Solar sizing from electricity bills means using the customer’s real consumption, demand and tariff pattern to decide a sensible rooftop capacity. It is not roof-area guessing. A responsible EPC first checks whether the site can consume the solar generation, whether approvals allow it and whether the payback logic survives the actual bill structure.
For a factory owner, this is the difference between a proposal that looks nice and a plant that matches operations. For a consultant, it is the difference between a confident meeting and a painful argument after commissioning.
Why is one bill not enough for solar sizing?
One electricity bill is only one month of behaviour. It may show a shutdown month, a festival season, a maintenance period, a low production month or an unusually high cooling month. Sizing solar from that single page can push the proposal in the wrong direction.
Collect a full year of bills where possible. If the unit recently expanded or changed shifts, collect the older bills but mark the change clearly. Do not silently mix past and future operating patterns.
The bill set should show:
- monthly kWh or kVAh consumption
- maximum demand and contract demand
- tariff category and connection type
- time-of-day rows, if present
- power factor or derived PF
- arrears, adjustments and fuel surcharge rows
- sanctioned load or connected load references
The article on factory load profiles explains why this monthly picture still needs operating context. Bills show totals. Solar output depends heavily on when the load is present.
Which bill fields matter most?
Monthly units are the starting point, not the whole answer. A client may ask for a plant that wipes out the light bill, but the bill may contain demand charges, fixed charges, duties, meter rent, adjustments and penalties that solar may not remove.
Demand is especially important for factories. Rooftop solar can reduce grid import during sunny hours, but it does not automatically remove the maximum demand recorded during cloudy periods, evening shifts, early morning starts or simultaneous large loads. A proposal that treats the entire bill as unit-linked savings will overstate the benefit.
Power factor also needs care. If the customer has kVAh billing or PF penalty, solar does not replace APFC maintenance. Read solar payback mistakes in India before using a blended bill rate.
| Bill field | Why it matters for sizing | Practical caution |
|---|---|---|
| Monthly units | Sets broad energy opportunity | Compare across seasons |
| Maximum demand | Shows peak import behaviour | Solar may not reduce it reliably |
| ToD rows | Shows timing value | Match generation hours to tariff slots |
| Tariff category | Controls billing treatment | Verify current DISCOM category |
How do daytime load and export risk change the answer?
Solar generation arrives when the sun is available. If the plant runs mainly in the day, a larger share can be self-consumed. If the plant runs mostly at night, or has long shutdowns on sunny days, export and settlement rules become central.
Ask the owner simple operating questions:
- Which shifts run on normal days?
- Which loads run during lunch breaks?
- Does the plant shut on weekly holidays?
- Are compressors, pumps or chillers left on when production stops?
- Is there a seasonal production cycle?
For a school, warehouse or office, weekend and vacation behaviour may matter more than the headline monthly units. For a textile or plastics unit, the shift pattern may be more useful than the roof size.
Where interval data is available from a meter, portal or logger, use it. Where it is not available, use production schedules, operator interviews and bills together. Be honest about the uncertainty.
What about sanctioned load and DISCOM approvals?
Every rooftop solar proposal must pass through the local approval route. Many Indian owners discover late that connection records, sanctioned load, name mismatch, meter category or old paperwork can slow down the project.
Do not treat approval as a back-office formality. Before freezing capacity, check the current net metering or net billing process for that DISCOM. The policy explainer on net metering and rooftop solar policy is useful for the roles of MNRE, MoP, ERCs and DISCOMs, but the final process is local.
For an EPC, the safer language is: “This is the technically suggested size from bills and load pattern. Final approved capacity depends on DISCOM rules, site documents and inspection.”
That sentence protects trust. It also prevents the common mistake of selling a capacity before checking the consumer number, load sanction and meter details.
Tips from the field
- Ask for the complete bill PDF for every month, because screenshots usually hide demand, PF, ToD and adjustment rows.
- Mark shutdown months separately before averaging consumption, because a closed plant should not define normal solar size.
- Check whether the customer’s major loads run during solar hours, because monthly units alone do not prove self-consumption.
- Verify sanctioned load, consumer name and tariff category before promising the final rooftop capacity.
- Explain which bill charges solar can reduce and which charges may remain after installation.
- Keep a separate note for assumptions, such as future expansion, shift additions and expected roof availability.
How should the EPC present a credible size?
A credible sizing note should show the reasoning in plain language. It does not need a fancy model, but it must connect bills, site operation and approval risk.
Include these points in the proposal:
- the bill months reviewed
- the normal and abnormal months identified
- the expected self-consumption logic
- the charges excluded from solar savings
- the approval assumptions
- the next data required before final design
The solar EPC bill analysis checklist can sit before the site survey. It helps the consultant avoid a weak first meeting.
Sizing is not about finding the largest number the roof can hold. It is about proposing the size the customer can use, approve, finance and defend when the first post-solar bill arrives.