Covens have more energy because they have more life, each individuals energy mixes with the same desire/focus. Its like the difference between one person lifting a heavy rock and 5-10 lifting that same heavy rock. This is also why witches use familiars and call upon spirits and deities to aid them.
Candles do not have energy unless you give them energy. Once lit, the fire is the energy. Fire has energy, therefore if you are using the power of fire, more candles would give you more fuel of source to use. But it is mostly just representation and a tool for focus.
Doing magick in a field, forest, or cemetary can give you more power because you are channelling the energy that comes from those places. The more energy surrounding you, the more that you can channel. This is why location can be very important for a witch. And some places that are frequently used for magick or where the veil is thin are primary picks for witches.
But candles are dead objects. They never carried life. Even the wax, if you were to buy pure beeswax candles are a product of a living being (bees) and not sxomething with a spirit or "life" energy itself.
What can give you more energy if you are stuck indoors are things with "spirit" or "life" such as fresh herbs, plants, gemstones/crystals, etc. These things grew and were taken from a source of life (our earth). Elements are also equally energy filled, so fire and water. But they will only aid you if they match the nature of your spellwork.