BatteryCap.java

package com.example.examplemod;

import net.minecraft.nbt.IntNBT;
import net.minecraft.util.Direction;
import net.minecraftforge.common.capabilities.Capability;
import net.minecraftforge.common.capabilities.ICapabilitySerializable;
import net.minecraftforge.common.util.LazyOptional;
import net.minecraftforge.energy.CapabilityEnergy;
import net.minecraftforge.energy.IEnergyStorage;

import javax.annotation.Nonnull;
import javax.annotation.Nullable;

public class BatteryCap implements ICapabilitySerializable<IntNBT> {
    SettableEnergyStorage storage = new SettableEnergyStorage(1000);
    LazyOptional<IEnergyStorage> energyCap = LazyOptional.of(()-> storage);
    @Nonnull
    @Override
    public <T> LazyOptional<T> getCapability(@Nonnull Capability<T> cap, @Nullable Direction side) {
        return CapabilityEnergy.ENERGY.orEmpty(cap, energyCap);
    }

    @Override
    public IntNBT serializeNBT() {
        return IntNBT.valueOf(storage.getEnergyStored());
    }

    @Override
    public void deserializeNBT(IntNBT nbt) {
        storage.setEnergy(nbt.getInt());
    }
}

BatteryItem.java

package com.example.examplemod;

import net.minecraft.item.Item;
import net.minecraft.item.ItemStack;
import net.minecraft.nbt.CompoundNBT;
import net.minecraftforge.common.capabilities.ICapabilityProvider;

import javax.annotation.Nullable;

public class BatteryItem extends Item {
    public BatteryItem(Properties properties) {
        super(properties);
    }

    @Nullable
    @Override
    public ICapabilityProvider initCapabilities(ItemStack stack, @Nullable CompoundNBT nbt) {
        return new BatteryCap();
    }
}