Keeping more solar electricity on site and covering more household demand are different ratios. Both require the same measurement period and a clear system boundary.
1. Collect three compatible energy totals
This simple balance is for one grid-connected household with PV as its only generator, no battery and no vehicle-to-home energy. Use existing authorized meter or inverter readouts; do not open electrical equipment or change meter or inverter settings.
Record AC solar generation G, gross grid export X and gross grid import I, all in kWh for exactly the same start and end times. For cumulative registers, subtract each starting reading from its ending reading separately. Do not combine a lifetime inverter total, a monthly bill and a different period’s export, or use a net import-minus-export figure as gross import. Document resets, gaps, units and timezone changes.
2. Keep the two denominators separate
Self-consumption compares locally used PV with PV generation; the solar share compares locally used PV with household demand. SMA distinguishes these two measures in its energy-balance documentation.
For this no-storage model, directly used solar S = G − X and household electricity D = S + I. Self-consumption is 100 × S / G; solar share is 100 × S / D. All totals must be non-negative and X must not exceed G. If G = 0, the first percentage is undefined; if D = 0, the second is undefined. Record “not applicable”, not an invented zero or infinity. Differences smaller than the records’ resolution are not evidence of precise flows.
SMA Sunny Home Manager — Energy Balance (definitions; English) ↗3. Compare like-for-like scenarios
Use the worked example below as arithmetic, not as a measured home or a recommended target. With the same 600 kWh generation and 700 kWh household demand, a hypothetical 250 kWh export and 350 kWh import give S = 350 kWh, self-consumption 58.3% and solar share 50.0%. This describes a changed energy balance; it does not prove that shifting loads can achieve it.
Exporting nothing does not mean grid independence. For example, G = 600, X = 0 and I = 450 kWh give 100% self-consumption but only 57.1% solar share. More solar consumption caused by unnecessary extra use is not automatically an improvement. Compare weather, occupancy and useful loads as well as the percentages.
4. Stop when the measurement boundary changes
A battery, another generator, vehicle discharge, DC generation figures or materially different measurement points require a fuller balance, including storage changes and losses. Do not treat G − X as direct household use in those cases. Even here, losses between the chosen AC measurement points are neglected; preserve that limitation rather than forcing inconsistent figures to agree.
Period totals do not establish peak demand, hourly matching, a future annual yield, a bill or an outage backup design. Ordinary solar generation alone does not guarantee electricity during a grid outage. Keep backup capability separate and follow the equipment documentation and competent installer’s advice.
US Department of Energy — Solar and Resilience Basics ↗A minimal matched-period record
| Record | What to retain | Do not substitute |
|---|---|---|
| Generation G | AC generation kWh, source and both timestamps | Array kWp, DC yield or lifetime total |
| Export X | Gross exported kWh for that same interval | Generation or an estimated payment |
| Import I | Gross imported kWh for that same interval | Net import after subtracting export |
| Boundary | No storage/other generator; known meter points and gaps | An unexplained dashboard percentage |
Worked example
One fictional period, two different percentages
Assume matched AC records of 600 kWh generation, 350 kWh export and 450 kWh import, with no storage or other generation.
S = 600 − 350 = 250 kWh D = 250 + 450 = 700 kWh 100 × 250 / 600 = 41.7% 100 × 250 / 700 = 35.7%
41.7% of generation was used locally; solar supplied 35.7% of household electricity. Import and export can both occur within one period. Neither percentage proves outage autonomy or financial savings.
What to have ready
- Same start/end times, timezone and kWh units
- Separate gross generation, import and export records
- Confirmed no-storage, single-generator boundary
- Missing readings, resets, resolution and meter points recorded
- Useful household demand and weather context retained
Common mistakes
- Dividing by generation for both percentages.
- Using net billing data as gross grid import.
- Applying a no-storage balance to a battery system.
- Treating 100% self-consumption as outage protection.
Use and limitations. Sources include US public agencies. Principles are general; regulations and design values must be checked where you live.Worked examples use stated hypothetical inputs.