Guía práctica para dimensionar baterías en aislada y on-grid

Practical guide to sizing batteries in off-grid and on-grid systems

Sizing batteries (off-grid and hybrid) explained simply

Reading time: 8–9 minutes · Updated

With this guide, you’ll learn how to choose battery capacity (kWh), system voltage (12V, 24V, 48V or high voltage), and charge/discharge power (kW) for your off-grid solar or grid-tied hybrid installation, with or without a back-up system.

Quick summary

  • Basic formula: Nominal capacity (kWh) ≈ Daily energy / (efficiency × DoD). Use 0.85 efficiency as standard and DoD according to battery type.
  • Recommended voltage: Up to 1000 W is comfortable with 12V; up to 2500 W with 24V; over 3000 W, better 48V or HV. In hybrids, 48V or high voltage is standard.
  • DoD (depth of discharge): Current lithium, 80–100%; lead, maximum 50% usable.
  • Charge/discharge: Choose a battery and inverter that can handle solar production and cover consumption peaks without issues.
  • Back-up: You can power only critical appliances (fridge, lights, router) to extend autonomy, or the whole house if your back-up output allows.

Index

Key concepts: daily consumption, DoD, efficiency, C-rate

  • Daily consumption (kWh/day): Add up the power of your devices multiplied by daily usage hours. This is what you really need to cover.
  • DoD (“Depth of Discharge”): Shows the % of the battery you can use. With lithium, you can use up to 90–100% of capacity, but with lead only half.
  • Efficiency: How much energy you recover compared to what you put in when charging the battery. Use 0.85 as a standard for overall calculations (battery + losses).
  • C-rate: Indicates the “rate” of charge and discharge the battery can handle. Very important for peaks and system safety.

How to choose the right voltage

  • 12 V: Up to about 1000 W, ideal for caravans, boats, or small cabins. Short cables and moderate currents.
  • 24 V: Between 1500 and 2500 W (small or medium home).
  • 48 V or high voltage: Over 3000 W. Less current and fewer losses.
  • Hybrid installations: Commonly 48 V or high voltage.

Step-by-step method for off-grid systems

  1. Calculate your daily consumption (kWh/day).
  2. Define the days of autonomy you want (1–3 basic, 3–5 if the location is cloudy or you need critical use).
  3. Multiply consumption × days = usable capacity (kWh).
  4. Divide by efficiency × DoD. Example: consumption 6 kWh/day × 2 = 12 kWh usable; efficiency = 0.85; DoD = 0.90 ⇒ 12/(0.85×0.90) = 15.7 kWh nominal.
  5. If the battery will often be below 10°C, add 10–20% to the final capacity.

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

  1. Analyze when you consume more: morning, afternoon/night (you need a battery to shift solar energy to those hours) or mainly during the day (enough panel capacity and a battery for the sunless period).
  2. Calculate the energy you want to cover “without sun.” For example, what you need at night.
  3. Divide by efficiency and DoD as before. Use 0.90–0.95 for hybrids, or 0.85 if you want a global safety margin.
  4. Check that the discharge power of the battery and inverter is enough for your nighttime peaks and that the battery can charge properly with your PV system.

Back-up: You can power only some devices (fridge, router, lights) if you limit consumption, so your autonomy lasts longer. If your inverter allows, you can power the whole house, but be cautious not to drain the battery too soon.

Charging and discharging power: practical rules

  • Make sure the battery supports the maximum power of your peaks, and the same for the inverter.
  • The battery must be able to charge with what your PV produces, and if not, the system should limit it.
  • For LiFePO₄ batteries, the typical C-rate is 0.5C–1C, but always follow the lowest value of the system (inverter or battery).

Series and parallel explained simply

  • In series, voltage adds up but ampere-hours remain the same.
  • In parallel, ampere-hours add up but voltage stays the same.
  • Always follow the connections allowed by the manufacturer.

Example: 2 batteries of 12 V 100 Ah.
Series connection: 24 V, 100 Ah (2400 Wh).
Parallel connection: 12 V, 200 Ah (2400 Wh).

Practical examples

Off-grid - Caravaning/Marine 12 V

Consumption: 0.8 kWh/day; autonomy: 2 days; LiFePO₄ DoD 90%, efficiency 0.85.
Nominal = 1.6/(0.85×0.90) ≈ 2.1 kWh → You’ll need about 2.5 kWh at 12 V (≈200 Ah LiFePO₄).

Off-grid - Home 24 V

Consumption: 3.5 kWh/day; autonomy: 2 days; DoD 90%, efficiency 0.85.
Nominal = 7/(0.85×0.90) ≈ 9.15 kWh → Recommended: 9–10 kWh at 24 V.

Off-grid - Home 48 V

Consumption: 6 kWh/day; autonomy: 2 days; DoD 90%, efficiency 0.85.
Nominal = 12/(0.85×0.90) ≈ 15.7 kWh → Recommended: ~16 kWh at 48 V.

Grid-tied hybrid - Without backup

Consumption without sun: 4 kWh; DoD 90%, efficiency 0.90.
Nominal = 4/(0.90×0.90) ≈ 4.94 kWh → Recommended: battery 5–7 kWh (48 V or HV).

Grid-tied hybrid - With backup

Goal: 10 h at 0.6 kW (fridge+lights+router) = 6 kWh usable; DoD 90%, efficiency 0.90.
Nominal = 6/(0.90×0.90) ≈ 7.4 kWh → Recommended: battery 7–10 kWh (48 V or HV).

Table / Basic checklist

Step / Parameter What to check Reference
Consumption (kWh/day) Sum of your equipment Meter/bill
Days of autonomy 1–3 normal; 3–5 if you need margin Critical climate/use
DoD LiFePO₄ 80–100%; lead 50% Battery
Efficiency 0.85 standard Overall system
Peaks and C-rate Sufficient charge/discharge Battery+inverter
Voltage 12/24/48 V/HV Equipment
Series/parallel Approved connection Manufacturer manual
Backup Critical loads Inverter output

Typical mistakes to avoid

  • Estimating capacity “by eye” and not leaving margin: use the formula and add 10–20% for safety.
  • Only looking at kWh and forgetting the kW of charge/discharge.
  • Wrong DoD chosen: remember lithium 80–100% and lead only 50% usable.
  • Trying to put too much power at low voltage: switch to 48 V or high voltage if there is excess power.
  • Do not add margin for cold (<10°C): add 10–20%.
  • “Creative” series/parallel connections not recommended by the manufacturer.

Frequently asked questions

  • How many kWh do I need? Off-grid: (kWh/day × days) / (efficiency × DoD). Hybrid: (kWh “without sun”) / (efficiency × DoD).
  • What voltage is best? Up to 1000 W: 12 V. Up to 2500 W: 24 V. Over 3 kW: 48 V or high voltage.
  • What DoD should I use? For current lithium, usually 80–100%. For lead, maximum 50% usable.
  • More battery or more panel? If your main consumption is during the day, prioritize panels; if at night, prioritize battery and watch the discharge power.
  • Can it be expanded later? Yes, but many brands require identical modules and expansion within certain timeframes. Check before buying.

Related Products

Can we help you? Tell us your location, consumption (day/night), peaks, and whether your system is off-grid or on-grid (with or without backup). We will send you the exact calculation and recommended equipment.

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