One installer offers solar for $24,000. Another wants $38,000 and includes a battery. Is backup power worth the extra $14,000? You cannot tell yet. The second installer may also be offering more panels, a different inverter or electrical work the first bid excludes. Ask the second installer for a cash price on the same array with the battery option removed. That one number lets you separate the price of choosing storage from the price of choosing a different solar design.
Put the two proposals beside each other
This example uses invented US cash prices before incentives. They illustrate the calculation, not prevailing prices. Assume both installers include permitting, installation and the same roof-work scope. Proposal B’s solar-only option is a written alternate from that installer.
| What the homeowner gets | A: solar only | B: solar + backup |
|---|---|---|
| Total cash price | $24,000 | $38,000 |
| Array capacity (DC) / estimated first-year output (AC)¹ | 8 kW / 11,200 kWh | 9 kW / 11,700 kWh |
| Cash price for that array without storage | $24,000 | $24,300 |
| Additional cost to include installed backup | No backup offered | $13,700 |
| Solar-only cash price per DC watt | $24,000 ÷ 8,000 = $3.00 | $24,300 ÷ 9,000 = $2.70 |
¹ DC kW is the panel array’s rated size. AC kWh is the electricity the design is expected to deliver over a year. The estimate is not a meter reading.
The decision is now clearer. Of the $14,000 difference, $300 buys a different solar design and $13,700 buys the backup package. If you only want solar, compare A with B’s $24,300 alternate. If you want backup, ask A to price the same protected circuits before comparing complete projects. Dividing $38,000 by 9,000 watts gives $4.22/W, but that number answers neither question.
If the installer will not separate the battery price
Ask for two versions of the same design: identical panels, roof layout and production assumptions, with and without storage. Highlight anything else that changes. A hybrid inverter, gateway and critical-load panel may all be part of the upgrade. The price difference is then the installed package premium, not the price of the battery box. If removing storage requires a different design and no comparable alternate is available, mark the storage premium “unknown.” Do not fill the gap with a battery retail price from another website.
Put a value on the extra solar output
B adds 1 kW of panels but only 500 projected kWh per year. Its projected yield is 1,300 kWh per installed kW, compared with 1,400 for A. Ask for the roof-plane layout and monthly production estimates. Extra panels on a different slope may have a different yield. Check that both estimates use the same shading assumptions and reporting period.
For a simple illustration, suppose 300 of those additional 500 kWh replace purchases at $0.25/kWh and 200 are exported at $0.06/kWh. Their first-year energy value is $75 + $12 = $87, before any other bill effects. Multiplying all 500 by the retail rate would give $125. Use your utility’s actual import and export terms and the hours the extra energy arrives. This is a comparison of the two solar-only designs; do not count that same exported energy again as a battery saving. How to distinguish production, self-use and exports on your bill.
Test the backup promise against one night in your house
Suppose B specifies 10 kWh usable, and the battery starts the outage at 70% charge. If its discharge floor during backup is 10%, the available window is 6 kWh. With an illustrative 90% delivery efficiency, that gives 5.4 kWh at the loads. At a measured average of 450 W, the simple estimate is 5.4 ÷ 0.45 = 12 hours, before additional overhead or solar recharge. If the quoted capacity is already stated as delivered AC energy, do not apply the same conversion loss twice. A grid-connected reserve setting and a discharge floor during an outage are not necessarily the same setting.
Now check power separately. A pump requiring 3 kW to start alongside 0.8 kW of other loads briefly demands 3.8 kW. The 12-hour estimate says nothing about whether it starts. Ask the installer to identify the backed-up circuits, continuous output, surge rating and supported surge duration for the installed configuration. A pump omitted from the backup panel receives no power, however large the battery.
Use this worksheet before choosing a bid
| Copy from each written offer | What to flag |
|---|---|
| Cash price before incentives; financed principal; down payment; term; total payments | If one figure is a monthly payment, request the cash alternate. Show assumed incentives separately. |
| Annual and monthly AC kWh; panel layout; tariff used in savings projection | Flag annual savings that value every export at the full purchase rate without support from your tariff. |
| Usable battery kWh; output kW; backed-up circuit list; gateway and panel work | Mark each line included, excluded or unknown. Ask who pays if a panel change is needed. |
| Equipment and labor warranty; service contact; exclusions | Ask who handles diagnosis and labor if the original installer stops trading. Record the answer in writing. |
A message you can send to both installers
“Please send the cash price before incentives for your proposed array, both with and without battery backup. Keep the array layout the same or identify what changes. For each version, include monthly AC production, the import and export rates used for savings, and any excluded work. For backup, list the circuits served, usable energy, continuous and startup power, and the assumed starting charge behind your runtime estimate. Please show financing terms separately.”
If your priority is bill reduction, compare the solar-only alternatives first and ask for the incremental bill savings from storage as a separate estimate. If your priority is outages, write down the appliances and hours you need before looking at the battery brand. In either case, an unanswered item on the worksheet gives you a concrete reason to request a revised proposal. For questions about installer credentials, roof work and warranty responsibility, see the US Department of Energy’s installer guide.
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