Separate three energy flows
For a grid-connected site without a battery, PV generation either serves the load at the same time or leaves the site as export. When the load exceeds PV, the grid supplies the difference. Adding those flows over a year hides when they occur.
Site consumption = direct solar use + grid import
These balances are a useful audit. If a proposal counts the same generated unit as both avoided import and paid export, it overstates value. A battery adds another path, conversion losses and a changing inventory of stored energy.
Compare two sites with the same PV output
Both illustrative sites consume 4,000 kWh/year and generate 4,000 kWh/year. Assume an avoided variable import rate of ₹8/kWh and an export payment of ₹3/kWh. These are teaching assumptions, not a current utility tariff or a net-metering rule.
| Annual quantity | Daytime-heavy site | Evening-heavy site |
|---|---|---|
| Direct solar use | 3,000 kWh | 1,500 kWh |
| Export | 1,000 kWh | 2,500 kWh |
| Grid import | 1,000 kWh | 2,500 kWh |
| Avoided import cost | ₹24,000 | ₹12,000 |
| Export value | ₹3,000 | ₹7,500 |
| Total variable-energy benefit | ₹27,000 | ₹19,500 |
The same equipment and generation produce a ₹7,500 difference because the load timing is different. Fixed charges, demand charges, taxes and settlement adjustments are outside this comparison. If exports have no value, the gap grows to ₹12,000. If every exported unit receives the same effective value as an imported unit, the simple annual value converges—but actual settlement rules still matter.
Read the settlement rules before projecting savings
Find the current utility or regulator document that applies to your connection category. Record whether credits are in units or currency, the period over which they can carry forward, the export rate, any capacity limit and what happens to unused credits. A residential illustration should not be silently reused for a common-area or commercial meter.
Monthly settlement can make a summer surplus less useful than annual accounting suggests. Slab tariffs can also make the avoided rate different from your average bill rate. Use actual billing periods rather than treating a nominal annual offset as guaranteed value.
Ask what a battery actually changes
A battery can move otherwise exported PV into an evening load. That is an incremental benefit over the solar-only case; it is not a second sale of the same electricity. If one kWh diverted from export returns 0.9 kWh to the load, using the illustrative rates above gives 0.9 × ₹8 − ₹3 = ₹4.20 per kWh diverted before battery ownership and operating costs.
This small example is an energy-value comparison, not an investment recommendation. Include lifetime throughput, replacement, financing and the value of backup separately. Battery capacity that sits reserved for outages may have fewer daily shifting opportunities.
A practical measurement plan
- Obtain interval load data if your meter supports it; otherwise identify major daytime and evening loads.
- Use an hourly solar profile for your location, roof direction and shading.
- Match load and generation within each interval, then aggregate imports and exports by billing period.
- Apply the actual settlement rules and retain fixed charges.
- Repeat with weaker solar yield and lower export compensation.
The solar sizing calculator screens capacity with annual energy. It does not perform this tariff reconciliation. Use its savings output as a scenario, and attach the settlement assumptions when sharing it.
Sources and scope
- U.S. DOE: Solar energy and storage basics ↗
Background on solar timing and storage losses. All tariff values and calculations above are illustrative.
- European Commission PVGIS ↗
Hourly PV output can support a time-matched load comparison.
Reference links provide technical context; the worked examples and interpretation are specific to this guide. No external source certifies these tools.