, consumption (day/night), peaks, and whether your system is off-grid or on-grid (with or without back-up). We will send you the calculation and the recommended materials.