Surplus Solar Power: Selling Back, Credits and Storage

A bright spring Saturday is a fair test. The sun is high, the house is quiet, and a 7 kW system may be producing far more than the fridge, lights and laptop need. Where does the rest go? Mostly to the street, and what you earn for it depends on where you live.

Step one: the house uses what it can

Solar power flows to wherever demand is closest. Your own appliances take it first, with no meter needed to decide. Whatever is left over leaves the house through the meter. That is why using more power while the sun shines is the most reliable way to benefit, since every kilowatt-hour consumed directly is one you do not buy.

Step two: export to the grid

The surplus travels onto the local network and is used by your neighbours. A bidirectional meter records the amount. Then it depends on your province and utility.

• Under net metering, each exported kilowatt-hour earns a bill credit worth roughly what you pay for power. Credits often roll over for about 12 months.
• Under net billing, exports are valued at a separate rate that is usually lower. Alberta and Manitoba are reported to use arrangements of this kind.
• Some programmes pay out unused credits at the end of a period at a set rate instead of letting them lapse.

Most people picture “selling back” as a cheque. More often it is a reduced bill. For the rules by province, read net metering across Canada, and for how the credit rate changes payback, see net billing vs net metering.

Step three: limits and curtailment

Sometimes the system cannot export. A utility may cap how much your connection can send, and if the local lines are congested the inverter may reduce output. This is called curtailment. For a typical home on a typical feeder it is uncommon, but it can happen with larger systems or in areas with lots of rooftop solar. Ask your utility whether any export limit applies to your connection.

Using more of it yourself

You can raise your share of self-use without buying anything.

• Run the dishwasher, washing machine and dryer in the middle of the day.
• Heat water when the sun is out, if you have an electric tank with a timer.
• Pre-cool or pre-heat the house during bright hours, where it suits you.
• Charge an electric vehicle at noon rather than overnight. Our page on solar and EV charging goes into this.

These habits matter most when export credits are low. Under generous net metering they matter less, because exports are worth about as much as direct use.

Storing surplus in a battery

A battery captures what the house does not use at noon and returns it in the evening. Typical home batteries range from about 5 to 15 kWh. Whole-home backup requires a much larger setup. Costs often add several thousand dollars up to well over ten thousand, and lithium iron phosphate is a common chemistry.

The economics are weakest where net metering is generous, since the grid already acts as a free, unlimited battery. They improve for backup during outages and for arbitrage under time-of-use pricing. Our battery storage basics has the fuller picture, and time-of-use rates and solar explains the pricing side.

A rough picture of a year

In a net metering province, summer surplus helps pay for winter. In a net billing province, the summer surplus earns a smaller credit and the winter draw is paid at the full rate. This is why sizing matters so much.

Sizing so surplus does not go to waste

If most of your output will be exported at a low credit, a slightly smaller system may give a better return per dollar. If credits are generous and roll over, a larger one can make sense. See how many solar panels you need for a worked example.

Surplus and the neighbourhood

There is a simple physical point behind all this. Electricity cannot be held in the wires, so the power you send out is consumed almost immediately by the nearest houses, shops or street lighting. That is useful to the grid, which is why utilities credit exports at all, though the credit rarely equals the value of the electricity to the person who receives it. It also explains why a street full of solar homes can strain local equipment on a sunny afternoon, which is where export limits come from.

What to check with your utility

1. How are exports credited, and at what rate?
2. How long do credits last before they reset?
3. Are there export limits or curtailment rules on my connection?
4. What happens to leftover credits at the end of the period?

You can read the broader context in our net metering and grid hub. The short answer to the opening question is that surplus does not vanish. It either earns something, gets stored, or occasionally goes unproduced, and a little planning moves it toward the first two.